Action and reaction force pairs

Answer Key - Grade 8 Science Worksheet - Challenge Me

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

1. 1.

   Bo (30 kg) and a cousin (45 kg) push apart on ice with 180 N. What is Bo's acceleration during the push?

   6 m/s²

2. Read this first

   Use this

   Force arrow diagram, one dimension, arrows drawn to scale by size. Box A labeled 2 kg: push from B 24 N pointing left. Box B labeled 3 kg: push from A 24 N pointing right.

3. 2.

   A spring between a 2 kg cart and a 3 kg cart pushes them apart with 24 N. What is the 2 kg cart's acceleration, Esme?

   12 m/s²
4. 3.

   Jun pulls a wagon. A friend says the wagon pulls back equally, so it can't move. What is wrong?

   • The pulls act on different objects
   • The wagon pulls back less
   • Nothing: the forces cancel
   • Friction and the pull form the pair
5. 4.

   A book sits still on a shelf. Kira says it is still because the book and Earth pull each other equally. What is wrong?

   • Earth pulls much harder
   • Those pulls are on different objects
   • Nothing: equal pulls cancel
   • The shelf's push is the reaction
6. 5.

   Luz says a canoe can't move forward, since water pushes back on the paddle as hard as the paddle pushes water. What is wrong?

   • The paddle push is larger
   • The paddle and water pushes both act on the canoe
   • The push on the paddle moves the canoe
   • Nothing: the forces cancel
7. 6.

   A heavy cart and a light cart bump. Which one's motion changes more, and why, Milo?

   • Neither: the forces cancel
   • The light cart: same force, less mass
   • The light cart: the heavy one pushes harder
   • The heavy cart: it has more mass

8. Use this

   Force arrow diagram, one dimension, arrows drawn to scale by size. Box A labeled 2 kg: force from B 12 N pointing left. Beside it: cart moving right fast. Box B labeled 3 kg: force from A 12 N pointing right. Beside it: cart moving left slowly.

9. 7.

   Two carts meet head-on. Which statement about the bump is correct, Pia?

   • The faster cart pushes harder
   • They cancel, so nothing changes
   • Equal forces; both change the same
   • Equal forces; the 2 kg cart changes more

10. Use this

    Investigation card (test results). Force sensors on two lab carts read the push on each cart during a bump. Trial 1: 1 kg cart and 2 kg cart, 6 N and 6 N. Trial 2: fast cart and slow cart, 9 N and 9 N. Trial 3: a cart and a wall, 4 N and 4 N.

11. 8.

    Ren reads the sensor results. Which conclusion do they support?

    • A wall pushes back only after a delay
    • The heavier or faster one pushes harder
    • The two forces are always equal
    • The forces cancel in each bump
12. 9.

    Tala looks at Trial 3. What does it show about the wall?

    • It pushed after the cart stopped
    • It pushed harder, since it is bigger
    • It didn't push, since it didn't move
    • It pushed back as hard as the cart
13. 10.

    Zee predicts a new trial with a 1 kg cart and a 4 kg cart pushing 10 N on each other. What will the readings be?

    • 10 N first, the other a moment later
    • 0 N, since the forces cancel
    • 10 N on the 1 kg cart, 40 N on the 4 kg cart
    • 10 N on each cart