e
Equilibrium occurs when
a. all the forces acting on an object are balanced.
b. the sum of the +x forces on an object equals the sum of the -x forces.
c. the net force on the object is zero.
d. the sum of the upward forces equals the sum of the downward forces.
e. all of the above
b
A girl whose weight is 500 N hangs from the middle of a bar supported by two vertical strands of rope. What is the tension in each
strand?
a. 0 N.
b. 250 N.
c. 500 N.
d. 750 N.
e. 1000 N.
d
The weight of a person can be represented by a vector that acts
a. in a direction that depends on where the person is standing.
b. perpendicular to the ground underneath the person.
c. parallel to the ground.
d. straight down, even if the person is standing on a hill.
e. all of the above
c
What would be the safest way to put up a clothesline?
a. It doesn’t make any difference which way the line is strung.
b. With the line very tight
c. With some slack in the line
e
A barge is being pulled along a canal by two ropes that make equal angles with the direction in which the barge points. Assuming
the two pulls on the barge are equal, in what direction does the barge move?
a. It oscillates back and forth between the two banks.
b. It moves straight ahead.
c. It moves in the direction of the resultant force on it.
d. both A and B
e. both B and C
c
A 100-N lantern is suspended by a pair of ropes with 120 degrees between them (each 60 degrees from the vertical). The tension in
each rope is
a. less than 100 N.
b. more than 100 N.
c. 100 N.
b
You are helping your aunt move a piano on wheels from one room to another. When you push the piano horizontally, it moves at
constant speed. What can you say about the piano?
a. It is in static equilibrium.
b. It is in dynamic equilibrium.
c. It is in equilibrium because it doesn’t experience a friction force.
d. It is in equilibrium because it experiences net force opposite to the friction force.
d
In order to find the components of a vector, you should
a. draw the vector with correct magnitude and orientation.
b. measure the sides of the rectangle.
c. draw a rectangle so that the vector is the diagonal.
d. all of the above
e
In physics, work is defined as
a. force times time.
b. force divided by distance.
c. distance divided by time.
d. force divided by time.
e. force times distance.
d
If you lift two loads up one story, how much work do you do compared to lifting just one load up one story?
a. One quarter as much
b. One half as much
c. The same amount
d. Twice as much
e. Four times as much
c
The unit of work is the
a. watt.
b. meter.
c. joule.
d. newton.
e. second.
c
Power is defined as the
a. force on an object divided by the time the force acts.
b. work done times the time taken to do that work.
c. work done on an object divided by the time taken to do the work.
d. distance divided by the time taken to move that distance.
e. force on an object times the distance the object moves.
e
The unit of power is the
a. newton.
b. meter.
c. joule.
d. second.
e. watt.
c
Potential energy is the energy an object has because of its
a. density.
b. size.
c. location.
d. speed.
e. temperature.
d
The amount of potential energy possessed by an elevated object is equal to
a. the power used to lift it.
b. the distance it is lifted.
c. the force needed to lift it.
d. the work done in lifting it.
e. the value of the acceleration due to gravity.
b
Kinetic energy of an object is equal to
a. its mass multiplied by its acceleration squared.
b. one half the product of its mass times its speed squared.
c. its mass multiplied by its speed.
d. one half the product of its mass times its speed.
e. its mass multiplied by its acceleration.
d
An arrow in a bow has 70 J of potential energy. Assuming no loss of energy to heat, how much kinetic energy will it have after it
has been shot?
a. 0 J
b. 35 J
c. 50 J
d. 70 J
e. 140 J
c
A ball is thrown into the air with 100 J of kinetic energy, which is transformed to gravitational potential energy at the top of its trajectory. When it returns to its original level after encountering air resistance, its kinetic energy is
a. 100 J.
b. more than 100 J.
c. less than 100 J.
d. Not enough information given.
c
An object that has kinetic energy must be
a. at rest.
b. falling.
c. moving.
d. elevated.
c
An object that has kinetic energy must have
a. acceleration.
b. a force applied to maintain it.
c. momentum.
d. none of the above
a
An object at rest may have
a. energy.
b. speed.
c. velocity.
d. momentum.
e. none of the above
e
How many joules of work are done on a box when a force of 25 N pushes it 3 m?
a. 1 J
b. 3 J
c. 8 J
d. 25 J
e. 75 J
c
How much power is required to do 40 J of work on an object in 5 seconds?
a. 0 W
b. 5 W
c. 8 W
d. 40 W
e. 200 W
A vector is a quantity that has direction and magnitude; a scalar has only magnitude. Force, velocity, acceleration, and weight are all vectors. Mass, time, temperature, and speed are scalars.
Briefly distinguish between vectors and scalars, giving examples of each.
Aristotle thought there were two kinds of motion—natural and violent. Natural motion referred to things falling and moving through space. Violent motion referred to motion under imposed forces. Copernicus thought that Earth moves around the sun. Galileo developed the modern ideas of force and inertia. Force is needed to change motion.
Write a short paragraph about what the early scientists from Aristotle to Galileo thought about the nature of motion.
The law of inertia says that an object in motion will continue in its state of motion unless an outside force acts on it. This means that a book at rest on a table will stay at rest on the table until someone or something picks it up, bumps it, or in some other way forces it to move. It also means that once we throw a ball in deep space, the ball will continue in a straight line until it runs into something.
State the law of inertia in its full form and give examples of how it works.
Mass generally refers to the quantity of matter in an object, whereas weight is the force due to gravity on an object. The mass of an object is the more basic quantity, for it is the same regardless of gravity. For example, a 1-kg brick has a mass of 1 kg on Earth, 1 kg on the moon, and 1 kg in deep space. Its weight, however, is 10 N on Earth, 1.6 N on the moon, and 0 N in deep space.
Write a short paragraph on the difference between mass and weight. Give examples.
You, the coin, and the air are all moving horizontally at the same speed. When you flip the coin into the air, it will continue moving horizontally at that speed (Newton’s first law).
Suppose you are on an airplane moving at high speed. If you flip a coin straight up it will land in your lap rather than a great distance behind you. Explain.
Speed is the rate at which an object covers distance. Velocity is speed in a direction. When you get in a car and travel on a highway at 90 km/hr that is your speed. If you are traveling south, then your velocity is 90 km/h S.
Write a short paragraph explaining the difference between speed and velocity, and give examples of both.
Acceleration is the rate of change of velocity. Since velocity has both speed and direction, if the direction of a moving object changes, as in rounding a corner, then the object is accelerating.
Write a short paragraph explaining what acceleration is and why a car is accelerating as it rounds a corner.
The acceleration due to gravity on Earth is 10 m/s2. This means that when an object falls straight down on Earth, it gains 10 m/s of speed in each second. On the moon it gains a speed of 1.6 m/s each second. On the moon, it is roughly one sixth of that, or 1.6 m/s2
Write a short paragraph on how fast things fall on Earth. Compare this to motion on the moon, where the acceleration due to gravity is about 1.6 m/s2.
Resultant velocity of the plane is the square root of (2302 + 802) km/h which equals 244 km/h. In general, the graphical method will not be as precise as the analytical method. However, the answers should be within a few percent of each other.
A plane flies north at 230 km/h. At the same time, there is wind of 80 km/h from the west. Use both a scaled diagram and the Pythagorean Theorem to find the resultant velocity of the plane. How closely do your answers agree?
One ball is projected from the top of a table with horizontal velocity. Another ball is dropped from the same starting location. Both balls hit the ground at the same time. Both balls fall the same distance in the same time interval, the only difference being that the projected ball travels horizontally at the same time the vertical motion takes place. Motion in one direction is independent of motion in the other direction. The only thing in common between the two motions is the length of time they are falling. The only force on each ball is the force of gravity, which accelerates the balls only downward.
Figure 3.9 in the text shows two balls released simultaneously from a mechanism that allows one ball to drop freely
while the other is projected horizontally. Explain why the balls fall farther and farther each successive time interval.
Explain why the projected ball has the same vertical location as the falling ball.
while the other is projected horizontally. Explain why the balls fall farther and farther each successive time interval.
Explain why the projected ball has the same vertical location as the falling ball.
Newton’s second law, a = F / m reminds us that the acceleration of an object depends on two things: the force that acts on it and the object’s mass. The force that acts on a freely falling object is the force of gravity—its weight. It would seem that the greater the weight, the greater should be the acceleration of the fall. However, the mass of the object resists acceleration. This is inertia. The force of gravity is greater on the more massive object, but its inertia is correspondingly greater. This results in the same ratio of force to mass for the two falling objects. Hence their accelerations are the same.
Explain why the acceleration of two freely-falling objects having different masses is the same. Is the force on each mass the same?
Friction depends on the nature of the two surfaces that are in contact and also on how much force is pushing the two surfaces together. It always acts in a direction to oppose motion. If it didn’t, then once we started an object sliding, it would move faster and faster and never stop, until it ran out of surface or ran into something
What two things does the force of friction depend on? In which direction does friction act? What would happen if friction were to act in the same direction as the motion?
Terminal speed is the maximum speed of an object when it falls through a fluid. A skydiver will reach terminal speed when the force of air friction equals his or her weight. At that point the weight of the diver and the force of air friction on the diver are balanced, the net force is zero, and he or she falls with no acceleration (i.e. constant speed).
What is terminal speed? When a skydiver has reached terminal speed, what is the air resistance equal to? What is the skydiver’s acceleration?
The force of impact in catching a ball will be less if the time of impact is lengthened. Other situations in which you want to lengthen the time of impact might be in a car or bicycle collision, in catching a soccer ball, and in hitting a baseball (to give it the most momentum you can).
In terms of momentum change, explain why it is best to “give” when catching a baseball. Provide other examples of situations in which you want to lengthen the impact time in a collision.
Conservation of momentum means that in a given system and situation where no external forces are acting, net momentum is neither created nor destroyed. For example, momentum is conserved in collisions and explosions, where the forces that act are internal. When a firecracker bursts, the momentum of its fragments add up to the firecracker’s momentum just before bursting. On the other hand, when you drop a rock, the momentum of the rock changes because the force of gravity acts on it; however, if you consider the rock and Earth as one system, then Earth and the rock each acquire the same amount of momentum, but in opposite directions, so the total momentum of the system is and remains zero.
What does it mean to say that momentum is conserved? Give at least two examples where momentum is conserved in a collision.
7.2 N
What is the magnitude of the resultant of a 6.0-N force acting vertically upward and a 4.0-N force acting horizontally?
2254 N
A 230.0 kg bear grasping a vertical tree slides down at constant velocity. What is the friction force between the tree and the bear?
14 N
Two forces of 10 N both act on an object. The angle between the forces is 90. What is the magnitude of their resultant?
70 N
The following forces act on an object: 9 N north, 52 N south, and 55 N west. What is the magnitude of the net force?
100 N
How much (in Newton’s) does a 10.0-kg bag of grass seed weigh?
65 kg
A person weighs 650 N. What is the mass of the person?
12 N
On the surface of Jupiter, the acceleration due to gravity is about 3 times that on Earth. How much would a 0.40-kg rock weigh on Jupiter?
170 kg
On the surface of Jupiter, the acceleration due to gravity is about 3 times that of Earth. What would be the mass of a 170-kg rock on Jupiter?
17.6 m/s
What is the average speed of a cheetah that runs 88 m in 5 seconds?
7.5 km/h(s)
What is the average acceleration of a car that goes from rest to 60 km/h in 8 seconds?
20 m/s
A jet on an aircraft carrier can be launched from 0 to 40 m/s in 2 seconds. What is the acceleration of the jet?
4 m/s
A skateboarder starting from rest accelerates down a ramp at 2 m/s2 for 2 s. What is the final speed of the skateboarder?
5 m/s
An apple falls from a tree and 0.5 second later hits the ground. How fast is the apple falling when it hits the ground?
20 m/s
What speed must you toss a ball straight up so that it takes 4 s to return to you?
0.5 s
You toss a ball at 5 m/s straight upward. How much time will the ball take to reach the top of its path?
7 s
How much time does a car with an acceleration of 5 m/s2 take to go from 5 m/s to 40 m/s?
15.8 s
A crate falls from an airplane flying horizontally at an altitude of 1250 m. Neglecting air drag, how long will the crate take to strike the ground?
14 m/s
A stone is dropped from a cliff. After it has fallen 10 m, what is the stone’s velocity?
10.4 km/h
A motorboat is driven across a river at 3.0 km/h at right angles to a current that is flowing at 10.0 km/h. What is the resulting speed of the motorboat?
560 m
A package falls out of a helicopter that is traveling horizontally at 70 m/s. It falls into the water below 8.0 seconds later. Assuming no air resistance, what is the horizontal distance it travels while falling?
12.0 m/s
Kyle throws a ball horizontally from the top of a building that is 5.0 m high. He hopes the ball will reach a swimming pool that is at the bottom of the building, 12.0 m horizontally from the edge the building. If the ball is to reach the pool, with what initial speed must Kyle throw it with?
255 m and 335 km/h
An airplane whose airspeed is 295 km/h flies parallel to the direction of a wind with a speed of 40.0 km/h. What are the
two possible speeds of the plane relative to the ground?
two possible speeds of the plane relative to the ground?
20 m
A ball is thrown horizontally from the top of a tall cliff. Neglecting air drag, what vertical distance has the ball fallen 2.0 seconds later?
10.4 m/s
A snowball rolls off the edge of a horizontal roof at a velocity of 3.0 m/s. What is the speed of the snowball 1.0 s later?
2.0 m/s
You push with 10.0 N on a 5.0-kg block and there are no opposing forces. How fast will the block accelerate?
2 m/s
You push with 27 N on a 10-kg chest, and there is a 7-N force of friction. How fast will the chest accelerate?
0.4 m/s
A 400,000-kg airplane in takeoff uses the 40,000 N thrust of each one of its four engines. What is the acceleration of the plane during takeoff?
5.0 N
An unbalanced force of 30 N gives an object an acceleration of 6.0 m/s2 What force would be needed to give it an acceleration of 1.0 m/s2?
2.0 m
A net force of 1.0 N acts on a 4.0-kg object, initially at rest, for 4.0 seconds. What is the distance the object moves during that time?
7 m/s
When air resistance on a falling skydiver builds up to 0.3 the weight of the skydiver, what is the acceleration of the skydiver?
4 m/s
Suppose that you exert 300 N horizontally on a 50-kg crate on a factory floor, where friction between the crate and the floor is 100 N. What is the acceleration of the crate?
10 m/s
A 20-kg block of cement is pulled upward (not sideways!) with a force of 400 N. What is the acceleration of the block?
7.5 m/s
Bronco the skydiver, who’s mass is 80 kg experiences 200 N of air resistance. What is the acceleration of his fall?
400 kg * m/s
What is the average momentum of a 50-kg skateboarder who covers 400 m in 50 s?
not enough information to say
A cement truck with a mass 16,000 kg moving at 15 m/s slams into a cement wall and comes to a halt. What is the force of impact on the truck?
30 N
A 10-kg cement block moving horizontally at 6 m/s plows into a bank of sand and comes to a stop in 2 s. What is the average impact force on the bank?
1.5 m/s
An 8-kg blob of clay moving horizontally at 2 m/s hits a 3-kg blob of clay at rest. What is the speed of the two blobs stuck together immediately after the collision?
80 kg * m/s
A 10-kg bowling ball moving at 4 m/s bounces off a spring at about the same speed that it had before bouncing. What is the change in momentum of the bowling ball?
45.0 km/h
A 50-kg cart moving at 100 km/h collides head-on with an approaching 50-kg cart moving at 10 km/h (in the opposite direction). If the two carts stick together, what will be their speed?
5.0 m/s
A 30-kg girl and a 50-kg boy face each other on friction-free roller skates. The girl pushes the boy, who moves away at a speed of 3 m/s. What is the girl’s speed?
0.04 m/s
A 70-kg free-floating astronaut fires 0.10-kg of gas at a speed of 30 m/s from her propulsion pistol. What is the astronaut’s recoil speed?
240 N*s
Assume that a 15-kg ball moving at 8 m/s strikes a wall perpendicularly and rebounds elastically at the same speed. What is the amount of impulse given to the wall?
The work you do when lifting something may be stored as gravitational potential energy. Then the force times the distance is equal to the weight times the height. If the lifted object is released, this energy transforms to motion energy (or kinetic energy). The kinetic energy as it returns to its starting point equals the gravitational potential energy at its highest point, which in turn equals the work done on it in the first place.
You do work on something when you lift it against gravity. How does this work relate to gravitational potential energy? If the lifted object is released, what becomes of this energy? Be sure to define all terms that you use.
Net energy is never created or destroyed. It can change from one form to another form. A swinging pendulum has the most gravitational potential energy at the top of its swing. At that point it has no kinetic energy. At the bottom of its swing its potential energy is at a minimum, or zero relative to that lowermost point, and its kinetic energy is at a maximum. Halfway down, it has half kinetic and half potential energy. Everywhere along the swing the sum of the kinetic and potential energies is the same. When air resistance and friction are taken into account, energy is transferred from the pendulum to the surroundings in the form of heat.
Discuss how energy conservation applies to a pendulum. Where is potential energy the most? The least? Where is kinetic energy the most? The least? Where is it moving the fastest? Stopped?
1200 J
What is the work done in lifting 60 kg of blocks to a height of 20 m?
40 J
A toy cart moves with a kinetic energy of 10 J. If its speed is doubled, what will its kinetic energy be?
144,000 J
What amount of work can a 600-W motor do in 4 minutes?
188 W
A 30-kg girl runs up the staircase to a floor 5 m higher in 8 seconds. What is her power output?
3200 N
The 4.0-kg head of an ax is moving at 4.0 m/s when it strikes a log and penetrates 0.01 m into the log. What is the average force the blade exerts on the log?
