# Physics Semester 1 HW Final Review

The friction force is equal and opposite to your push
When you push horizontally on a crate that doesn’t slide on a level floor, how great is the force of friction on the crate?
The friction force increases and is opposite in direction to your push.
As you increase your push on a stationary crate, will friction on the crate increase also?
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You push with a force equal to and opposite the dynamic friction force.
Once the crate is sliding, how hard do you push to keep it moving at constant velocity?
Acceleration is proportional to the net force
How does acceleration depend on the net force?
Static friction is normally greater than dynamic friction
Which is normally greater: static friction or sliding friction on the same object?
The force of friction is approximately independent of speed.
How does the force of friction for a sliding object vary with speed?
Fluid friction increases as speed increases.
Does fluid friction vary with speed?
Weight
Which depends on gravity?
Shaking measures mass, whereas lifting measures weight
Shake something to and fro and you’re measuring its what? Lift against gravity and you’re measuring its what?
Mass is kilograms; force is newtons
The Standard International unit for mass is ____. The Standard International unit force is _____.
One newton
What is the approximate weight of a quarter-pound hamburger after it is cooked?
10 N
What is the weight of a 1 – kg brick resting on a table
Weight
In the string-pull illustration, a gradual pull of the lower string results in the top string breaking. does this occur because of the ball’s weight or its mass
Mass
In the string-pull illustration, a sharp jerk on the bottom string results in the bottom string breaking. Does this occur because of the ball’s weight or mass.
Acceleration is inversely proportional to mass
How does acceleration depend on mass?
Acceleration is proportional to net force and inversely proportional to mass.
State Newton’s second law of motion.
If the mass doubles, then the weight doubles, too
Give an example of what it means to say mass and weight are proportional to each other?
The acceleration triples
If the net force acting on a sliding block is somehow tripled, what happens to the acceleration?
The acceleration remains the same
If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change?
The acceleration is at right angles to the net force.
How does the direction of acceleration compare with the direction of the net force that produces it?
When gravity is the only force acting on an object, it is in free fall.
What is the condition for an object experiencing free fall?
The ratio is g
The ratio circumference/ diameter for all circles is pie. hat is the ratio force/mass for freely falling bodies.
The ratio of the weight to mass is the same for all objects in the same locality
Why doesn’t a heavy object accelerate more than a light object when both are freely falling?
The net forces are 6 N and 0 N, respectively
What is the net force that acts on a 10 N falling object when it encounters 4 N of air resistance? When it encounters 10 N of air resistance?
10 N
What is the net force that acts on a 1 kg falling object?
The force depends mostly on frontal area and speed
What two principal factors affect the force of air resistance on a falling object?
The acceleration is 0
What is the acceleration of a falling object that has reached its terminal velocity?
Terminal speed must be higher to make air drag equal the gravity force for a heavier person.
Why does a heavy parachutist fall faster than a lighter parachutist who wears a parachute of the same size?
The faster object encounters more air resistance
If two objects of the same size move through the air at different speeds, which encounters the greater air resistance?
You exert an action force and the wall exerts an equal and opposite reaction force.
When you push against a wall with your fingers, they bend. Identify the force or forces involved.
The boxer can only hit the tissue paper with a force as large as the tissue paper can exert on the boxer, and the low-mass tissue can only exert a weak force
A boxer can hit a heavy bag with great force. Why can’t he hit a piece of tissue paper in midair with the same amount of force?
Two forces, an action and a reaction, are needed for an interaction
How many forces are required for an interaction?
For every action there is an equal and opposite reaction
State Newton’s third law of motion.
The reaction force is the force by the ball on the bat
Consider hitting a baseball with a bat. If we call the force on the bat against the ball the action force, identify the reaction force.
Yes, there must be an external net force on the system
To produce a net force on a system, must there be an externally applied net force?
If you kick it, there is a net force; if both of you kick it, there is not
Consider the system of a single football. If you kick it, is there a net force to accelerate the system? If a friend kicks it at the same time with an equal and opposite force, is there a net force to accelerate the system?
Yes, you pull up on Earth with the same force
Earth pulls down on you with a gravitational force that you call your weight. Do you pull up on Earth with the same amount of force?
Remember F=ma, and note that the cannon has a much greater mass than the cannon ball, so the cannon accelerates less for the same force
If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why d the cannonball and cannon have very different accelerations?
The rocket is propelled by the reaction force from the particles accelerated out the rear by an action force from the rocket.
Identify the force that propels a rocket
the helicopter exerts downward forces on air; the reaction forces of the air on the helicopter are upward and called lift
How does a helicopter get its lifting force?
No, when you touch a person, they must touch you with an equal and opposite force
Can you physically touch a person without the person touching you with the same amount of force?
A vector can be broken into two components at right angles that add together to make the original vector
What is meant by the term vector resolution?
The magnitude of the normal vector decreases
What happens to the magnitude of the normal vector on a block resting on an incline when the angle of the incline increases?
It is equal and opposite
What is the force of friction acting on a shoe at rest on an incline compared with the resultant of the vectors mg and N?
The vertical component decreases in magnitude until it reaches the maximum height and then increases; the horizontal component is constant
How does the magnitude of the vertical component of velocity for a ball tossed at an upward angle change as the ball travels upward? How about the horizontal component of velocity when air resistance is negligible?
Inertia, acceleration, action-reaction
Newton’s first law is often called the law of _____; Newton’s second law is the law of _____ and Newton’s third law is the law of ____
Newton’s third law
Which of Newton’s three laws focuses on interactions?
The moving skateboard
Which has a greater momentum: a heavy truck at rest or a moving skateboard?
Impulse is force times a time interval
Distinguish between force and impulse
Increasing the force or increasing the time interval
An impulse can be increased by
The force is applied for a longer time in the long cannon
For the same force, why does a long cannon impart more speed to a cannonball than a short cannon?
exert the greatest force over the longest time
To impart the greatest momentum to an object, you must
Long contact time decreases the magnitude of the average force and is favorable
When you are struck by a moving object, is it favorable that the object makes contact with you over a short time or over a long time?
The average force is increased
Why is a force that is applied for a short time more effective in karate?
Rolling with the punch increases contact time, which decreases the force
Why is it advantageous to roll with the punch in boxing?
A baseball that is caught and then thrown back
Which undergoes the greatest change in momentum (if all of the baseballs have the same speed just before being caught and just after being thrown)?
A baseball that is caught and then thrown back
In the preceding question, which case requires the greatest impulse?
No, never
Can you produce a net impulse on an automobile if you sit inside and push on the dashboard
Yes, always
Is it correct to say that, if no net impulse is exerted on a system, then no change in the momentum of the system will occur?
It means momentum does not change
What does it mean to say that momentum is conserved?
the momentum of the cannon is equal to the magnitude of the momentum of the cannon ball and points in the opposite direction
When a cannonball is fired, the momentum of the system (cannon + cannonball) is conserved if
Momentum is conserved in elastic and inelastic collisions
In which is momentum conserved: an elastic collision?
The speed of car B is equal to the initial speed of A
Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?
Their speed is half the initial speed of car A
If the equally massive cars of the preceding question stick together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A?
1.41 kg m/s
Suppose a ball of putty moving horizontally with 1 kg m/s of momentum collides with and sticks to an identical ball of putty moving vertically with 1 kg m/s of momentum. What is the magnitude of their combined momentum?
1.41 kg m/s before and after
In the preceding question, what is the total momentum of the balls of putty before and after the collision?
joule
What is the unit of work?
Work
A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, and an impulse changes the momentum of that object. What do we call the quantity force multiplied by distance?
Zero
How much work is done on a satellite in a circular orbit about Earth?
Both take the same 1000 J
Which requires more work: lifting a 50 kg sack a vertical distance of 2 m or lifting a 25 kg sack a vertical distance of 4 m?
Lifted in the same time, the power is the same. The light sack moving in half the time requires double the power.
If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for each compare? How about for the case in which the lighter sack i moved the same distance in half the time?
Energy
Exactly what is it that enables an object to do work?
It would have twice as much potential energy
A car is raised a certain distance in a service-station lift, thus giving it potential energy relative to the floor. if it were raised twice as high, how much more potential energy would it have?
The car with twice the mass has twice the potential energy
Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, compare their gains of potential energy
When the potential energy changes
When is the potential energy of something significant?
It has four times as much
When the speed of a moving car is doubled, how much more kinetic energy does it have?
It takes four times the work and four times the stopping distance
Compared with a car moving at some original speed, how much work must the brakes of a car supply to stop a car that is moving twice as fast? how will the stopping distances compare?
300 J
If you push a crate horizontally with 100 N across a 10 m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?
Speed has no effect on the friction
How does speed affect the friction between a road and a skidding tire?
10 kJ
What will the kinetic energy of a pile driver ram be if it starts from rest and undergoes a 10 kJ decrease in potential energy?
The energy is kinetic energy before it hits the ground; it is thermal energy after
An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground? When it hits the ground?
Thermonuclear fusion energy
What is the source of energy in sunshine?
Reused energy that otherwise would be wasted
What is recycled energy?
Force and distance, but not energy
Can a machine multiply input force? Input distance? Input energy?
Distance is diminished to one-quarter
If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?
150 N
A force of 50 N is applied to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, how much force does it exert?
100%
What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?
60%
If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?
The Sun
What is the ultimate source of the energy from fossil fuels, dams, and windmills?
Nuclear power in Earth’s interior
What is the ultimate source of geothermal energy?
No. It takes energy to make hydrogen
Can we correctly say that hydrogen is a new source of energy? Why or why not?
m/s for tangential, RPM for rotational
What are the units of measurement for tangential speed? For rotational speed?
Tangential speed increases with distance. Rotational speed is constant
On a rotating turntable, how do tangential speed and rotational speed vary with distance from the center?
The tangential speed of the wide end is faster
A tapered cup rolled on a flat surface makes a circular path. What does this tell you about the tangential speed of the rim of the wide end of the cup compared with that of the rim of the narrow end?
The tangential speed is proportional to the radius. The inside of the wheel rolls on a larger radius than the outside of the wheel
How does the tapered rim of a wheel on a railroad train allow one part of the rim to have a greater tangential speed than another part when it is rolling on a track?
Most of the mass is concentrated far from the axis
How is a flywheel constructed to maximize its rotational inertia?
The distribution of mass about the axis of rotation
Inertia depends on mass; rotational inertia depends on mass and something else. What?
It increases
As distance increases between most of the mass of an object and its center of rotation, how does rotational inertia change?
Axis along the lead, through the center, through one end
Consider three axes of rotation for a pencil: along the lead, at right angles to the lead of the middle, and at right angles to the lead at one end. Rate the rotational inertias about each axis from smallest to largest.
The easiest is chocked up and a short up
Which is easier to get swinging: a baseball bat held at the narrow end or a bat held closer to the massive end? Also answer for a short bat versus a long bat.
Bending your legs shortens them, thus reducing rotational inertia
Why does bending your legs when running enable you to swing your legs to and fro more rapidly?
The solid disk will because the mass is closer to the axis of rotation
Which will have the greater acceleration rolling down an incline: a hoop or a solid disk? Why?
Torque tends to twist or change the state of rotation of the object
What does a torque tend to do to an object?
It is the perpendicular distance from the rotational axis to the line along which the force acts.
What is meant by the “lever arm” of a torque?
They are equal in magnitude and opposite in direction
How do clockwise and counterclockwise torques compare when a system is balanced?
it will rotate about the center of mass
If you toss a stick into the air, it appears to wobble all over the place. Specifically, about what place does it wobble?
The center of mass and the center of gravity are both at the center of the spherical ball.
Where is the center of mass of a baseball? Where is its center of gravity?
Somewhere straight below where your hands grab the rope
If you hang at rest by your hands from a long vertical rope, where is your center of gravity with respect to the rope
In the center of the ball
Where is the center of mass of a hollow soccer ball?
A vertical line through the center of gravity must pass inside the support base of the object
What is the relationship between the center of gravity and the support base for an object that is in stable equilibrium?
A vertical line through the center of gravity passes inside its support base
Why doesn’t the Leaning Tower of Pisa topple over?
A vertical line through the center of gravity of your body is in front of the tips of your toes, which is outside your support base
Why can’t you stand with your heels and back to a wall and then bend over to touch your toes and return to your stand-up position?
Toward the center of the circle
When you whirl a can at the end of a string in a circular path, what is the direction of the force you exert on the can?
The force is inward on clothes and there is none on water
Is it an inward force or an outward force that is exerted on the clothes during the spin cycle of an automatic washing machine? what about the direction of the force on the water?
No force; Newton’s first law
If the string that holds a whirling can in its circular path breaks, what kind of force causes it to move in a straight-line path: centripetal, centrifugal, or no force? What law of physics supports your answer?
There is no force as viewed by someone outside the car. To them you move in a straight line
If you are not wearing a seat belt in a car that rounds a curve, and you slide across your seat and slam against a car door, what kind of force is responsible for your slide: centripetal, centrifugal, or no force?
It is not a fundamental force of nature. instead, it is a force that only appears in an accelerating frame of reference
Why is centrifugal force in a rotating frame called a fictitious force?
1) Spin a station shaped like a bicycle wheel
2) Spin two pods connected by a cable
3) Rotate a cylinder to create centrifugal force as viewed by a person on the inside of the curved out wall
How can gravity be simulated in an orbiting space station?
Angular momentum depends on the distribution of mass, whereas linear momentum depends on the total mass
Distinguish between linear momentum and angular momentum
Angular momentum does not change without an applied external torque
What is the law of inertia for rotating systems in terms of angular momentum?
Her angular momentum does not change. Her spin rate doubles
If a skater who is spinning pulls her arms in so as to reduce her rotational inertia by half, by how much will her angular momentum change? By how much will her rate of spin change?
1) The equation for the force of gravity
2) The law of universal gravitation
3) The Newtonian synthesis
What did Newton discover about gravity?
The union of terrestrial laws and cosmic laws
What is the Newtonian synthesis?
It falls away from the straight path that it would fallow if there were no forces acting on it
In what sense does the Moon “fall”?
6.67 x 10^-11
What is the magnitude of the gravitational force between two 1 kg bodies that are 1 m apart?
10 N
What is the magnitude of Earth’s gravitational force on a 1 kg body of Earth’s surface?
With a known mass, a scale, and the radius of Earth, you can calculate the unknown mass of Earth if you know G.
When G was first measured by Henry Cavendish, newspapers of the time hailed his experiment as the “weighing Earth experiment.” Why?
The force decreases to 1/4 of its initial value
How does the force of gravity between two bodies change when the distance between them is doubled?
It is 1/4 as thick
How does the thickness of paint sprayed on a surface change when the sprayer is held twice as far away?
You weigh more in Death Valley because you are closer to the center of Earth
Where do you weigh more: at the bottom of Death Valley or atop one of the peaks of the Sierra Nevada? Why?
More compressed while accelerating upward, but less compressed while accelerating downward?
Would the springs inside a bathroom scale be more compressed or less compressed if you weighed yourself in an elevator that was accelerating upward? Downward?
There would be no more compression and no more expansion for both upward and downward motion
Would the springs inside a bathroom scale be more compressed or less compressed if you weighed yourself in an elevator that was moving upward at constant velocity? Downward at constant velocity?
For a non-accelerating mass near the surface of Earth
When is your weight measured at mg?
Your weight is greater than mg when you are accelerating upward. your weight is zero when you are in free fall
Give an example of when your weight is greater than mg. Give an example of when your weight is zero.
They are in free fall
Why are occupants of the International Space Station weightless?
Tides depend on the difference
Do tides depend more on the strength of gravitational pull or on the difference in strengths? Explain
The Sun exerts a stronger force on the side of Earth nearest the Sun, and the Moon exerts a stronger force on the side nearest the Moon
Describe how the gravitational forces from the Sun and the Moon compare from one side of Earth to the other.
Spring tides are higher, because the tides from the Moon and Sun add together
Which has the higher tides: spring tides or neap tides? Why?
Yes, due to the difference in gravitational force across the core
Do tides occur in the molten interior of Earth for the same reason that tides occur in the oceans?
At full Moon and new Moon, the tides from the Moon and the Sun add because they are in line with Earth
Why are all tides greatest at the tie of a full Moon or new Moon?
No. The torque requires a distance between the center of mass and the center of gravity
Would a torque on the Moon occur if the Moon were spherical, with both its center of mass and center of gravity in the same location? Why?
It is a force field on any body with mass. Its strength is the force per unit mass on a test mass
What is gravitational field, and how can its strength be measured?
Zero N/kg
What is the magnitude of the gravitational field at Earth’s center?
g/2 m/s^2
For a planet of uniform density, how would the magnitude of the gravitational field halfway to the center compare with the field at the surface?
Zero N/kg
What would the magnitude of the gravitational field be anywhere inside a hollow, spherical planet?
Moving in curved 4-dimensional space time
Newton viewed the curving of the path of a planet as being caused by a force acting on the planet. How did Einstein view the curved path of a planet?
It would increase
If Earth shrank, but there was no change in its mass, then what would happen to your weight at the surface?
It increases
What happens to the strength of the gravitational field at the surface of a star that shrinks?
The escape velocity of the black hole is greater than the speed of light
Why is a black hole invisible?
Neptune caused the perturbations, and later Pluto was discovered
What was the cause of perturbations discovered in the orbit of the planet Uranus? What later discovery did this lead to?
An object that continues in motion by its own inertia
What exactly is a projectile?
Gravity is a purely vertical force
Why does the vertical component of velocity for a projectile change with time, whereas the horizontal component of velocity doesn’t change?
Rising or falling, it does not change
A stone is thrown upward at an angle. What happens to the horizontal component of its velocity as it rises? As it falls?
It decreases while rising, but it increases while falling.
A stone is thrown upward at an angle. What happens to the magnitude of the vertical component of its velocity as it rises? As it falls?
5 m, 20 m
A projectile falls beneath the straight-line path it would follow if there were no gravity. how many meters does it fall below this line if it has been traveling for 1 s? For 2 s?
NO
Do your answers to the preceding question depend on the angle at which the projectile is launched?
20 degrees
A projectile is launched upward at an angle of 70 degrees from the horizontal and strikes the ground a certain distance downrange. For what other angle of launch at the same speed would this projectile land just as far away?
100 m/s
A projectile is launched vertically at 100 m/s. If air resistance can be ignored, at what speed will it return to its initial level?
1) In a circular orbit around a spherical planet, the force and the fall are always toward the center
2) A projectile can “fall around Earth” if the distance it falls matches the curvature of Earth.
3) The projectile falls 5 m for every 8 km and so does Earth.
How can a projectile “fall around the Earth”?
5 m, 8 km/s
How far does a projectile drop in 1 second? What is the speed needed for a projectile to orbit Earth?
To avoid air resistance
Why is it important that the projectile in the preceding question be 100 km or higher above Earth?
The force is at a right angle to the motion
Why doesn’t the force of gravity change the speed of a bowling ball as it rolls along a bowling lane?
The force is at a right angle to the velocity
Why doesn’t the force of gravity change the speed of a satellite in circular orbit?
90 min
How much time does it take for a complete revolution of a satellite in close orbit about Earth?
The period is longer, and the speed is slower
For orbits of greater altitude, is the period longer or shorter? Is the speed faster or slower?
Gravity slows the satellite as it moves away and speeds it up on its return
Why does the force of gravity change the speed of a satellite in an elliptical orbit?
Greatest nearest Earth; lowest furthest from Earth
At what part of an elliptical orbit does an Earth satellite have the greatest speed? The lowest speed?
Brahe, Kepler, Newton
Who gathered the data that showed planets traveling in elliptical paths around the Sun? Who discovered elliptical orbits? Who explained them?
The period squared was proportional to the distance cubed
What did Kepler discover about the periods of planets and their distances from the Sun?
The force was parallel to the motion for Kepler, whereas the force was toward the planet for Newton
What was the direction of the force on a planet in a circular orbit in Kepler’s thinking? In Newton’s thinking?
The force of gravity is perpendicular to the motion in a circular orbit but not in an elliptical
Why is kinetic energy a constant for a satellite in a circular orbit but not for a satellite in an elliptical orbit?
Both
Is the sum of kinetic and potential energies a constant for satellites in circular orbits, in elliptical orbits, or in both
1.41
What is the ratio of escape speed from Earth to circular orbital speed? Ignore air resistance.
What is the condition for an object experiencing free fall?

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