Forces and Motion
Grade 8 · Science · Study Guide
Understand the relationships between force, motion, speed, velocity, and acceleration with real-world examples.
Motion is a change in an object's position over time relative to a reference point. Speed measures how fast an object moves (distance ÷ time), while velocity includes both speed and direction. Acceleration is any change in velocity — speeding up, slowing down, or changing direction.
Forces are pushes or pulls that can change an object's motion. A balanced force results in no change in motion (the object stays still or moves at constant velocity). An unbalanced force causes acceleration — a change in speed or direction.
Friction is a force that opposes the motion of objects sliding against each other. Gravity is a force that attracts all objects with mass toward each other. Both are examples of forces that regularly affect the motion of objects in everyday life.
| Quantity | Definition | Formula | Unit |
|---|---|---|---|
| Speed | Distance per time | d ÷ t | m/s |
| Velocity | Speed + direction | d ÷ t (with direction) | m/s north |
| Acceleration | Change in velocity per time | (v_f − v_i) ÷ t | m/s² |
| Force Type | Net Force | Effect on Motion |
|---|---|---|
| Balanced | 0 N | No change in motion (stays still or constant velocity) |
| Unbalanced | Not 0 N | Acceleration — change in speed or direction |
Key terms
- Speed
- The distance an object travels per unit of time (speed = distance ÷ time).
- Velocity
- Speed in a specific direction; a vector quantity.
- Acceleration
- The rate of change in velocity; can be a change in speed or direction.
- Friction
- A force that opposes the relative motion of surfaces in contact.
Practice questions
A car travels 150 km in 3 hours. What is its average speed?
- 450 km/h
- 50 km/h
- 0.02 km/h
- 153 km/h
Show answer
Speed = distance ÷ time = 150 km ÷ 3 h = 50 km/h.
Which of the following is a vector quantity?
- Speed
- Distance
- Velocity
- Temperature
Show answer
Velocity includes both magnitude (speed) and direction, making it a vector quantity. Speed and distance are scalar quantities.
An object moving at constant speed in a straight line has balanced forces acting on it.
Show answer
Constant velocity (same speed, same direction) means zero acceleration, which means net force = 0 — balanced forces.
The formula for speed is: speed = distance ÷ ___.
Show answer
Speed = distance divided by time (s = d/t). The SI unit for speed is meters per second (m/s).
A ball is thrown horizontally off a cliff. Describe the forces acting on it during its flight (ignoring air resistance).
Show answer
This is projectile motion — gravity acts as the only unbalanced force.
Fun facts
- A car going around a circular track at constant speed is still accelerating — because its direction is constantly changing.
- In the absence of friction and air resistance, a hockey puck would slide forever at constant velocity, demonstrating Newton's First Law perfectly.
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