# Forces & Motion — ACT Science
The ACT Science section tests your ability to interpret experimental data and understand scientific reasoning. Forces and motion concepts appear frequently in passages about velocity, acceleration, friction, and Newton's laws. You don't need to memorize formulas, but understanding the core concepts will help you answer questions quickly.
Key Concepts
Newton's Laws (Conceptual)
- First Law: Objects stay at rest or in motion unless a net force acts.
- Second Law: More force → more acceleration; more mass → less acceleration ().
- Third Law: Every action has an equal and opposite reaction.
Speed, Velocity, and Acceleration
- Speed: how fast (scalar).
- Velocity: how fast and in what direction (vector).
- Acceleration: rate of change of velocity.
- Positive acceleration in the direction of motion → speeding up.
- Negative acceleration (deceleration) → slowing down.
Friction
- Opposes motion.
- Greater on rougher surfaces.
- Static friction > kinetic friction.
Free Fall and Gravity
- All objects near Earth accelerate at (ignoring air resistance).
- Heavier and lighter objects fall at the same rate in a vacuum.
Reading Graphs on the ACT
- Position vs. time: slope = velocity.
- Velocity vs. time: slope = acceleration; steeper slope = greater acceleration.
- Flat line on velocity graph = constant speed.
Worked Example
Passage summary: Students measure the distance a cart travels down ramps of different angles.
| Angle (°) | Distance in 2s (m) |
|---|---|
| 10 | 0.6 |
| 20 | 1.2 |
| 30 | 1.7 |
| 40 | 2.1 |
Q: As the ramp angle increases, the cart's acceleration:
A: Increases. Steeper angles produce a larger component of gravitational force along the ramp, increasing acceleration.
Practice Questions
1. A graph shows velocity increasing linearly from 0 to 10 m/s over 5 seconds. What is the acceleration?
.
2. Two objects are dropped from the same height — one is 1 kg, the other is 5 kg. Ignoring air resistance, which hits the ground first?
They hit at the same time. In the absence of air resistance, all objects fall with the same acceleration.
3. In an experiment, adding weight to a cart increases its mass while the applied force stays constant. What happens to acceleration?
Acceleration decreases. ; as increases, decreases.
Want to check your answers and get step-by-step solutions?
Summary
- On the ACT, focus on interpreting data and trends rather than complex calculations.
- More force → more acceleration; more mass → less acceleration.
- Understand how to read position-time and velocity-time graphs.
- Friction opposes motion; steeper inclines increase the force component along the surface.