Physics- Ch. 11 Test Review

A model rocket with an initial mass of 3.0 kg is launched horizontally by burning and expelling 740 g of fuel with a velocity of 52 m/s. What is the velocity of the rocket after the fuel is expelled? (Assume the propellant is expelled instantaneously.)

• 17 m/s

• 13 m/s

• 85 m/s

• 64 m/s

• 0.16 m/s

Answer explanation

v_f=(0.740 kg/2.26 kg) x 52 m/s =17.0 m/s

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Car A is moving to the right with a speed of 14 m/s and car B is moving to the left with a speed of 12 m/s. Both cars have a mass of 1100 kg. What is the total momentum of the system consisting of cars A and B? (You may define positive direction to be to the right)

• 0 kg·m/s

• -2900 kg·m/s

• 2200 kg·m/s

• 1900 kg·m/s

• -2200 kg·m/s

Answer explanation

The car moving to the right (positive direction) has momentum 15400 kg⋅m/s.
The car moving to the left (negative direction) has momentum -13200 kg⋅m/s.
The total momentum is
15400 kg⋅m/s − 13200 kg⋅m/s = 2200 kg⋅m/s

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A moving object with a mass of 5.0 kg has linear momentum of magnitude 17 kg·m/s. What is the kinetic energy of the object?

• 29 J

• 58 J

• 14 J

• 1.7 J

• 120 J

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A spacewalking astronaut is stranded a few meters from the space station door. His jet pack is broken, but he can take it off and toss it in order to propel himself toward the door. In what direction should he toss it?

• It should be tossed toward the Earth.

• It should be tossed to the right of the door.

• It should be tossed away from the door.

• The direction depends on the astronaut's mass.

• The direction depends on the jet pack's mass.

Answer explanation

Initially, the momentum of the astronaut is zero. If he throws his jet pack, conservation of momentum tells us that he will travel in the opposite direction so that the total momentum of the moving jet pack and astronaut remains zero. Therefore, he should throw the jet pack in the opposite direction that he wants to go, away from the door.

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A bowling ball has a mass of 4.4 kg and is traveling with a velocity of 19 m/s down the lane. How much momentum does it have?

• 4.3 kg·m/s

• 84 kg·m/s

• 23 kg·m/s

• 370 kg·m/s

• 790 kg·m/s

Answer explanation

p = mv = (4.4 kg)(19 m/s) = 83.6 kg m/s

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A 1400 kg car with 310 kg of passengers and cargo is moving at 93 m/s. Suddenly, the driver applies the brakes and the car skids to a stop. What happens to the momentum that the car (and its contents) had before braking?

• It is transferred to the pavement (and the Earth).

• It is transferred to the passengers and cargo.

• It is transferred to the driver.

• We need to know the time frame in which the forces of the brakes was applied.

• It is destroyed.

Answer explanation

The moving wheels transfer their momentum to the pavement and the Earth that they come in contact with. Like any "collision," momentum is conserved. However, because the mass of the pavement and Earth is so much greater than that of the car, passengers, and cargo, the resulting velocity of the pavement is so small that we do not see anything.

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A spring-loaded ballistic cart measuring 300 g is in contact with a second 760 g cart. The carts are initially at rest on a level surface. The spring is released and the lighter cart is observed to move at -0.98 m/s afterward. What is the velocity of the other cart?

• -0.39 m/s

• 0.39 m/s

• -2.5 m/s

• 2.5 m/s

• -0.98 m/s

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A motionless 130 kg astronaut is holding a 11 kg tool while on a spacewalk. To get moving, the astronaut throws the tool forward at a speed of +4.3 m/s. How fast does the astronaut move backward?

• a) 4.3 m/s

• b) 1.1 m/s

• c) 0.85 m/s

• d) 0.033 m/s

• e) 0.36 m/s

Answer explanation

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May (75 kg) and Amber (60 kg) stand motionless on ice skates, facing each other at arm's length on a smooth, frozen pond. Which of the following actions will give Amber the least speed?

• a) Amber pushes May with 400 N of force for 0.01 s.

• b) Amber pushes May with 500 N of force for 0.02 s.

• c) Amber pushes May with 300 N of force for 0.03 s.

• d) May pushes Amber with 200 N of force for 0.04 s.

• e) May pushes Amber with 80 N of force for 0.2 s.

Answer explanation

Looking at impulse for this situation, we have that J = Δp = mΔv = FΔt. Since Amber is motionless in the beginning, her final speed is v = ^FΔt⁄_m. Because Amber's mass does not change, the smallest product of force and time will give her the least speed.

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A 3000 kg truck moving at 16 m/s collides with a stationary 950 kg passenger car, and the two stick together. Assuming that they are free to move afterward, what is the momentum of the truck+car object after the collision?

• a) 15000 kg m/s

• b) 12000 kg m/s

• c) 63000 kg m/s

• d) 48000 kg m/s

• e) 12 kg m/s

Answer explanation

Momentum is conserved in all collisions. In the beginning only the truck possesses momentum:

p = mv = (3000 kg)(16 m/s) = 48000 kg m/s which equals the momentum of the truck+car object after the collision.

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