Why things keep moving or stay still (balanced and unbalanced forces)

Answer Key - Grade 8 Science Worksheet - Challenge Me

Circle the best answer, or write your answer on the line.

1. Read this first

   Use this

   Force arrow diagram, one dimension, arrows drawn to scale by size. Box A: push 1 25 N pointing right; push 2 15 N pointing right; friction ? pointing left. Beside it: moving right at a steady speed.

2. 1.

   Ari and a friend push a box of books to the right at a steady speed. How strong is the friction?

   40 N

3. Use this

   Force arrow diagram, one dimension, arrows drawn to scale by size. Box A: push 1 30 N pointing right; push 2 18 N pointing right; friction ? pointing left. Beside it: moving right at a steady speed.

4. 2.

   Bo and a cousin push a big box of toys to the right at a steady speed. How strong is the friction?

   48 N

5. Use this

   Force arrow diagram, one dimension, arrows drawn to scale by size. Box A: push 1 22 N pointing right; push 2 14 N pointing right; friction ? pointing left. Beside it: moving right at a steady speed.

6. 3.

   Esme and a friend push a sled to the right across flat snow at a steady speed. How strong is the friction?

   36 N
7. 4.

   An ice skater glides at 3 m/s with friction and air draw a line almost zero. Luz says the skater keeps gliding. Agree or disagree?

   • Disagree: gliding needs a push
   • Agree: nothing changes the motion
   • Disagree: the skater must speed up
   • Agree: a force of inertia carries the skater
8. 5.

   Milo pulls a wagon of books; the wagon bumps a curb and stops. The books slide forward. Why?

   • The wagon's pull is still inside them
   • They keep moving; nothing stopped them yet
   • They have inertia because they move fast
   • A force called inertia pushes them forward
9. 6.

   Noah reads why school bus seats have belts. When the bus stops suddenly, why do riders move forward?

   • Fast riders have more inertia
   • The bus pushes riders forward as it stops
   • Inertia is a force that throws them
   • They keep moving until a force stops them
10. 7.

    Pia carries an empty cup and a full cup of water and stops suddenly. Which is harder to stop?

    • Both, since inertia pushes them equally
    • Neither, since stopping takes no force
    • The empty cup, since it moves faster
    • The full cup, since it has more mass
11. 8.

    Ren stops pushing a rolling cart. Now only friction acts. What happens to the cart?

    • It slows down bit by bit
    • It stops at once, with no push
    • It rolls back to the left
    • It keeps 2 m/s, since forces balance
12. 9.

    Force arrow diagram, one dimension, arrows drawn to scale by size. Tala shoves a box to get it moving, then pushes with exactly 10 N to match friction. What happens next?

    • It keeps moving at 1 m/s
    • It speeds up to the right
    • It stops, since balanced means still
    • It slows, since 10 N is not extra push
13. 10.

    A sled glides at a steady 2 m/s while Zee pulls with 30 N against 30 N of friction. What happens next, and why?

    • It keeps gliding: no net force changes it
    • It slows: the pull adds nothing extra
    • It slows to a stop: balanced means still
    • It moves left: friction wins