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. 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.

2. 1.

   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
3. 2.

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

   • Friction and the pull form the pair
   • Nothing: the forces cancel
   • The pulls act on different objects
   • The wagon pulls back less
4. 3.

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

   • Those pulls are on different objects
   • Earth pulls much harder
   • The shelf's push is the reaction
   • Nothing: equal pulls cancel
5. 4.

   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 push on the paddle moves the canoe
   • Nothing: the forces cancel
   • The paddle and water pushes both act on the canoe
6. 5.

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

   • Neither: the forces cancel
   • The light cart: the heavy one pushes harder
   • The heavy cart: it has more mass
   • The light cart: same force, less mass
7. 6.

   Noah (30 kg) and a 60 kg coach push apart on ice. Who speeds up more?

   • Both the same: the force is the same
   • Neither: equal pushes cancel
   • The lighter skater: same force, half the mass
   • The coach: the coach pushes harder

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?

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

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?

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

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

    • It pushed harder, since it is bigger
    • It pushed back as hard as the cart
    • It didn't push, since it didn't move
    • It pushed after the cart stopped
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 on each cart
    • 10 N on the 1 kg cart, 40 N on the 4 kg cart
    • 0 N, since the forces cancel
    • 10 N first, the other a moment later