Physics Practice Test: Friction and Inclines
- Friction and Inclines
- 10 questions
- 15 minutes
These 10 questions cover static and kinetic friction on level ground and on inclines. Use g = 9.8 m/s². On a ramp, split gravity into mg sin θ along the slope and mg cos θ into the slope; the normal force balances the second part.
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Question 1
A block slides down a ramp inclined at 30°. The coefficient of kinetic friction is 0.20. What is the block's acceleration?
Correct answer: D
a = g(sin 30° − μ cos 30°) = 9.8(0.500 − 0.20 × 0.866) ≈ 3.2 m/s². 4.9 m/s² ignores friction, and 2.9 m/s² forgets the cos 30° in the friction term.
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Question 2
A 10 kg box rests on a ramp inclined at 30°. What is the normal force on the box?
Correct answer: B
N = mg cos 30° = 98 × 0.866 ≈ 85 N. 49 N is mg sin 30°, the component along the ramp, not into it.
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Question 3
A coin lies on a book. The book is tilted slowly, and the coin starts to slide when the cover makes a 25° angle with the table. What is the coefficient of static friction?
Correct answer: C
At the point of slipping, mg sin θ = μs mg cos θ, so μs = tan 25° ≈ 0.47. 0.42 is sin 25°, which leaves out the normal force.
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Question 4
A crate slides down a 20° ramp at constant velocity. What is the coefficient of kinetic friction?
Correct answer: A
Constant velocity means friction balances mg sin 20°, so μk = tan 20° ≈ 0.36. 0.34 is sin 20° and 0.94 is cos 20°.
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Question 5
A 25 kg crate sits on a level floor. The coefficient of static friction is 0.40. What is the smallest horizontal push that will start it moving?
Correct answer: D
Maximum static friction is μsmg = 0.40 × 25 × 9.8 = 98 N, so the push must reach 98 N. 10 N multiplies μ by the mass and forgets g.
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Question 6
Once the 25 kg crate is moving, the coefficient of kinetic friction is 0.30. If the worker keeps pushing with 98 N, what is the crate's acceleration?
Correct answer: C
Kinetic friction is 0.30 × 245 = 73.5 N, so the net force is 98 − 73.5 = 24.5 N and a = 24.5/25 = 0.98 m/s². 3.9 m/s² ignores friction completely.
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Question 7
A car speeds up on a dry road without skidding. In which direction does the road's friction force on the driving tires point?
Correct answer: B
The tire pushes backward on the road, and by Newton's third law static friction from the road pushes the tire forward. This forward friction is what accelerates the car.
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Question 8
A box sits on the flat bed of a truck. The coefficient of static friction between box and bed is 0.35. What is the largest acceleration the truck can have without the box sliding?
Correct answer: A
Static friction is the only horizontal force on the box, so μsmg = ma and a = 0.35 × 9.8 ≈ 3.4 m/s². The mass cancels, so the box's mass is not needed.
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Question 9
A car moving at 20 m/s locks its wheels and skids to a stop. The coefficient of kinetic friction is 0.70. How far does it skid?
Correct answer: D
The deceleration is μg = 6.86 m/s², and d = v²/(2a) = 400/13.7 ≈ 29 m. 58 m forgets the 2 in the formula.
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Question 10
Two blocks of the same material slide across the same floor with the same starting speed. Block B has twice the mass of block A. How do their stopping distances compare?
Correct answer: A
Friction doubles with the mass, but so does the inertia, so the deceleration μg is the same for both. Equal deceleration and equal starting speed give equal stopping distance.
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