Mechanics
Opposing forces
Watch the box slow down, stop, then reverse under a force smaller than its own momentum would need to stop it instantly.
Opposing forces — interactive Mechanics simulation. Watch the box slow down, stop, then reverse under a force smaller than its own momentum would need to stop it instantly.
Net force and deceleration
When a force opposes motion, the net force is smaller than either force alone and the object decelerates.
This block moves right but feels a leftward applied force. The net force is the vector sum of all forces, so the block slows, stops, and can reverse — a concrete case of Newton’s second law with opposing contributions.
- F_net = ΣF
- a = F_net / m
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
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?
Predictions to weigh
- 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
Variable roles
What you set:
- Mass m (kg)
- Initial speed v₀ (m/s)
- Opposing force F (N)
What you measure:
- Initial momentum p = mv (kg·m/s)
- Speed at t = 4 s (m/s)
How the investigation runs
- 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.
Governing equation
Momentum — p = m·v
The block's mass and initial speed set its momentum before the opposing force acts.
Newton's Second Law — F = m·a
The constant opposing force divided by mass gives the block's constant deceleration, which is what slows, stops, and eventually reverses it.
What the printable worksheet asks students to work out
- 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?
Where this shows up beyond the lab
- 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?
- AP Physics 1 — Unit 2: Force and Translational Dynamics
- AP Physics C: Mechanics — Unit 2: Force and Translational Dynamics
- IB Physics — A.2 Forces and momentum
- General High School Physics — Forces & Newton's laws
- NGSS High School Physics — Forces and Newton's second law
- Middle School Physical Science — Force, mass, and motion
- Welcome to Opposing Forces
- Select the block
- Press Play
- Slowing down
- Open the data
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.