Opposing Forces: Newton's Second Law & Momentum
1 · Predict
A block moving to the right is pulled by a constant force in the opposite direction. What happens to its motion as time goes on?
- It slows down and comes to a permanent stop
- It slows down, stops, then speeds up moving in the opposite direction
- It slows down briefly, then continues at a constant reduced speed
2 · Set Up
- Open the opposing-forces simulation and press Reset.
- Enable the speed probe on the block.
- For each trial, set the block's initial speed, press Play, pause at t = 4 s using the timeline, and record the speed reading.
3 · Collect Data
| Mass m (kg) | Initial speed v₀ (m/s) | Opposing force F (N) | Initial momentum p = mv (kg·m/s) | Speed at t = 4 s (m/s) |
|---|---|---|---|---|
| 1 | 4 | 2 | ||
| 1 | 6 | 2 | ||
| 1 | 8 | 2 |
Plot the speed at t = 4 s (y-axis) against the initial speed v₀ (x-axis).
4 · Analyze
- Show your work computing the initial momentum p = mv for one trial.
- Using a = F/m, predict the block's velocity at t = 4 s for each trial and compare it to your table. Is the block still moving in its original direction, momentarily at rest, or moving backward?
5 · Extend
- If the block's mass were doubled while the opposing force stayed the same, would it take longer or shorter to stop? Explain using F = ma.
- Real blocks sliding on a real surface also feel friction. Would friction make the block stop sooner or later than this idealized simulation predicts?
The Physics Behind This Experiment
Momentum
The block's mass and initial speed set its momentum before the opposing force acts.
Newton's Second Law
The constant opposing force divided by mass gives the block's constant deceleration, which is what slows, stops, and eventually reverses it.