Chap. 6 – Physics

Which has a greater momentum: a heavy truck at rest or a moving skateboard?

the moving skateboard

A slowly moving ship can have a greater momentum than a fast-moving racing car when _______.

its mass times velocity is greater than that of the car.

Distinguish between force and impulse.

Impulse is force times a time interval.

An impulse can be increased by

increasing the force or increasing the time interval.

The quantity that is called impulse can be measured by the _______.

product of force and time.

How is the impulse -momentum relationship related to Newton’s second law?

F=ma=mΔv/Δt, so Ft=Δ(mv)

To impart the greatest momentum to an object, you must

exert the greatest force over the longest time.

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?

Long contact time decreases the magnitude of the average force and is favorable.

Why is a force that is applied for a short time more effective in karate?

The average force is increased.

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?

Yes, always.

What does it mean to say that momentum is conserved?

It means momentum does not change.

When two vehicles collide, momentum is conserved _______.

whether the collision is elastic or inelastic.

When a big fish swims into an oncoming smaller fish and swallows it, the momentum of the two-fish system _______.

remains the same

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?

The speed of car B is equal to the initial speed of A. (since the cars have the same mass they will have the same velocity)

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?

Their speed is half the initial speed of car A.

When a falling firecracker explodes, the momenta of its pieces _______.

vectorally add up to equal the initial momentum of the firecracker

What is the momentum of an 7.0-kg bowling ball rolling at 1.6m/s ?

11.2 kg(m)/s

What impulse occurs when an average force of 14N is exerted on a cart for 2.3s

32.3 N⋅s

What is the magnitude of the impulse on an 8.0-kg ball rolling at 2.2m/s when it bumps into a pillow and stops?

18 N⋅s

A 5.4-kg fish swimming 0.80m/s swallows an absentminded 1.0-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch

4.3 m/s

In what direction and at what speed does the wreckage move if one car was driving north and one south.

The wreckage after collision is motionless.

In what direction does the wreckage move if one car was driving north and one east

northeast

At what speed does the wreckage move if one car was driving north and one east?

18 m/s

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

momentum

The speed of a 4-kg ball with a momentum of 12 kg m/s is

3 m/s

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

less momentum

The total momentum of a flock of identical birds could be zero only if the birds are

flying in different directions.

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

3,000 kg m/s

It is correct to say that impulse is equal to

a corresponding change in momentum.

Padded dashboards in cars are safer in an accident than non-padded ones because passengers hitting the dashboard encounter

lengthened time of contact.

Compared with falling on a stone floor, a wine glass may not break when it falls on a carpeted floor because the

stopping time is longer on the carpet.

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

longer time

Whether a truck comes to a stop by crashing into a haystack or a brick wall, the stopping force is

greater with the brick wall.

Whether a truck comes to a stop by crashing into a haystack or a brick wall, the impulse is

both the same

A tiny gun made of a strong but very light material fires a bullet more massive than the gun itself. For such a weapon

the target would be safer than the shooter.

A cannon recoils while firing a cannonball. The speed of the cannon’s recoil is relatively small because the

cannon has much more mass than the cannonball.

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

greater

Freddy Frog drops vertically from a tree onto a horizontally-moving skateboard. The reason he doesn’t slip off the skateboard is due to

friction between his feet and the board.

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?

both the same

An astronaut floating alone in outer space throws a baseball. If the ball moves away at 20 m/s, the astronaut will

move in the opposite direction at a lower speed.

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

2/3 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?

6 m/s

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?

all the same

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

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 magnitude of momentum for the combined wreck is

greater than the magnitude of momentum of either car before collision.

A cannonball following a parabolic path explodes into fragments. The momentum of the fragments

continue along the path as if the explosion didn’t occur.

When a vertically falling firecracker bursts, the vector sum of momentum fragments

both of these *in the vertical direction equals the momentum of the firecracker before bursting. *in the horizontal direction cancels to zero.

In which is momentum conserved: an elastic collision or an inelastic collision?

Momentum is conserved in elastic and inelastic collisions.

When a cannonball is fired, the momentum of the system (cannon + cannonball) is conserved if

the momentum of the cannon is equal to the magnitude of the momentum of the cannon ball and points in the opposite direction.

Which has the greater momentum when moving?

either of these depending on speed *a container ship *a bullet

When Peter tosses an egg against a sagging sheet, the egg doesn’t break due to

neither reduced momentum or reduced impulse.

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

1.5 g

The force that accelerates a rocket in outer space is exerted on the rocket by the

exhaust gases.

As Bronco Brown steps off a hovering high-flying helicopter and falls through the air, he experiences

an impulse. an increasing speed. a decreasing acceleration. ALL OF THE ABOVE

Two billiard balls having the same mass and speed roll toward each other. What is their combined momentum after they meet?

zero

If a monkey floating in outer space throws his hat away, the hat and the monkey will both

move away from each other, but at different speeds.

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

decreases.

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 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

1/6 m/s

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

6 m/s. 8 m/s. 4 m/s. 3 m/s. NONE OF THESE