Momentum Test Review

#1 - Units What are the units for Impulse?

#1 - Answer N * s (Impulse is Force x Time, so the units are Newtons x Seconds)

#2 – Conservation of Momentum A firework shell explodes, causing fragments to fly off. How does the explosion affect the total momentum?

#2 - Answer No effect. The explosion is not an external force, so the total momentum does not change.

#3 - Separation A single object breaks into two parts. One part is twice as massive as the other. Which part has the higher velocity?

#3 - Answer The smaller part. When an object separates, the part with the lower mass will travel faster due to conservation of momentum. (i.e. recoil speed vs bullet speed)

#4 - Formula What is the equation used for the Law of Conservation of Momentum?

#4 - Answer m 1 v 1i + m 2 v 2i = m 1 v 1f + m 2 v 2f The total momentum before an event is equal to the total momentum after an event. (in the absence of an outside force)

# 5 - Collision The light rail rams into a passing car and carries it along the track. What type of collision is this?

#5 - Answer Inelastic Collision The two objects stick together and act as one object.

#6 - Units What are the units for momentum?

#6 - Answer kg*m/s Momentum is mass * velocity so the units are kg*m/s.

#7 - Momentum A 15 kg heart-decorated cube is hurled through the air at 12.5 m/s. What is its momentum?

#7 - Answer kg*m/s Remember: p=m*v

#8 – Inelastic Collision J.J. Watt charges a stationary receiver at 6 m/s. After he tackles the receiver, they both fly forward at 3.5 m/s. J.J. Watt has a mass of 134 kg. What was the mass of the receiver?

#8 - Answer 95.7 kg The formula for an inelastic collision is m 1 v 1i + m 2 v 2i = (m 1 + m 2 )v f. Plug in your values and solve for m 2. Remember that “stationary” = 0 velocity.

#9 - Impulse Two identical cars stop from 30 mph to 0 mph. The first car slams on the breaks and stops in 1 second. The second car slowly stops over 8 seconds. Which car had the greater impulse?

#9 - Answer They both had the same Remember: Impulse is NOT a Force. It is just the change in momentum. Both cars had the same change in momentum. ∆p=m∆v

#10 - Impulse If you decrease your impact time, what must increase?

#10 - Answer The Force Impulse = F*t. If time decreases then force must increase and vice versa. This is an inverse relationship.

# 11 - Separation A 20 kg cannonball is fired from a 300 kg cannon. If the cannon has a recoil speed of 2.5 m/s, what is the speed of the cannonball?

#11 - Answer 37.5 m/s The formula for a separation is (m 1 + m 2 )v f = m 1 v 1i + m 2 v 2i. Plug in the values and solve for v 2i. Notice the total momentum is zero.

#12 - Impulse What part of the egg drop contraption managed to increase the duration of the impulse when the egg hit the ground?

#12 - Answer Examples: Padding, Bubble wrap, Sponges, Foam, etc. (NOT “parachutes”) By adding padding, the time of impact for the egg was increased, thus reducing the stopping force. (parachutes changed the magnitude of the impulse, not it’s duration)

#13 –Momentum An innocent cyclist is struck head- on by a speeding car! Who experiences a greater change in momentum, the cyclist or the car?

#13 - Answer They experience the same change in momentum. Because the car has a larger mass, its change in velocity will be lower, but the total change in momentum stays the same.

#14 - Impulse Manjit Singh can bring a stationary 7000 kg bus to a speed of 1.5 m/s by pulling it with his ears for 20 s. What is the average force he applies to the bus?

#14 - Answer 525 N Impulse = change in momentum, and Force x time. The bus starts with zero momentum and ends up with kg*m/s. Dividing that by time yields the average force.

#15 - Impulse In the formula, Ft=m∆v, what does Ft stand for? What does m∆v stand for? (Not just “force * time” or “mass * velocity”)

#15 - Answer Ft = “Impulse” m∆v = “change in momentum” The entire formula, Ft=m∆v, can be called the “Impulse- Momentum Therom”

#16 – Elastic Collision A.6 kg basketball bounces off a.05 kg tennis ball. If both balls had an initial speed of 4 m/s, what is the final speed of the basketball if the tennis ball has a final speed of 8 m/s?

#16 - Answer 3.67 m/s The formula for an elastic collision is m 1 v 1i + m 2 v 2i = m 1 v 1f + m 2 v 2f. Plug in your values and solve for v 1f

#17 - Collision Describe a collision where Kinetic Energy is NOT conserved.

#17 - Answer Any Inelastic collision KE is not conserved in inelastic collisions. The energy is lost as heat. In elastic collisions, however, KE is conserved.

#18 - Momentum A.04g golfball is hit with a force of 2 N over a course of 1.4 seconds. What is the final velocity of the golfball?

#18 - Answer 70 m/s Use Ft=m∆v. Plug in the values and solve for ∆v. Since the golfball starts at rest, ∆v is equal to the final velocity.

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