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. 3.

   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: without a push it would stop
   • Agree: the force of inertia pushes it
   • Disagree: the ice pushes it forward
6. 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: the skater must speed up
   • Agree: a force of inertia carries the skater
   • Disagree: gliding needs a push
   • Agree: nothing changes the motion
7. 5.

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

   • A force called inertia pushes them forward
   • They have inertia because they move fast
   • They keep moving; nothing stopped them yet
   • The wagon's pull is still inside them
8. 6.

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

   • Fast riders have more inertia
   • They keep moving until a force stops them
   • Inertia is a force that throws them
   • The bus pushes riders forward as it stops
9. 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
   • Both, since inertia pushes them equally
   • Neither, since stopping takes no force
   • The empty cup, since it moves faster
10. 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 rolls back to the left
    • It stops at once, with no push
    • It slows down bit by bit
11. 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 stops, since balanced means still
    • It keeps moving at 1 m/s
    • It speeds up to the right
    • It slows, since 10 N is not extra push
12. 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 slows: the pull adds nothing extra
    • It slows to a stop: balanced means still
    • It keeps gliding: no net force changes it
    • It moves left: friction wins