Newton's third law: force pairs

Answer Key - Grade 11 Science Worksheet - Challenge Me

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

1. 1.

   A 1.5 kg cart at 2 m/s hits a 0.5 kg cart at rest and they stick. The contact lasts 0.1 s. What average force acts on the 0.5 kg cart, Ari?

   7.5 N
2. 2.

   A 2 kg cart at 3 m/s hits a 1 kg cart at rest and they stick. The contact lasts 0.05 s. What average force acts on the 1 kg cart, Bo?

   40 N
3. 3.

   The contact lasts 0.050 s. Using the table, what average force does each cart feel during the contact, Jun?

   • 25 N on each cart, in opposite directions
   • 25 N on cart B and 12.5 N on cart A, since A has half the mass
   • 50 N on cart A and 25 N on cart B, since A changed speed more
   • 0 N on each cart, since the system's total momentum is unchanged
4. 4.

   A cart's momentum changes by 5 kg·m/s against a spring in 0.10 s. A softer spring takes 0.20 s. What happens to the forces, Kira?

   • Both forces stay at 50 N, since the momentum change is the same
   • The cart's force halves, but the spring's force stays at 50 N
   • Both forces double, from 50 N to 100 N, since the time doubled
   • The cart's force and the spring's force both fall from 50 N to 25 N
5. 5.

   The two sensors do not give identical readings in any trial. Do the data contradict the third law, Luz?

   • No: the third law requires the readings to match to the last digit
   • Yes: cart B read higher in two trials, so B pushed harder
   • No: the gaps are 0.3 N or less, within the sensors' 0.5 N accuracy
   • Yes: the readings differ, so the forces were not equal
6. 6.

   Milo says a rocket's thrust and the exhaust's force are equal and opposite, so the rocket cannot accelerate. Agree?

   • Agree: the thrust works because the exhaust pushes the air
   • Disagree: the gas pushes harder than the rocket, so it moves forward faster
   • Agree: equal and opposite forces cancel, so the rocket stays put
   • Disagree: the forces act on different objects, the rocket and the gas
7. 7.

   Block A rides on block B without slipping, and both accelerate at 3 m/s² on a smooth table. Noah says block A pushes nothing on block B, since A just rides along. Using the diagrams, agree or disagree?

   • Agree: a block that rides along without slipping exerts no force on the other
   • Agree: the hand's 18 N is the one force that acts on block B
   • Disagree: A pushes B with 2 N, its mass times 1 m/s²
   • Disagree: A needs 6 N from B, so A pushes back on B with 6 N
8. 8.

   The contact lasts 0.050 s. Pia says the heavier cart in the table gains more momentum than the lighter cart loses. Agree or disagree?

   • Agree: B is twice as heavy, so it gains twice as much
   • Disagree: B gains 1.25 kg·m/s, exactly what A loses
   • Agree: B ends faster than A did, so it gained more
   • Disagree: A loses 2.5 kg·m/s, twice what B gains
9. 9.

   A 10 N push drives a 2 kg block into a 3 kg block; only the 3 kg block feels 4 N of friction. How hard do the blocks push each other, Ren?

   • 6 N, from 10 N shared by mass, ignoring the friction
   • 7.6 N, since the 3 kg block needs 3 × 1.2 N plus 4 N
   • 2.4 N, the net force on the 2 kg block alone
   • 3.6 N, the 3 kg block's mass times its acceleration
10. 10.

    A 50 kg skater on smooth ice throws a 2 kg ball forward at 5 m/s. How fast does the skater move backward, Zee?

    • 5 m/s, since the skater and ball feel equal and opposite forces
    • 0.04 m/s, since the mass ratio must be squared
    • 0.1 m/s, since the skater's mass is spread over 100 kg of load
    • 0.2 m/s, since the skater gets the same momentum as the ball