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

Answer Key - Grade 8 Science Worksheet - Building Confidence

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.

2. 1.

   Bo pushes a moving box across the floor. The forces point opposite ways, so subtract them. What is the net force?

   15 N
3. 2.

   Esme pushes a laundry basket across the floor. The forces point opposite ways, so subtract them. What is the net force?

   12 N
4. 3.

   Jun slides a box across the classroom floor. Are the forces balanced, and what happens to the box?

   • Balanced; it keeps the same speed
   • Unbalanced; it speeds up
   • Balanced; it must stop
   • Unbalanced; it slows down

5. Use this

   Force arrow diagram, one dimension, arrows drawn to scale by size. Box A: pull 12 N pointing right; friction 12 N pointing left. Beside it: rolling right at a steady speed.

6. 4.

   Kira pulls a wagon along the sidewalk. Read the arrows. What happens to the wagon's motion?

   • It keeps rolling at the same speed
   • It stops, since the forces are balanced
   • It speeds up to the right
   • It slows, since the pull is used up
7. 5.

   Luz pushes a chair with wheels that is sitting still. Are the forces balanced, and what happens?

   • Unbalanced; it starts moving right
   • Balanced; it stays still
   • Unbalanced; it moves left
   • Balanced; it moves at a steady speed
8. 6.

   Noah swings an empty backpack and a full one. Which has more inertia?

   • The empty one, since it is easier to swing
   • Neither, since inertia is a push
   • The one swung faster
   • The full one, since it has more mass
9. 7.

   Pia reads that a bowling ball has a lot of inertia. What does that mean?

   • It is hard to start or stop
   • It rolls very fast
   • A force inside it keeps pushing it
   • It feels no friction
10. 8.

    Ren bikes at a steady 5 m/s. The forces on the bike balance. What happens next?

    • It keeps going 5 m/s
    • It slows down, since it needs more push
    • It speeds up a little
    • It stops soon, since forces balance

11. Use this

    Force arrow diagram, one dimension, arrows drawn to scale by size. Box A: push from the wind 3 N pointing right; water drag 3 N pointing left. Beside it: moving right at a steady 2 m/s.

12. 9.

    Tala's toy sailboat crosses a pond at a steady 2 m/s with balanced forces. What happens to its motion?

    • It stops, since balanced means still
    • It speeds up to the right
    • It stays the same, 2 m/s
    • It slows, since it is not pushed harder
13. 10.

    A dictionary sits on Zee's desk. Gravity and the desk's support balance. What happens to the book?

    • It stays at rest
    • It starts to slide
    • It needs a push to stay put
    • It sinks into the desk