Newton's first law (AQA GCSE Physics): 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. It says that an object will stay still or keep moving in a straight line at the same speed unless the forces acting on it become unbalanced.
The key idea is balanced forces. When all the forces pushing and pulling on an object cancel each other out, the object's motion doesn't change.
Understanding balanced and unbalanced forces is fundamental to all of Newton's laws. This concept will appear throughout your physics studies, so make sure you grasp it thoroughly.
Objects that aren't moving (stationary bodies)
When an object is sitting still, all the forces acting on it are balanced. This means the forces cancel each other out completely.
Worked Example: Book on a Table
Consider a book resting on a table:
- The book's weight pulls it down: downwards
- The table pushes up with normal force: upwards
- Net force:
Since the forces are balanced, the book remains stationary.
The important thing to remember is that even though the object isn't moving, there are still forces acting on it - they're just perfectly balanced.
Objects moving at constant speed
Here's where it gets interesting. An object can also have balanced forces when it's moving! If something moves in a straight line at the same speed, the forces on it are balanced too.
Worked Example: Car at Steady Speed
A car driving at constant velocity:
- Engine provides driving force: forwards
- Air resistance and friction: backwards
- Car's weight: downwards
- Road's normal force: upwards
All forces are balanced, so the car maintains constant speed and direction.
Because all forces are balanced, the car keeps moving at the same speed in a straight line. It won't speed up, slow down, or change direction. This is a key insight - moving objects can have balanced forces just like stationary ones.
What happens with unbalanced forces?
When forces don't balance out, we get a resultant force. This means there's more force in one direction than the other. When this happens, the object will accelerate (speed up, slow down, or change direction).
Consider a simple example:
- force pushing right
- force pushing left
- Resultant force = to the right
The object will accelerate to the right because of this unbalanced force.
Working out resultant forces
You can calculate resultant forces by adding and subtracting the forces. The mathematical approach is straightforward: forces in the same direction add together, while opposing forces subtract from each other.
Worked Example: Calculating Resultant Forces
Case 1: Accelerating Car
- Driving force: forwards
- Resistive force: backwards
- Resultant force = forwards
- Result: The car will speed up
Case 2: Braking Car
- Forwards momentum force:
- Braking force: backwards
- Resultant force = backwards
- Result: The car will slow down
Case 3: Constant Speed
- Forwards force:
- Drag force: backwards
- Resultant force =
- Result: The car maintains constant speed
Common Mistake to Avoid
Many students think that if the resultant force is zero, the object must be stationary. This isn't true!
When forces are balanced (resultant force = 0N):
- The object might be stationary (not moving at all)
- OR the object might be moving at constant speed
Both situations have balanced forces with zero resultant force.
Key Points to Remember:
- Balanced forces mean no change in motion - objects stay still or keep moving at constant speed
- Unbalanced forces create a resultant force that makes objects accelerate
- Objects moving at constant speed have balanced forces, just like stationary objects
- To find resultant force: add forces in the same direction and subtract opposing forces
- Zero resultant force doesn't always mean the object is stationary - it could be moving at steady speed