Section 1 of 9What is a force?
A force is a push or a pull that one object exerts on another.
- so when you name a force you should be able to say which object exerts it and which object feels it.
Force is a vector: it has a size and a direction. We draw forces as arrows whose length shows the size and whose direction shows the way the force acts.
Section 2 of 9Contact forces
Contact forces need the two objects to touch.
- Normal contact force (reaction) the push of a surface on an object resting on it, acting at right angles to the surface.
- A book on a table is pushed upwards by the table.
- Friction a force that opposes sliding between two surfaces in contact.
- It acts along the surfaces, opposite to the motion.
- (or the tendency to move)
- Drag (air resistance and water resistance) friction-like force from a fluid that opposes the motion through it.
- It grows as speed increases.
- Tension the pull along a stretched rope, string or cable.
- Upthrust the upward push of a fluid on an object placed in it.
Contact forces arise because the atoms of two touching surfaces interact electrically.
- which holds an object still, can be anywhere from zero up to a maximum value,
- while sliding (kinetic) friction is usually a little smaller than that maximum.
Lubricants such as oil put a thin layer of liquid between surfaces and reduce friction and wear, which is why engines contain oil.
Section 3 of 9Non-contact forces
These act across a gap, with nothing touching.
- Gravitational force (weight) the pull of a planet or other large mass on an object.
- Always attractive.
- Electrostatic force between charged objects.
- Like charges repel, unlike charges attract.
- Magnetic force between magnets, or between a magnet and a magnetic material such as iron.
- Like poles repel, unlike poles attract.
A field is a useful way of picturing non-contact forces: a mass has a gravitational field around it, a charge has an electric field, and a magnet has a magnetic field.
- but because planets have enormous masses and gravity is always attractive, it dominates on astronomical scales.
Section 4 of 9Forces and their effects
The effect of a force depends on whether it is balanced by others.
Elastic deformation means the object goes back to its original shape when the force is removed; plastic deformation means it stays permanently changed.
Section 5 of 9Drawing free-body diagrams
To show all the forces on one object, draw the object as a dot or box and add an arrow for each force starting from the object.
Remember that the arrows must all act on the object you are studying. The force the car exerts on the road is a different force, and belongs on a diagram of the road.
A useful check on a free-body diagram is to ask for each arrow, "what object is exerting this?" If you cannot name one, the arrow does not belong.
Section 6 of 9Links to other topics
Weight is studied in more detail in the lesson on weight; the combined effect of several forces is the resultant force, and how forces change motion is covered under unbalanced forces. Electrostatic and magnetic forces return in the electricity and magnetism topics.
Section 7 of 9Worked examples
A box rests on a table. State the two forces acting on it, their directions, and how they compare in size.
Weight of the box acts downwards (gravitational pull of Earth). The normal contact force from the table acts upwards.
The box is at rest, so the forces are balanced: the two forces are equal in size and opposite in direction.
A skydiver of weight 800 N falls with a drag force of 300 N. Name each force as contact or non-contact and give the size of the net force.
Weight: non-contact (gravity). Drag: contact force from air.
Net = 800 N − 300 N = 500 N downwards.
A magnet pulls a steel paperclip across a gap of 2 cm. Which type of force is this?
The paperclip moves without touching the magnet, so it is a non-contact (magnetic) force. The force attracts because steel becomes magnetised in the magnet's field.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Force | A push or pull measured in newtons (N). |
| Contact force | A force that acts only when two objects touch. |
| Non-contact force | A force that acts across a gap, such as gravity or magnetism. |
| Friction | A force opposing sliding between surfaces. |
| Tension | The pulling force along a stretched rope or string. |
| Upthrust | The upward force from a fluid on an object in it. |