P5.1 Forces and Their Interactions
Scalars and vectors
| Scalar | Vector |
|---|---|
| Magnitude only | Magnitude AND direction |
| Speed | Velocity |
| Distance | Displacement |
| Mass | Force |
| Temperature | Acceleration |
| Energy | Momentum |
Forces are vectors — direction matters. Two forces of equal magnitude but opposite direction cancel.
Types of forces
Contact forces (require physical contact):
- Normal contact force / reaction force
- Friction
- Tension
- Air resistance / drag
- Compression
Non-contact forces (act at a distance):
- Gravitational force (attraction between masses)
- Electrostatic force (between charges)
- Magnetic force (between magnets / charges in motion)
Gravity
Every mass exerts a gravitational attractive force on every other mass. The size of the gravitational force depends on:
- The masses of both objects (larger mass → larger force)
- The distance between the objects (larger distance → smaller force — inverse square law)
Weight is the gravitational force acting on an object:
W = mg
W = weight (N), m = mass (kg), g = gravitational field strength (N/kg)
On Earth: g ≈ 9.8 N/kg (use 10 N/kg in many GCSE calculations)
Resultant force
The resultant force is the single force that has the same effect as all the individual forces acting on an object combined.
Collinear forces (same line):
- Same direction: add magnitudes.
- Opposite direction: subtract.
Perpendicular forces (vector addition — Higher only):
- Use Pythagoras: F = √(F₁² + F₂²)
- Use scale drawing to find resultant magnitude and direction.
Free body diagrams show all forces acting on an object as arrows:
- Length represents magnitude
- Arrow direction represents force direction
✦Worked example
A box is pushed right with 50 N and friction acts left with 30 N.
Resultant = 50 − 30 = 20 N to the right
The box accelerates to the right (Newton's 2nd Law).
Newton's Third Law (introduced here)
Every force has an equal and opposite reaction force. Forces always come in pairs — acting on different objects.
Example: You push down on the ground (action). The ground pushes up on you with equal force (reaction).
Note: Newton's 3rd Law pairs act on different objects; they do NOT cancel each other.
Common exam errors
- Confusing mass and weight — mass is in kg (scalar), weight is a force in N (vector).
- Calling non-contact forces "no forces" — gravity, electrostatics and magnetism act at distance.
- Saying Newton's 3rd Law pairs cancel — they act on different objects, so cannot cancel.
- Forgetting direction when working with forces — always state the direction.
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