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MCAT Newton's Laws, Friction and Inclined Plane Practice Questions

By StudyStore · Published 2026-11-04 · 8 original questions · about 12 min

Eight free MCAT-style physics practice questions on net force, free-body diagrams, normal force, static and kinetic friction and inclined planes, with worked explanations.

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Quick concept primer

Newton's second law, net force = mass x acceleration (F = ma), is the workhorse. Draw a free-body diagram, choose axes (tilted along the slope for inclines), and sum forces along each axis. Newton's first law says zero net force means constant velocity (including rest). Newton's third law says forces come in equal and opposite pairs acting on different objects.

Normal force is perpendicular to the surface and is not always mg. On an incline of angle theta, weight splits into mg sin(theta) along the slope and mg cos(theta) into it, so N = mg cos(theta). Friction opposes motion or attempted motion: static friction is at most mu_s N, and kinetic friction is mu_k N, usually smaller. Use g = 10 m/s^2 for quick estimates.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

A 5 kg block on a frictionless surface is pushed by a net horizontal force of 20 N. What is its acceleration?

  1. 0.25 m/s^2
  2. 100 m/s^2
  3. 15 m/s^2
  4. 4 m/s^2
Show answer and explanation

Answer: D. 4 m/s^2

From F = ma, a = F/m = 20 N / 5 kg = 4 m/s^2. Dividing the other way gives 0.25, and multiplying gives 100, which are common mistakes. Always check that the units work out to m/s^2.

Question 2

A box moves at constant velocity across a rough floor under a horizontal push. Which statement is correct?

  1. The push is greater than the friction force
  2. The push is less than the friction force
  3. The push equals the friction force in magnitude
  4. No friction force acts on the box
Show answer and explanation

Answer: C. The push equals the friction force in magnitude

Constant velocity means zero acceleration, so by Newton's first and second laws the net force is zero. The push and kinetic friction are equal in magnitude and opposite in direction. If the push were larger, the box would accelerate.

Question 3

A 10 kg block rests on a horizontal surface with a coefficient of static friction of 0.40. Using g = 10 m/s^2, what is the largest horizontal force that can be applied before the block begins to slide?

  1. 4 N
  2. 25 N
  3. 100 N
  4. 40 N
Show answer and explanation

Answer: D. 40 N

The normal force equals the weight, 10 kg x 10 m/s^2 = 100 N. Maximum static friction is mu_s N = 0.40 x 100 N = 40 N. A larger push overcomes static friction and the block starts to move, after which kinetic friction applies.

Question 4

A block slides down a frictionless incline of 30 degrees. Using g = 10 m/s^2, what is its acceleration along the incline?

  1. 10 m/s^2
  2. 8.7 m/s^2
  3. 5 m/s^2
  4. 0 m/s^2
Show answer and explanation

Answer: C. 5 m/s^2

The component of gravity along the incline is g sin(theta) = 10 x sin 30 = 10 x 0.5 = 5 m/s^2. The mass cancels. The value 8.7 would come from using the cosine, which gives the component perpendicular to the surface.

Question 5

An elevator speeds up while rising, with an upward acceleration of 2 m/s^2. A bathroom scale on its floor supports a 60 kg rider. Using g = 10 m/s^2, what force in newtons does the scale display?

  1. 480 N
  2. 600 N
  3. 720 N
  4. 120 N
Show answer and explanation

Answer: C. 720 N

Take upward as positive: N - mg = ma, so N = m(g + a) = 60 x (10 + 2) = 720 N. The scale reads the normal force, which is greater than the weight (600 N) when accelerating upward. It would read 480 N if accelerating downward at 2 m/s^2.

Question 6

A book rests on a table. Which force is the Newton's third law partner of the book's weight (the Earth's gravitational pull on the book)?

  1. The normal force of the table on the book
  2. The book's gravitational pull on the Earth
  3. The friction force of the table on the book
  4. The force of the book on the table
Show answer and explanation

Answer: B. The book's gravitational pull on the Earth

Third law pairs act on different objects and are the same type of force. The Earth pulls on the book, so the book pulls on the Earth. The normal force balances weight only because the book is at rest, which is a second law result and not a third law pair.

Question 7

A 2 kg block and a 3 kg block are connected by a light string on a frictionless table. A 10 N horizontal force pulls the 2 kg block (the string is attached to the other side of it and pulls the 3 kg block along). What is the tension in the string?

  1. 6 N
  2. 4 N
  3. 10 N
  4. 2 N
Show answer and explanation

Answer: A. 6 N

The system accelerates together: a = 10 N / (2 + 3) kg = 2 m/s^2. The string is the only horizontal force on the 3 kg block, so T = ma = 3 x 2 = 6 N. Checking the other block, 10 - T = 2 x 2 gives T = 6 N as well.

Question 8

A block is on the verge of sliding down an incline of 37 degrees (sin 37 = 0.60, cos 37 = 0.80). What is the coefficient of static friction?

  1. 0.60
  2. 0.75
  3. 0.80
  4. 1.33
Show answer and explanation

Answer: B. 0.75

At the point of slipping, mg sin(theta) = mu_s mg cos(theta), so mu_s = tan(theta) = 0.60/0.80 = 0.75. The mass and g cancel, which is why the angle of repose is a direct measure of the friction coefficient.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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