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

Answer Key - Grade 8 Science Worksheet

Circle the best answer, or write your answer on the line.

1. 1.

   Ari and a friend push a heavy box across the gym floor. What is the net force on it?

   10 N
2. 2.

   Force arrow diagram, one dimension, arrows drawn to scale by size. Bo and a teacher push a loaded cart of chairs to the stage. What is the net force on it?

   20 N
3. 3.

   Esme and a friend push a sled across packed snow. What is the net force on it?

   24 N
4. 4.

   Jun pushes a shopping cart that is already rolling right. Read the diagram. What happens to its speed?

   • It stays at 1 m/s, since it is moving
   • It speeds up
   • It slows down
   • It stops, since friction acts
5. 5.

   Kira's hockey puck slides right across an indoor rink with only friction on it. What happens to its motion?

   • It stops at once with no push
   • It slows down
   • It speeds up to the left
   • It keeps 3 m/s, since forces balance
6. 6.

   Force arrow diagram, one dimension, arrows drawn to scale by size. Luz's toy train rolls right at 2 m/s. The motor's pull matches friction. What happens next?

   • It speeds up to the right
   • It comes to a stop
   • It keeps rolling at 2 m/s
   • It slows, since a balanced pull is not enough
7. 7.

   Milo pulls an empty wagon fast and a wagon full of books slowly. Which has more inertia?

   • The full wagon, since it has more mass
   • Neither, since inertia is a force
   • They have the same inertia
   • The empty wagon, since it is faster
8. 8.

   Two 5 kg balls sit side by side, one rolling and one at rest. Which has more inertia, Noah?

   • The rolling ball
   • The ball at rest, since it is harder to move
   • The rolling ball, since inertia pushes it
   • They have the same inertia

9. Read this first

   Use this

   Investigation card (test results). Question: how far does a cart roll on different floors? Changed: the floor (carpet, wood, smooth plastic). Measured: how far the cart rolls after leaving the ramp. Kept the same: cart, ramp height, release point. Results: carpet 1.2 m, wood 3.5 m, smooth plastic 6.8 m.

10. 9.

    Tala's class tested a cart on three floors. Why did the cart stop soonest on carpet?

    • Carpet gave the most friction
    • The cart was heavier on carpet
    • Carpet made the forces balance, so it stopped
    • The cart's push ran out fastest there
11. 10.

    Zee reads the class results. Which claim do they support?

    • Friction is what stops the rolling cart
    • Wood floors push carts forward
    • Balanced forces stop a cart
    • A cart needs a push to keep rolling