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 30 N pointing right; push 2 18 N pointing right; friction ? pointing left. Beside it: moving right at a steady speed.

2. 1.

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

   48 N
3. 2.

   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
4. 3.

   A space probe far from any planet coasts with its engine off. Kira says it will slow down and stop. Agree or disagree?

   • Disagree: with no net force it keeps coasting
   • Agree: everything stops in the end
   • Disagree: inertia is a force pushing it on
   • Agree: its engine push ran out
5. 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: a force of inertia carries the skater
   • Agree: nothing changes the motion
   • Disagree: the skater must speed up
6. 5.

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

   • They keep moving; nothing stopped them yet
   • The wagon's pull is still inside them
   • They have inertia because they move fast
   • A force called inertia pushes them forward
7. 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
8. 7.

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

   • The empty cup, since it moves faster
   • Both, since inertia pushes them equally
   • The full cup, since it has more mass
   • Neither, since stopping takes no force
9. 8.

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

   • It slows down bit by bit
   • It rolls back to the left
   • It keeps 2 m/s, since forces balance
   • It stops at once, with no push
10. 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
11. 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 to a stop: balanced means still
    • It slows: the pull adds nothing extra
    • It keeps gliding: no net force changes it
    • It moves left: friction wins