Newton’s first law (AQA GCSE Physics Combined Science): Revision Notes
Newton's first law
What is Newton's first law?
Newton's first law tells us how objects behave when forces act on them. The law states that a body will stay at rest or keep moving in a straight line at constant speed as long as the forces acting on it are balanced.
This means that when all the forces pushing and pulling on an object cancel each other out, the object won't change what it's doing. If it's still, it stays still. If it's moving, it keeps moving at the same speed in the same direction.
When forces are balanced, we say the object is in equilibrium. This is a fundamental concept in physics that applies whether the object is moving or stationary.
Stationary bodies
When an object is not moving, all the forces acting on it must be balanced. This creates what we call equilibrium.
For example, imagine a ball hanging from a string:
- The tension in the string pulls upwards (25 N)
- The weight of the ball pulls downwards (25 N)
- These forces are equal and opposite, so they balance out
- The ball stays still because the forces are balanced
This happens with any stationary object. The forces might be different (like friction, air resistance, or contact forces), but they always add up to zero when the object isn't moving.
Bodies moving at constant speed
Here's where Newton's first law gets interesting. An object can be moving and still have balanced forces acting on it. When this happens, the object continues moving at the same speed in a straight line.
Think about a car driving at a steady 30 mph on a straight road:
- Driving force from the engine pushes forwards (20 kN)
- Resistive force from air resistance and friction pushes backwards (20 kN)
- Weight pulls down (15 kN)
- Reaction force from the road pushes up (15 kN)
All these forces are balanced, so the car keeps moving at constant speed. It won't speed up or slow down until something changes these forces.
This is why objects in space continue moving indefinitely - there are no significant resistive forces to create imbalance, so they maintain their motion according to Newton's first law.
Common mistake to avoid
Many students think that if the resultant force on an object is zero, the object must be stationary. This is wrong!
Remember: when forces are balanced (resultant force = 0), the object might be:
- Stationary (not moving at all)
- Moving at constant speed in a straight line
Both situations follow Newton's first law because there's no change in the object's motion.
Unbalanced forces
When forces acting on an object are not balanced, there is a resultant force. This resultant force will cause the object to accelerate in the direction of that force.
For example:
- If there's a 5 N force to the left and a 10 N force to the right
- The resultant force is 5 N to the right
- The object will accelerate to the right
The relationship between unbalanced forces and acceleration forms the foundation of Newton's second law, which we'll explore in future topics.
Worked examples
Worked Example: Calculating Resultant Forces on a Moving Car
Let's look at how to calculate what happens to a car with different forces:
Example 1: 300 N driving force, 200 N resistive force
- Resultant force = 300 N - 200 N = 100 N forwards
- The car will accelerate forwards and speed up
Example 2: 200 N driving force, 400 N braking force
- Resultant force = 200 N - 400 N = -200 N (200 N backwards)
- The car will decelerate and slow down
Example 3: 300 N driving force, 300 N drag force
- Resultant force = 300 N - 300 N = 0 N
- The car will continue at the same speed (balanced forces)
Summary
Key Points to Remember:
- Newton's first law: Objects stay at rest or move at constant speed when forces are balanced
- Balanced forces mean the resultant force equals zero
- Balanced forces can act on both stationary objects and objects moving at constant speed
- Unbalanced forces create a resultant force that causes acceleration
- Don't assume that zero resultant force always means the object is stationary