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?

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

   • Yes: the readings differ, so the forces were not equal
   • 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: cart B read higher in two trials, so B pushed harder
6. 6.

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

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

   • Agree: B ends faster than A did, so it gained more
   • 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 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
   • 2.4 N, the net force on the 2 kg block alone
   • 7.6 N, since the 3 kg block needs 3 × 1.2 N plus 4 N
   • 3.6 N, the 3 kg block's mass times its acceleration
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?

   • Neither team moves, since the rope forces on them are equal and opposite
   • Team B slides toward team A, since B pulls with the larger force
   • Team A wins, since the ice lets its team pull the rope more easily
   • Team A slides toward team B, since only B's feet can hold 150 N
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?

    • 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
    • 5 m/s, since the skater and ball feel equal and opposite forces