Friction and the Coefficient of Friction · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How hard surfaces can grip

Mathematics · Mechanics · ages 17-18
Name ______________________   Date ____________

  1. A block presses on a surface with normal reaction 40 N. The coefficient of friction is 0.5. What is the maximum static friction?

    • 20 N
    • 10 N
    • 40 N
    • 90 N

  2. A block presses on a surface with normal reaction 40 N. The coefficient of friction is 0.5. What is the maximum static friction?

    • 10 N
    • 20 N
    • 40 N

  3. A box stays put until pushed past a 40 N limit. It is pushed horizontally with 15 N. What friction force acts on it?

    • 40 N
    • 0 N
    • 15 N

  4. A 10 kg box rests on a horizontal floor. Take g equal to 10 N per kg. The coefficient of friction is 0.4. Give the maximum static friction in newtons.

    Answer: ______________

  5. Mia says friction always equals the coefficient times the normal reaction. Is Mia right?

    Circle one:   True   False

  6. A block has normal reaction 50 N and coefficient of friction 0.6. It is pushed horizontally with 35 N. What happens?

    • It slides
    • It stays put
    • It moves at a steady speed

  7. Mia says friction always equals the coefficient times the normal reaction. Is Mia right?

    Circle one:   True   False

  8. The box from the last question (limit 40 N) is pushed horizontally with 15 N. What friction force acts on it?

    • 15 N
    • 20 N
    • 0 N
    • 35 N

  9. A block is pushed with 10 N and stays still. Its friction limit is 30 N. What is the friction force?

    • 30 N
    • 10 N
    • 20 N

  10. A block has normal reaction 50 N and coefficient of friction 0.6. It is pushed horizontally with 35 N. What happens?

    • It slides
    • It stays put
    • It moves at a steady speed
    • It stays put because friction always matches the push
LightMySky · lightmysky.comW1-mt_EwFUd9N24W-s1

Answer key

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

How hard surfaces can grip W1-mt_EwFUd9N24W-s1

  1. 20 N · Limiting friction is the coefficient times the normal reaction: 0.5 times 40 N is 20 N.
  2. 20 N · The limit is the coefficient times the normal reaction: 0.5 times 40 N.
  3. 15 N · Static friction matches the push below the limit, so it supplies exactly 15 N.
  4. 40 · The normal reaction is 100 N, and 0.4 times 100 N is 40 N.
  5. False · Mia is wrong. Friction obeys an inequality, so it can be smaller than the limit. It equals the limit only at the point of slipping.
  6. It slides · The limit is 30 N, and a 35 N push beats it, so the block slides.
  7. False · Mia is wrong. Friction obeys an inequality and equals the limit only when slipping starts.
  8. 15 N · Static friction matches the push up to the limit, so it supplies exactly 15 N and the box stays put.
  9. 10 N · Below the limit, friction equals the push that it balances.
  10. It slides · The limit is 0.6 times 50 = 30 N, and a 35 N push beats it, so the block slides.
Worksheet · LightMySky