Lesson focus: frictionA force that tries to stop surfaces sliding over each other, like when you rub your hands together. opposes relative sliding; dragA force that acts against an object moving through air or water, slowing it down; air resistance is one kind of drag. opposes motion relative to a fluidA liquid or a gas: anything that can flow..
Explain useful and resistive friction without assuming it always points backward.
Build on earlier learning
Introduction to forcesName forces from interactions and draw them on the chosen object.
Practical
Measuring friction
- Pull a block across a surface with a newtonmeter at a steady speed.
- Read the force needed: at steady speed it equals the friction force.
- Repeat on different surfaces, keeping the block the same.
Record: A table of surface against friction force, with a bar chart.
You will be successful when you can…
- Distinguish friction and drag.
- Explain forward static friction.
- Explain braking friction.
- Relate shape and speed to drag.
Words that carry the science
- friction
- Contact force opposing relative sliding or its tendency.
- static friction
- Friction where surfaces do not slide relative to each other.
- drag
- Resistance to motion relative to a fluid.
- fluid
- Liquid or gas able to flow.
- grip
- Frictional interaction preventing unwanted sliding.
Choose force direction from relative motion
01Friction direction is contextual
A walking foot pushes ground backward; ground exerts forward static frictionFriction that stops an object from starting to slide. on the foot. During braking, road frictionA force that tries to stop surfaces sliding over each other, like when you rub your hands together. on wheels acts to reduce forward motion.
Think about walking step by step. Your foot pushes backwards on the ground. Friction from the ground pushes your foot forwards. That forward push is what moves you forward. On ice there is very little friction, so your foot slips back instead.
02Drag needs relative motion
Air or water dragA force that acts against an object moving through air or water, slowing it down; air resistance is one kind of drag. acts opposite movement relative to the fluidA liquid or a gas: anything that can flow. and usually increases with speed and frontal area.
03Useful or wasteful
gripWhen friction stops something from slipping, like a shoe on a dry floor. enables walking and braking; lubrication reduces heating and wear in bearings.
Friction on different surfaces
A block is pushed and let go. Friction acts against its motion, so it slows down and stops. Rougher surfaces give more friction, so the block stops sooner.
Controls
Values are illustrative. Friction also wastes energy by heating the surfaces. Lubricants such as oil reduce friction.
Explain walking
Why can friction on a walker point forward?
- The foot pushes backward on the ground.
- The foot tends to slip backward relative to ground.
- Static friction from ground on foot opposes that tendency.
- The forward friction force accelerates/supports the walker forward.
How to make the reasoning visible: Name the object receiving the force.
Interaction sorter
State the object and direction.
| Situation | Chosen object | Force | Direction/effect |
|---|---|---|---|
| Walking | Foot/person | Static friction | Forward |
| Car braking | Car | Road friction | Backward |
| Cyclist | Cyclist+bike | Air drag | Opposite air-relative motion |
| Bearing | Axle | Sliding friction | Reduced by lubricant |
Why is an icy path hard to walk on?
Why streamline a vehicle?
Use the table. Why is friction useful when you walk?
Check your understanding
Answer from memory, then check against the suggested answer.
What is friction?
What is static friction?
What is drag?
In which direction does friction act?
What is a lubricant?
A conveyor belt speeds a box forward without slipping. Which way is friction on the box?
Why can tyre tread improve wet-road grip but increase rolling losses?
Cyclists crouch low when racing. Explain why.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A person is walking forwards. Answer the parts to explain how friction helps.
Which way does the foot push on the ground?
(1)What pushes back on the foot?
(1)Which way does friction push the foot?
(1)What does this let the person do?
(1)Total for this question: 4 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., give each step in order, joined with “so” or “because”.