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 22 N pointing right; push 2 14 N pointing right; friction ? pointing left. Beside it: moving right at a steady speed.

4. 2.

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

   36 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?

   • Agree: without a push it would stop
   • Disagree: no net force is needed to keep moving
   • Agree: the force of inertia pushes it
   • Disagree: the ice pushes it forward
6. 4.

   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: its engine push ran out
   • Disagree: inertia is a force pushing it on
   • Agree: everything stops in the end
7. 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: nothing changes the motion
   • Agree: a force of inertia carries the skater
   • Disagree: the skater must speed up
   • Disagree: gliding needs a push
8. 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
   • They have inertia because they move fast
   • A force called inertia pushes them forward
   • They keep moving; nothing stopped them yet
9. 7.

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

   • Inertia is a force that throws them
   • Fast riders have more inertia
   • The bus pushes riders forward as it stops
   • They keep moving until a force stops them
10. 8.

    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
    • Neither, since stopping takes no force
    • The empty cup, since it moves faster
    • Both, since inertia pushes them equally
11. 9.

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

    • It rolls back to the left
    • It keeps 2 m/s, since forces balance
    • It stops at once, with no push
    • It slows down bit by bit
12. 10.

    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 slows, since 10 N is not extra push
    • It stops, since balanced means still
    • It speeds up to the right