# Mastering physics Ch. 6

The moving skateboard
Which has a greater momentum: a heavy truck at rest or a moving skateboard?
Its mass times velocity is greater than that of the car.
A slowly moving ship can have a greater momentum than a fast-moving racing car when?
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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?
product of force and time.
The quantity that is called impulse can be measured by the?
F=ma=mΔv/Δt, so Ft=Δ(mv)
How is the impulse -momentum relationship related to Newton’s second law?
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?
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?
whether the collision is elastic or inelastic.
When two vehicles collide, momentum is conserved
Remains the same
When a big fish swims into an oncoming smaller fish and swallows it, the momentum of the two-fish system
The trucks will stick together after the collision and not move
predict what will happen if the red and green truck with equal mass and equal magnitude of velocity collide.
The trucks will stick together after the collision and move to the left.
Predict what will happen if you reduce the magnitude of the velocity of the green truck to a magnitude smaller than that of the red truck.
The trucks will stick together after the collision and move to the right.
predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is negative.
The trucks will stick together after the collision and move to the right.
predict what will happen if you change the velocity of the red truck so that it has a magnitude smaller than that of the green truck but is positive.
The trucks will stick together after the collision and move to the right.
predict what will happen if you increase the mass of the green truck so that it is greater than the mass of the red truck.
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?
vectorally add up to equal the initial momentum of the firecracker
When a falling firecracker explodes, the momenta of its pieces
p =
26 kg⋅ms
What is the momentum of an 8.8-kg bowling ball rolling at 3.0m/s ? Momentum = mv
p =
30 kg⋅ms
What impulse occurs when an average force of 15N is exerted on a cart for 2.0s ?
Impulse = Ft
impulse =
15 N⋅s
What is the magnitude of the impulse on an 8.6-kg ball rolling at 1.8m/s when it bumps into a pillow and stops? Impulse = change in momentum: Ft=Δmv
v =
7.5 ms
A 5.0-kg fish swimming 1.5m/s swallows an absentminded 1.0-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Find the speed of the approaching smaller fish before lunch.
The wreckage after collision is motionless.
Two automobiles, each of mass 1100kg , are moving at the same speed, 22m/s , when they collide and stick together. In what direction and at what speed does the wreckage move if one car was driving north and one south.
to the northeast
Two automobiles, each of mass 1100kg , are moving at the same speed, 22m/s , when they collide and stick together. In what direction does the wreckage move if one car was driving north and one east.
v =
16 ms
Two automobiles, each of mass 1100kg , are moving at the same speed, 22m/s , when they collide and stick together. At what speed does the wreckage move if one car was driving north and one east?
Momentum
A same-size iron ball and wooden ball are dropped simultaneously from a tower and reach the ground at the same time. The iron ball has a greater
3 m/s
The speed of a 4-kg ball with a momentum of 12 kg m/s is
Less momentum
A large heavy truck and a small baby carriage roll down a hill. Neglecting friction, at the bottom of the hill, the baby carriage will likely have
flying in different directions.
The total momentum of a flock of identical birds could be zero only if the birds are
3,000 kg m/s.
A motorcycle of mass 100 kilograms slowly rolls off the edge of a cliff and falls for three seconds before reaching the bottom of a gully. Its momentum upon hitting the ground is
a corresponding change in momentum.
It is correct to say that impulse is equal to
lengthened time of contact.
Padded dashboards in cars are safer in an accident than non-padded ones because passengers hitting the dashboard encounter
stopping time is longer on the carpet.
Compared with falling on a stone floor, a wine glass may not break when it falls on a carpeted floor because the
longer time
If you can’t avoid being hit by a fast-moving object, you’ll suffer a smaller contact force if you can extend that force over a
greater with the brick wall
Whether a truck comes to a stop by crashing into a haystack or a brick wall, the stopping force is
both the same
Whether a truck comes to a stop by crashing into a haystack or a brick wall, the impulse is
the target would be safer than the shooter.
A tiny gun made of a strong but very light material fires a bullet more massive than the gun itself. For such a weapon
cannon has much more mass than the cannonball.
A cannon recoils while firing a cannonball. The speed of the cannon’s recoil is relatively small because the
greater
A heavy truck and a small car rolling down a hill at the same speed are forced to stop in the same amount of time. Compared with the force that stops the car, the force needed to stop the truck is
friction between his feet and the board.
Freddy Frog drops vertically from a tree onto a horizontally-moving skateboard. The reason he doesn’t slip off the skateboard is due to
Both the same
You’re driving down the highway and a bug spatters into your windshield. Which undergoes the greater change in momentum during the time of contact?
move in the opposite direction at a lower speed.
An astronaut floating alone in outer space throws a baseball. If the ball moves away at 20 m/s, the astronaut will
2/3 m/s.
A 5000-kg freight car moving at 2 m/s collides with a 10,000-kg freight car at rest. They couple upon collision and move away at
6 m/s
A 5000-kg freight car collides with a 10,000-kg freight car at rest. They couple upon collision and move at 2 m/s. What was the initial speed of the 5000-kg car?
All the same
A red car has a head-on collision with an approaching blue car with the same magnitude of momentum. A green car driving with the same momentum as the other cars collides with an enormously massive wall. Which of the three cars will experience the greatest impulse?
at 45 degrees to the direction of either car before collision.
Two vehicles with equal magnitudes of momentum traveling at right angles to each other undergo an inelastic collision. The combined wreck moves in a direction
greater than the magnitude of momentum of either car before collision.
Two vehicles with equal magnitudes of momentum traveling at right angles to each other undergo an inelastic collision. The magnitude of momentum for the combined wreck is
continue along the path as if the explosion didn’t occur.
A cannonball following a parabolic path explodes into fragments. The momentum of the fragments
C
When a vertically falling firecracker bursts, the vector sum of momentum fragments?
A) in the horizontal direction cancels to zero.
B) in the vertical direction equals the momentum of the firecracker before bursting.
C) both of these
D) neither of these
Momentum is conserved in elastic and inelastic collisions.
In which is momentum conserved: an elastic collision or an inelastic collision?
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
C
Which has the greater momentum when moving?
A) a bullet
B) a container ship
C) either of these depending on speed
D
When Peter tosses an egg against a sagging sheet, the egg doesn’t break due to?
A) reduced impulse.
B) reduced momentum.
C) both of these
D) neither of these
1.5 g
A bungee jumper attains a speed of 30 m/s just as the bungee cord begins to stretch. If the period of stretch is 2 s while coming to a halt, the jumper’s average deceleration is about
Exhaust gases
The force that accelerates a rocket in outer space is exerted on the rocket by the
D
As Bronco Brown steps off a hovering high-flying helicopter and falls through the air, he experiences?
A) an impulse.
B) a decreasing acceleration.
C) an increasing speed.
D) all of the above
Zero
Two billiard balls having the same mass and speed roll toward each other. What is their combined momentum after they meet?
move away from each other, but at different speeds.
If a monkey floating in outer space throws his hat away, the hat and the monkey will both
Decreases
An open freight car rolls friction free along a horizontal track in a pouring rain that falls vertically. As water accumulates in the car, the car’s speed
1 m/s
A 2-kg rifle that is suspended by strings fires a 0.01-kg bullet at 200 m/s. The recoil velocity of the rifle is about
1/6 m/s
A 5-kg shark swimming at 1 m/s swallows an absent-minded 1-kg fish swimming toward it at 4 m/s. The speed of the shark after his meal is
E
A sandbag motionless in outer space is hit by a three-times-as massive sandbag moving at 12 m/s. They stick together and move at a speed of?
A) 6 m/s.
B) 4 m/s.
C) 8 m/s.
D) 3 m/s.
E) none of the above

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