How pushes, pulls and rubbing surfaces change motion
A force is a push or a pull. Forces can start or stop motion, change the speed or direction of a moving object, or change its shape, as when you squeeze a ball of dough. Contact forces need objects to touch, like muscular force and friction. Non-contact forces act from a distance: magnets attract iron, gravity pulls everything towards Earth, and a rubbed balloon attracts small bits of paper through electrostatic force.
Friction is the force that opposes motion between two surfaces in contact. It arises because even smooth-looking surfaces have tiny bumps that catch on each other. Rougher surfaces produce more friction, and heavier objects pressed together produce more as well. Static friction stops a resting object from starting to move, sliding friction acts while it slides, and rolling friction is the smallest, which is why wheels and ball bearings make moving things easier.
Why it matters
Without friction we could not walk, write with a pencil or stop a bicycle, yet too much friction wears out shoe soles and machine parts and wastes energy as heat. People increase friction with treaded tyres and grip on sports shoes, and reduce it with oil, grease, wheels and streamlined shapes. This topic builds towards Newton’s laws of motion.
Key terms
- Force
- A push or pull that can change an object’s state of motion or its shape.
- Friction
- A force that opposes relative motion between two surfaces in contact.
- Non-contact force
- A force, such as gravity or magnetism, that acts without objects touching.
- Lubricant
- A substance such as oil or grease used between surfaces to reduce friction.