P2 Momentum Revision

P2 Momentum Revision

Ms Azrina

Multiple Choice

Which of these is the correct definition for momentum and its unit?

• A

• B

• C

• D

Explanation

• The correct answer is C

• Because momentum is defined by equation : momentum (p) = mass (m) x velocity (v)

• Therefore the correct unit kg m/s (the unit of mass multiplied by the unit of velocity)

Multiple Choice

Which statement correctly describes the law of conservation of momentum?

• The momentum before the collision is lesser than the momentum after

• The momentum before the collision is as same as the momentum after

• The momentum before the collision is as more than the momentum after

• The sum of the momentum before and after the collision is zero

Explanation

• The answer is B because it states that the momentum before and after collision is the s

Multiple Choice

Complete the sentence:

Impulse is equal to....

• Momentum

• Velocity multiplied by time

• A corresponding change in momentum

• Force multiplied by the distance it acts

Explanation

• The correct answer is C because....

• Impulse = mv - mu

Multiple Choice

A dummy is used in a car crash test to investigate the safety of a new car. The deceleration of the dummy and the passenger compartment is measured.

The deceleration of the dummy is less than the deceleration of the passenger compartment.

Why is this the benefit for the safety of passenger?

• Seat belts, crumple zones, and airbags all absorbs energy from the crash

• The crash happens quickly so the passenger doesn't feel anything

• The force of impact on the passenger, and therefore harm, is reduced.

• The time over which the collision happens is elongated for the passenger

Explanation

• The answer is C because the question is specifically asking why a shorter deceleration benefits the passenger

• Since F = ma, decreasing the value of a will subsequently decrease the force of the impact

• Why A is incorrect? factually correct but doesn't answer the question

• Why B is incorrect? The shorter the time of the collision, the larger the force of impact. And the passenger certainly will feel something!

• Why D is incorrect? Factually correct but doesn't answer the question

Multiple Choice

A rollercoaster speeds along a track with considerable momentum.

If a different rollercoaster travels at four times the speed but has half as much mass, its momentum would be:

• Zero

• Twice as much

• Four times as much

• Unchanged

Explanation

• Using p = mv and substituting m for 1/2m and v for 4v

• So the momentum of the second rollercoaster will be doubled.

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Multiple Choice

Two identical balls move at the same speed as shown in the diagram.

What momentum would the ball possesses?

• ball 1: mv , ball 2: mv

• ball 1: -mv , ball 2: -mv

• ball 1: mv , ball 2: -mv

• ball 1: 0 , ball 2: 0

Explanation

• The answer is C because they have the same mass but different velocities

• They are moving in the opposite direction so one ball has a positive momentum and the other one has negative momentum

Multiple Choice

A passenger of the mass of 90 kg is involved in a minor car crash.

The car approaches a solid barrier at 32 m/s. It crashes into the barrier and stops in 0.2s.

Determine the impulse that must be applied to the car to bring it to rest.

• 2.8 Ns

• 14 Ns

• 580 Ns

• 2900 Ns

Explanation

• Impulse = change in momentum = mv - mu

• So Impulse = 90 kg x 32 m/s = 2880 kg m/s or 2900 Ns

Multiple Choice

Padded dashboards in cars are safer in an accident than non-padded ones because a passenger hitting a dashboard would experience

• lengthened time of contact

• shorter time of contact

• decreased impulse

• increased momentum

Explanation

• A padded dashboard will absorb some of the energy from the collision with the passenger's head.

• This, in turn, reduces the force encountered by the passenger

• When the force reduces, the time of contact will be lengthened

Multiple Choice

After a car crash, the car driver's inflates.

The airbag then deflates when it is hit by the driver's head.

How does an airbag reduce the risk of injury?

• Collision time increases, which increases the rate of change of momentum

• Collision time increases, which reduces the rate of change of momentum

• Collision time decreases, which increases the rate of change of momentum

• Collision time decreases, which reduces the rate of change of momentum

Explanation

• Airbags are designed to increase the time taken for the momentum of the head to reduce to zero which subsequently reduces the force exerted on the head.

Multiple Choice

A bullet is shot from a gun. The bullet moves forward and the gun moves in the opposite direction.

Which of the following statement is true?

• They move with the same velocity in the opposite direction

• The bullet moves with a slower velocity due to its mass

• The total momentum of the system is zero

• The total momentum of the system does not stay constant before and after the collision.

Explanation

• the total momentum of the bullet and gun before shooting = the total momentum of the bullet and gun after shooting.

• Before shooting the bullet and gun are stationary, so overall the total momentum will be zero

Multiple Choice

A toy train is rolling at a constant speed on a level track. The train collides with a stationary toy truck and joins with it.

Before the collision, the toy train is travelling at 0.5 m/s. The train and the stationary truck both have a mass of 75 g.

Calculate the momentum of the toy train before the collision.

• 37.5 kg m/s

• 0.04 kg m/s

• 0.38 kg m/s

• 0.08 kg m/s

Explanation

• p = mv

• p = 0.5 m/s x 0.075 kg

• p = 0.0375 kg m/s

• p = 0.04 kg m/s

Multiple Choice

An object of mass 150 kg accelerates from a velocity of 10 m/s in the same direction.

What is the impulse provided to cause this acceleration?

• 750 Ns

• 1500 Ns

• 2250 Ns

• 7500 Ns

Explanation

• Impulse = change in momentum

• Impulse = mv - mu = m (v-u)

• Impulse = 150 (10 - 5) = 150 x 5

• Impulse = 750 Ns

Multiple Choice

A gas molecule strikes the wall of a container with a speed of 200 m/s. It rebounds with the same kinetic energy as it had before striking the wall.

What is its final velocity?

• 100 m/s

• -100 m/s

• 200 m/s

• -200 m/s

Explanation

• the question states that the gas molecule rebound with the same kinetic energy, therefore the molecule will rebound with the same speed

• However, the question asked for the velocity so we need to take into consideration the direction

• Since the molecule rebounds in the opposite direction, its final velocity is -200 m/s