Contact and non-contact forces
Every force is a push or a pull that one object exerts on another. We measure force in newtons, \( \text{N} \), using a device called a newtonmeter. Forces come in two families.
- Contact force
- Acts only when two objects touch, such as friction, air resistance, tension in a rope, or the push of one hand on a door.
- Non-contact force
- Acts across a gap without touching, such as gravity, magnetism, and electrostatic (charge) forces.
Weight is the everyday example of a non-contact force: the Earth pulls down on you through gravity even though nothing is touching you from above.
Friction and air resistance
Two contact forces come up in almost every problem, and both act to oppose motion.
- Friction
- A force between two surfaces that are sliding, or trying to slide, past each other. It always acts in the direction that opposes the movement.
- Air resistance (drag)
- Friction with the air. It grows as an object moves faster, which is why a falling object eventually reaches a steady terminal velocity.
Friction is not always the enemy
Without friction you could not walk, grip a pencil or stop a bike. It wastes energy as heat, but it is also what lets tyres push against the road.
Free-body force diagrams
A free-body diagram shows a single object with an arrow for every force acting on it. The length of each arrow shows the size of the force, and the arrow points in the direction the force acts. For a book resting on a table there are two forces: weight pulling down and the normal contact force pushing up.
One object at a time
Draw forces acting on your chosen object only, never the forces it pushes back on other things. Mixing these up is the most common free-body diagram slip.
Resultant force
When several forces act at once, we add them (taking direction into account) to get one overall force called the resultant. Forces along a line simply add if they point the same way and subtract if they oppose.
A sledge is pulled forwards with \(50\ \text{N}\) while friction pulls backwards with \(20\ \text{N}\). Find the resultant force.
Balanced and unbalanced forces
Everything comes down to the resultant.
- Balanced forces
- The resultant is \(0\ \text{N}\). The object stays at rest, or keeps moving at a constant velocity in a straight line.
- Unbalanced forces
- There is a non-zero resultant. The object changes its motion: it speeds up, slows down, or changes direction.
This is exactly Newton's first law in plain words: an object keeps doing what it is doing unless an unbalanced force acts on it. A car cruising at a steady speed has its driving force balanced by friction and drag; press the accelerator harder and the forces become unbalanced, so it speeds up.
Where this is assessed
Identifying and naming the forces on an object shows Criterion A (knowing and understanding), while drawing an accurate free-body diagram and calculating the resultant is Criterion C (processing).
Check yourself
1. Is magnetism a contact or a non-contact force? +
A magnet attracts or repels across a gap without touching, so magnetism is a non-contact force, like gravity and electrostatic forces.
2. A box has a \(40\ \text{N}\) push forwards and \(40\ \text{N}\) of friction backwards. What is the resultant, and what happens? +
Resultant \(= 40 - 40 = \mathbf{0\ \text{N}}\). The forces are balanced, so the box stays at rest or continues at a constant velocity. Its motion does not change.
3. A rocket has \(500\ \text{N}\) of thrust up and \(300\ \text{N}\) of weight down. Find the resultant and describe the motion. +
Taking up as positive: \( 500 - 300 = \mathbf{200\ \text{N upwards}} \). The forces are unbalanced, so the rocket accelerates upwards.
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