Newton's third law: force pairs

Answer Key - Grade 11 Science Worksheet - Building Confidence

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

1. 1.

   Ari pushes block A (2 kg) with 15 N. It pushes block B (3 kg) ahead of it on a smooth table. How hard does A push B?

   9 N
2. 2.

   Esme pushes block A (4 kg) with 18 N. It pushes block B (5 kg) ahead of it on a smooth table. How hard does A push B?

   10 N
4. 3.

   Using the two diagrams, which cart has the larger acceleration, and by how much, Jun?

   • Both carts: 3 m/s², since the two forces are equal
   • Cart A: 6 m/s², since 6 N acts on it alone
   • Cart A: 3 m/s², twice cart B's 1.5 m/s²
   • Cart B, since the same force acts on a heavier cart
5. 4.

   Do the two 6 N forces cancel, so that the carts stay put, Kira?

   Use the picture in question 3.

   • No: the heavier cart pushes harder, so the carts both move
   • Yes, but just while the spring is still pushing on them
   • No: each force acts on a different cart, so both accelerate
   • Yes: equal and opposite forces cancel, so neither cart moves
6. 5.

   Two free-body diagrams side by side (example data). The skaters push off each other on smooth ice. After one second of pushing, which skater has the greater speed, Luz?

   • Neither: the two 60 N forces cancel, so both skaters stay put
   • Skater A, the 50 kg skater, who gains 1.2 m/s against 0.6 m/s
   • Both gain 1.2 m/s, since the two forces are equal in size
   • Skater B, since the 100 kg skater is pushed with more force
7. 6.

   A car's tires push backward on the road. What is the partner of that force, Milo?

   • Air resistance pushing back on the car
   • The car's weight pulling it down onto the road
   • The engine pushing the wheels around
   • The road pushing forward on the tires
8. 7.

   A book rests on a table, and the table pushes up on it. What is the partner of that force, Noah?

   • The book pulling Earth up with the same size of force
   • Air pressure pushing down on the book from above
   • The book pushing down on the table
   • Earth pulling the book down, which is its weight
9. 8.

   A rocket's engine pushes burning gas downward. What is the partner of that force, Pia?

   • The air around the rocket pushing it upward
   • The fuel tank pushing the engine downward
   • The gas pushing the rocket upward
   • Earth's gravity pulling the rocket downward
10. 9.

    A bug hits a moving car's windshield. How do the force on the bug and the force on the car compare, Tala?

    • They are equal, but the bug's acceleration is far greater
    • The bug's force on the car is greater, since it hits at speed
    • They are equal, so the bug and car accelerate the same amount
    • The car's force on the bug is far greater, since it is bigger
11. 10.

    A heavy puck hits a light puck on an air-hockey table. How do their forces on each other compare during contact, Zee?

    • The light puck pushes harder, since it bounces back faster
    • Equal at the start, then the heavy puck pushes harder
    • Equal at each instant of the contact
    • The heavy puck pushes harder, since it has more mass