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.

   A hockey puck glides across smooth ice at a steady 4 m/s. Jun says a forward force must keep pushing it. Agree or disagree?

   • Disagree: no net force is needed to keep moving
   • Agree: the force of inertia pushes it
   • Disagree: the ice pushes it forward
   • Agree: without a push it would stop
8. 5.

   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?

   • Agree: a force of inertia carries the skater
   • Agree: nothing changes the motion
   • Disagree: gliding needs a push
   • Disagree: the skater must speed up
9. 6.

   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
   • A force called inertia pushes them forward
   • They keep moving; nothing stopped them yet
   • They have inertia because they move fast
10. 7.

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

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

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

    • It keeps 2 m/s, since forces balance
    • It slows down bit by bit
    • It stops at once, with no push
    • It rolls back to the left
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 speeds up to the right
    • It stops, since balanced means still
    • It keeps moving at 1 m/s
    • 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 moves left: friction wins
    • It slows to a stop: balanced means still
    • It keeps gliding: no net force changes it
    • It slows: the pull adds nothing extra