به انگلیسی

Checking momentum and energy in collisions

Science · Forces & Motion · ages 17-18

Name ______________________ Date ____________

What defines an elastic collision?
- Both objects leave at the same final speed
- Total kinetic energy is unchanged
- Total momentum increases
For two colliding objects, net external impulse during impact is negligible. Which total is approximately unchanged in both elastic and inelastic collisions?
- Total momentum
- Total kinetic energy
- Sum of speeds
Objects must stick together for a collision to be inelastic.
Circle one: True False
A 2 kg trolley at +3 m/s hits a stationary 1 kg trolley. After impact, their velocities are +1 and +4 m/s, respectively. Net external impulse is zero. Treat the data as exact. Calculate total KE before and after. How is the collision classified?
- Inelastic, with a gain in total KE
- Elastic, with no change in total KE
- Inelastic, with a decrease in total KE
Right is positive. A 2 kg trolley at +5 m/s collides with a 1 kg trolley at -2 m/s. Net external impulse is zero. The 2 kg trolley leaves at +1 m/s. What is the signed final velocity of the 1 kg trolley, in m/s?
Answer: ______________
Total kinetic energy decreases during an inelastic collision. Which statement describes a possible transfer of that energy?
- It may heat or deform objects, or become sound
- It is destroyed during impact
- It becomes momentum without any other energy change
Right is positive. Two 2 kg trolleys move at +2 and -2 m/s. Tom says zero total momentum means zero total kinetic energy. Which calculation checks his claim?
- Each trolley has -4 J, giving a total of -8 J
- Each trolley has 4 J, giving a total of 0 J
- Each trolley has 4 J, giving a total of 8 J
A 2 kg trolley moving right at 6 m/s hits a stationary 4 kg trolley and sticks. Net external impulse is zero. Use momentum to find their shared speed, then calculate the decrease in total kinetic energy. Enter only the decrease, in J.
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

Checking momentum and energy in collisions W1-mt_JMwUuh1YUq-s1

Total kinetic energy is unchanged · An elastic collision conserves the sum of the objects' kinetic energies. Their individual speeds can change.
Total momentum · Negligible net external impulse leaves total momentum approximately unchanged. Kinetic energy need not stay the same.
False · Objects can bounce apart in an inelastic collision. Sticking together is the special case called perfectly inelastic.
Elastic, with no change in total KE · KE before is 0.5 × 2 × 3² = 9 J. Afterward it is 0.5 × 2 × 1² + 0.5 × 1 × 4² = 1 + 8 = 9 J. Equal totals mean elastic.
6 · Initial momentum is +10 - 2 = +8 kg m/s. The 2 kg trolley carries +2 afterward, leaving +6 kg m/s for the 1 kg trolley: +6 m/s.
It may heat or deform objects, or become sound · The decrease can increase internal energy through heating or deformation, or transfer energy away as sound. Energy is not destroyed.
Each trolley has 4 J, giving a total of 8 J · The momenta +4 and -4 kg m/s cancel. Both squared velocities equal 4, so each trolley has 4 J of kinetic energy. Their total is 8 J.
24 · Momentum is 12 kg m/s and joined mass is 6 kg, giving 2 m/s. KE decreases from 36 J to 12 J. The decrease is 24 J.