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
   • 0 N on each cart, since the system's total momentum is unchanged
   • 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
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?

   • 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
   • Both forces stay at 50 N, since the momentum change is the same
5. 5.

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

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

   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?

   • Disagree: A needs 6 N from B, so A pushes back on B with 6 N
   • Agree: a block that rides along without slipping exerts no force on the other
   • Disagree: A pushes B with 2 N, its mass times 1 m/s²
   • Agree: the hand's 18 N is the one force that acts on block B
7. 7.

   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?

   • Disagree: B gains 1.25 kg·m/s, exactly what A loses
   • Disagree: A loses 2.5 kg·m/s, twice what B gains
   • Agree: B ends faster than A did, so it gained more
   • Agree: B is twice as heavy, so it gains twice as much
8. 8.

   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
   • 3.6 N, the 3 kg block's mass times its acceleration
   • 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
9. 9.

   Team A is on ice and slips at a rope tension of 150 N. Team B is on rough pavement, and its feet hold. Which way does the contest go, Tala?

   • Team A slides toward team B, since only B's feet can hold 150 N
   • Team B slides toward team A, since B pulls with the larger force
   • Neither team moves, since the rope forces on them are equal and opposite
   • Team A wins, since the ice lets its team pull the rope more easily
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
    • 0.1 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
    • 0.04 m/s, since the mass ratio must be squared