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.

   Bo pushes block A (3 kg) with 20 N. It pushes block B (2 kg) ahead of it on a smooth table. How hard does A push B?

   8 N
3. 3.

   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. 4.

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

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

   • Skater B, since the 100 kg skater is pushed with more force
   • 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
   • Neither: the two 60 N forces cancel, so both skaters stay put
6. 6.

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

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

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

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

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

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

   A 1,500 kg car hits a 15,000 kg truck head-on. Which statement about the crash is true, Ren?

   • The forces are equal; the car's acceleration is ten times larger
   • The car pushes harder, since it is the lighter vehicle
   • The forces are equal, so both vehicles accelerate equally
   • The truck pushes harder, so the car's acceleration is the larger one
10. 10.

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

    • 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
    • The light puck pushes harder, since it bounces back faster