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

• m1v1i + m2v2i = m1v1f + m2v2f

• 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

• 187.5 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 m1v1i + m2v2i = (m1+ m2)vf . Plug in your values and solve for m2 . 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 (m1+ m2)vf = m1v1i + m2v2i . Plug in the values and solve for v2i . 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 10500 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 m1v1i + m2v2i = m1v1f + m2v2f . Plug in your values and solve for v1f

#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.

#19 -

#19 - Answer

#20 –

#20 - Answer