AP → F=ma readiness · 25 min
A compact check for the force-direction, normal-force, friction, and circular-motion habits that decide early contest mechanics.
Draw a free-body diagram before writing equations.
Write one sentence naming the trap each question is testing.
Do not look at the solutions until you have a complete attempt.
Question 1
A 5.0 kg block rests on a frictionless 35° incline. Find the normal force.
Use the perpendicular direction.
N = mg cos35 = (5.0)(9.8)(0.819) = 40.1 N.
Trap tested: Normal force is not automatically mg.
Question 2
A 6 kg crate with μs = 0.50 is pushed with 20 N on a level floor and remains at rest. Find the friction force.
Compare required friction to max friction.
f_s,max = 0.50(6)(9.8) = 29.4 N. The required friction is 20 N, so f_s = 20 N opposite the push.
Trap tested: Static friction is not automatically at its maximum.
Question 3
A cyclist rides a circular path at constant speed. Is the net force zero? Explain.
Velocity includes direction.
No. The velocity direction changes, so acceleration and net force point inward.
Trap tested: Constant speed does not mean zero acceleration.
Question 4
A 70 kg person stands on a scale in an elevator accelerating upward at 1.8 m/s². What does the scale read?
The scale reads normal force.
N - mg = ma, so N = 70(9.8 + 1.8) = 812 N.
Trap tested: The scale reads contact force, not gravitational weight.
Question 5
A 15 N force pushes a 2 kg block into a 3 kg block on a frictionless floor. Find the contact force on the 3 kg block.
Find shared acceleration first.
a = 15/(2+3) = 3 m/s². The contact force on the 3 kg block is (3)(3) = 9 N.
Trap tested: The applied force is not the same as the internal contact force.
Question 6
A 1000 kg car turns on a flat road of radius 50 m at 10 m/s. Find the required static friction force.
Friction supplies the inward force.
F = mv²/r = 1000(100)/50 = 2000 N inward.
Trap tested: Static friction can point sideways and provide centripetal force.
score What it means
Move into the F=ma constraints path.
Redo the two missed lesson drills, then take the constraints mock.
Start with normal force, static friction, and circular motion before timing yourself again.
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.