Newton's third law: force pairs
Grade 11 Science Worksheet - Challenge Me
Circle the best answer, or write your answer on the line.
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?
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?
A 0.8 kg cart at 1.5 m/s hits a 1.2 kg cart at rest and they stick. The contact lasts 0.2 s. What average force acts on the 1.2 kg cart, Esme?
The contact lasts 0.050 s. Using the table, what average force does each cart feel during the contact, Jun?
- 0 N on each cart, since the system's total momentum is unchanged
- 25 N on each cart, in opposite directions
- 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
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 and the spring's force both fall from 50 N to 25 N
- The cart's force halves, but the spring's force stays at 50 N
- Both forces stay at 50 N, since the momentum change is the same
- Both forces double, from 50 N to 100 N, since the time doubled
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 third law requires the readings to match to the last digit
- No: the gaps are 0.3 N or less, within the sensors' 0.5 N accuracy
- Yes: cart B read higher in two trials, so B pushed harder
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
- 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
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
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?
- 2.4 N, the net force on the 2 kg block alone
- 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
- 3.6 N, the 3 kg block's mass times its acceleration
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?
- 0.04 m/s, since the mass ratio must be squared
- 0.2 m/s, since the skater gets the same momentum as the ball
- 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