EM1 · AP2 pts
Question 1
Net work
2 points
A force does +45 J on a block while friction does -12 J. Find the change in kinetic energy.
Hint
Use net work.
Worked solution
ΔK = W_net = 45 - 12 = 33 J.
Trap tested: Using only the named applied work.
AP → F=ma · 35 min
Energy and momentum mock
A decision-making set for choosing the right conservation law on the right interval.
rule 1
Draw vertical lines between time intervals.
rule 2
Check external impulse before using momentum.
rule 3
Check nonconservative work before using mechanical energy.
EM2 · AP3 pts
Question 2
Sticky collision
3 points
A 2 kg cart at 5 m/s sticks to a 3 kg cart at rest. Find final speed.
Hint
Momentum during the collision.
Worked solution
(2)(5) = (5)v, so v = 2 m/s.
Trap tested: Conserving kinetic energy in an inelastic collision.
EM3 · F=ma4 pts
Question 3
Ballistic pendulum
4 points
A projectile embeds in a block. The combined mass rises to height h. What equation gives the speed just after collision?
Hint
Apply energy during the swing after the collision.
Worked solution
½(M+m)V² = (M+m)gh, so V = √(2gh). The pre-collision projectile speed needs momentum in the earlier interval.
Trap tested: Using energy through the collision itself.
EM4 · AP3 pts
Question 4
Explosion recoil
3 points
A 4 kg object at rest explodes into a 1 kg piece moving right at 9 m/s and a 3 kg piece. Find the 3 kg piece's velocity.
Hint
Initial total momentum is zero.
Worked solution
0 = (1)(9) + (3)v, so v = -3 m/s. It moves left.
Trap tested: Forgetting internal energy can create kinetic energy while momentum stays fixed.
EM5 · AP/F=ma3 pts
Question 5
Rebound impulse
3 points
A 0.20 kg ball moving left at 10 m/s rebounds right at 6 m/s. Taking right as positive, find the impulse.
Hint
Write both velocities with signs.
Worked solution
v_i = -10 m/s, v_f = +6 m/s. J = m(v_f - v_i) = 0.20(16) = 3.2 N·s right.
Trap tested: Using speed change instead of velocity change.
EM6 · USAPhO intro FR5 pts
Question 6
Circular orbit energy
Free response derivation5 points
A satellite of mass m orbits mass M in a circular orbit of radius r. (a) Derive its speed. (b) Derive total mechanical energy. (c) Explain why losing energy lowers the orbit radius.
Hint
Use gravity as centripetal force first.
Worked solution
GMm/r² = mv²/r gives v = √(GM/r). Then K = GMm/(2r), U = -GMm/r, and E = -GMm/(2r). Losing energy makes E more negative, so the circular-orbit radius associated with that energy is smaller.
Trap tested: Reporting only potential energy or using K = |U|.
Rubric
- 2 pts for deriving v from the force model.
- 2 pts for combining K and U with signs.
- 1 pt for the qualitative energy-radius explanation.
score What it means
Use the score to choose the next repair.
17-20 pts
Strong interval control
Move to mixed mechanics and rotation conservation problems.
11-15 pts
Good but inconsistent
Review every missed interval choice and redo R24/R27/R28.
0-10 pts
Law-choice repair
Work untimed on energy vs momentum decisions before contest timing.
after Turn the mock into data
If you missed a question, do not just reread the solution. Open the linked lesson, redo one drill, then come back to the mock.