1Friction
Friction acts where surfaces rub or slide. Even smooth-looking surfaces have tiny bumps that catch on each other, so friction pushes against the motion.
friction
Lesson 3.3 notes
Friction, drag and springs
Friction, air and water resistance, streamlining, and how springs stretch: Hooke's law.
2Friction heats things
Friction transfers energy from a moving object to the thermal store of the rubbing surfaces. Bike brakes squeeze pads against the wheel to slow it, and the pads and rim get hot.
3Reducing friction
A lubricant such as oil keeps moving surfaces apart so they slide easily. Less friction means less wear, and less energy wasted heating the parts.
lubricant
4Air and water resistance
Anything moving through air or water has to push it out of the way. This air resistance or water resistance gets bigger the faster it goes.
air resistancewater resistance
5Streamlining
Streamlined shapes let air or water flow smoothly round them, so the resistance is smaller. Fish, fast cars and racing cyclists are streamlined.
streamlined
6Stretching a spring
A weight stretches a spring until the spring pulls up as hard as the weight pulls down. The extension is how much longer the spring gets.
extension
7Hooke's law
Double the force and the extension doubles: Hooke's law. Stretched past its elastic limit, a spring stays longer when the weight is taken off.
Hooke's lawelastic limit
Explore: Spring lab
Hang weights on a spring one at a time and plot force against extension. Choose a soft or a stiff spring, and try stretching it too far.
Common mistake: Extension is the increase in length
The extension is how much longer the spring gets, not its new length. A 10 cm spring stretched to 16 cm has an extension of 16 − 10 = 6 cm.
Remember
- Friction acts where surfaces rub, against the motion, and it heats them. Lubricants such as oil reduce it.
- Air and water resistance push against things moving through air or water. They get bigger at higher speeds, and streamlined shapes reduce them.
- Hooke's law: the extension of a spring is proportional to the force on it, as long as it isn't stretched too far. Past its elastic limit, it won't go back to its original length.
Going further at GCSE
GCSE: Work and power → GCSE: Energy stores and transfers → GCSE: Forces revision →