conservation(of(momentum( - ap physics at centennial...
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Practice 5B name _____________________________________________ Answers: posted in classroom, on the website, and linked from this QR code.
Conservation of Momentum 1. A 1.2-‐kilogram block and a 1.8-‐kilogram block are
initially at rest on a frictionless, horizontal surface. When a compressed spring between the blocks is released, the 1.8-‐kilogram block moves to the right at 2.0 meters per second, as shown.
What is the speed of the 1.2-‐kilogram block after the spring is released?
A. 3.6 m/s B. 3.0 m/s C. 2.0 m/s D. 1.4 m/s
2. Ball A of mass 5.0 kilograms moving at 20 meters
per second collides with ball B of unknown mass moving at 10 meters per second in the same direction. After the collision, ball A moves at 10 meters per second and ball B at 15 meters per second, both still in the same direction. What is the mass of ball B?
A. 6.0 kg B. 2.0 kg C. 10 kg D. 12 kg
3. In the diagram below, scaled vectors represent the
momentum of each of two masses, A and B, sliding toward each other on a frictionless, horizontal surface.
Which scaled vector best represents the momentum of the system after the masses collide?
A. B. C. D.
4. At the circus, a 100-‐kilogram clown is fired 15
meters per second from a 500-‐kilogram cannon. What is the recoil speed of the cannon?
A. 75 m/s B. 15 m/s C. 5.0 m/s D. 3.0 m/s
5. Which two quantities can be expressed using
the same units? A. energy and force B. impulse and force C. momentum and energy D. impulse and momentum
6. A 3.0-‐kilogram steel block is at rest on a
frictionless horizontal surface. A 1.0-‐kilogram lump of clay is propelled horizontally at 6.0 meters per second toward the block as shown in the diagram below.
Upon collision, the clay and steel block stick
together and move to the right with a speed of A. 1.5 m/s B. 2.0 m/s C. 3.0 m/s D. 6.0 m/s
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9. A 1.0 kg block sits near the top of a hill. A 20-‐gram bullet is fired at
200 m/s at the block. The bullet embeds in the block and the two objects slide along the frictionless surface. Then they slide down the ramp with height 1.8 m. Find their speed at the bottom.
10. A block is pushed at 4.0 m/s from its initial position at the top of a
hill of height H = 2.6 m. It slides down the ramp and collides with a second block, which is initially at rest. The two blocks stick together. Both blocks have the same mass. Find the speed of the two-‐block combination.
7.
8.
1.8 m
H
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TERMINOLOGY: Inelastic collision = nearly all collisions. The objects bounce and energy is “lost” in the collision. Completely inelastic collision. The objects stick together and energy is “lost” in the collision. Elastic collision. The objects bounce and energy is conserved.
11. A cart of mass m, traveling on a frictionless track with speed v collides with a stationary cart of mass 3m. The carts undergo and completely inelastic collision. (a) What is the speed of the resulting two-‐car system? (b) What fraction of the energy was “lost” during the collision?
12. At the same instant that a 1.0 kg ball is dropped from 25 m above the Earth, a second ball, with a mass of 3.0 kg, is
thrown straight upward from the Earth's surface with an initial velocity of 15 m/s. (a) Find the position above the Earth at which they collide. (b) Find the velocity of each ball just before the collision. (c) The balls stick to each other as a result of the collision. What is the resulting velocity of the 4.0-‐kg mass?
13. A cart with mass 500 g moving on a frictionless horizontal air track at an initial speed of 1.5 m/s undergoes a
completely inelastic collision with an initially stationary cart of unknown mass. After the collision, the combined carts move at 0.60 m/s. (a) Determine the mass of the second cart. (b) State the energy lost in the collision as a fraction of the initial energy.
14. A 3.2 kg box of chocolate slides on a horizontal frictionless table and collides
with a 2.0 kg box of peanut butter initially at rest on the edge of the table. The speed of the chocolate is 3.0 m/s just before the collision. If the two boxes stick together because of packing tape on their sides, what is their speed just as they strike the floor 0.80 m below the table’s surface?
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15. An object of mass m1 = 2.0 kg moving with velocity v1i = 12 m/s, collides head-‐on with a stationary object whose mass is m2 = 6.0 kg. Given that the collision is elastic, what are the final velocities of the two objects?
16. A particle of mass 4.0 kg, initially moving with a velocity of 2.0 m/s collides elastically with a particle of mass 6.0 kg
initially moving with a velocity of -‐4.0 m/s. What are the velocities of the two particles after the collision? 17. Block A with a mass of 2 kg, moving to the right on a frictionless table at 6.0 m/s, has a head-‐on collision with Block B
which is at rest and has a mass of 4.5 kg. (a) The graph shows the force exerted by block A on block B during the collision. What was the impulse applied to
block B? (b) What is the speed and direction of block B just after the collision? (c) What is the speed and direction of block A just after the collision? (d) Calculate the change in the energy for the system? Was the collision
elastic? (e) After the collision the blocks slide off the table and land on the floor. What
is the distance between the landing points of each block?
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For this problem, the solution is guided. When you finish feel free to adjust the method to fit your needs. A video of this problem is also available.
Ex. A 600 kg car is moving North at 20 m/s and a 1000 kg truck is moving 10 m/s 20° S of E. After they collide the car
moves at 5.0 m/s 10° N of E. What is the resulting velocity of the truck? 𝑝!"!#!$% = 𝑝!"#$% 𝑝 = 𝑚𝑣
(a) Find the momentum of each vehicle. The direction matches the direction of the velocity.
Before the crash: 𝒑𝑪𝑨𝑹 = (𝟔𝟎𝟎 𝒌𝒈)(𝟐𝟎 𝒎/𝒔) = 𝟏𝟐,𝟎𝟎𝟎 𝒌𝒈 ·𝒎/𝒔,𝑵𝒐𝒓𝒕𝒉 𝒑𝑻𝑹𝑼𝑪𝑲 = (𝟏𝟎𝟎𝟎 𝒌𝒈)(𝟏𝟎 𝒎/𝒔) = 𝟏𝟎,𝟎𝟎𝟎 𝒌𝒈 ·𝒎/𝒔,𝟐𝟎° 𝑺 𝒐𝒇 𝑬
After the crash: 𝒑𝑪𝑨𝑹 = (𝟔𝟎𝟎 𝒌𝒈)(𝟓.𝟎 𝒎/𝒔) = 𝟑𝟎𝟎𝟎 𝒌𝒈 ·𝒎/𝒔,𝟏𝟎° 𝑵 𝒐𝒇 𝑬
(b) You may want to create a vector diagram to help you visualize a problem. People usually picture this because it looks like This is a better way to picture the a crash, but it doesn’t represent the adding of total momentum of the system.
momentum values. (c) Using components is the key. Momentum is conserved along any axis you choose.
Before: 𝒑𝑪𝑨𝑹 = 𝟎 ! + 𝟏𝟐,𝟎𝟎𝟎 ! 𝒌𝒈 ·𝒎/𝒔
𝒑𝑻𝑹𝑼𝑪𝑲 = 𝟗𝟑𝟗𝟕 ! − 𝟑𝟒𝟐𝟎 ! 𝒌𝒈 ·𝒎/𝒔 After: 𝒑𝑪𝑨𝑹 = 𝟐𝟗𝟓𝟒 ! + 𝟓𝟐𝟏 ! 𝒌𝒈 ·𝒎/𝒔
𝒑𝑻𝑹𝑼𝑪𝑲 = 𝒑𝒙 ! + 𝒑𝒚 ! 𝒌𝒈 ·𝒎/𝒔
In the x-‐direction, 0 + 9397 = 2954 + px. So px = 6443 kg·m/s. Divide by mass, 1000 kg: vx = 6.443 m/s. In the y-‐direction, 12,000 -‐ 3420 = 521 + py. py = 8580 kg·m/s and vy = 8.580 m/s. Final answer: vTRUCK = 10.7 m/s, 53° N of E
18. A 1500 kg car is moving North at 16 m/s and a 4000 kg truck is moving at 10 m/s, 30° North of East. If they collide and
the vehicles interlock, what is the velocity of the combined system?
10,000 kgm/s
12,000 kgm/s
10,000 kgm/s
12,000 kgm/s
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19. A 60 kg man is ice-‐skating due North with a velocity of 6.0 m/s when he collides with a 38 kg child, who is also skating. The man and child stay together and have a velocity of 3.0 m/s at an angle of 35° north of east immediately after the collision. What are the magnitude and direction of the velocity of the child just before the collision?
20. A 0.30-‐kg ball moves along the +x-‐axis at an unknown speed, and strikes a 0.50-‐kg ball initially at rest. The 0.30-‐kg ball
is observed to travel at 6.0 m/s along a path 70° with respect to its original path, and the other ball moves along a path 40° on the other side.
(a) Determine the speed of the 0.50-‐kg ball after the collision. (b) Determine the speed of the 0.30-‐kg ball before the collision. 21. A bullet of mass 45 g is fired at a speed of 220 m/s into a 5.0 kg sandbag hanging from a string from the ceiling. The
sandbag absorbs the bullet and begins to swing. To what maximum vertical height will it rise?
70°
40°
before collision after collision