Friction D. Crowley, 2007

Friction To understand what friction is, and how this affects movement

What is friction? Rub your hands together quickly - what do you notice? This is first hand experience of friction!

Experiment Last lesson we looked at road surfaces affecting the speed of a falling object What could you have done to make the object fall faster? Can you explain your answer? Can you draw your experiment, adding the force arrows between the different surfaces Some surfaces limited the speed the marble rolled at, due to much greater frictional forces Some surfaces allowed the marble to roll quickly, as they had low frictional forces A steep angle might have meant very little of the marble actually touched the surface

Riding your bike Before we define what friction is, think about riding your bike What do you think affects how quickly your bike can stop? See if you and a partner can come of with three factors affecting a bike’s stopping potential

Stopping the bike Type of surface you are cycling on - road / gravel / ice / mud?! How quickly you are going - the faster you go, the longer it takes to stop Type of brakes you have - the better the brakes (e.g. disc brakes) the quicker they can stop you How good are your tyres? Do they grip the road well, or are they more like slicks? How much are you carrying - the more you have on your bike the more energy you will have at a given speed - so the more energy it will take to stop you!

Friction Friction is a force which occurs when two objects interact If an object has no force propelling it, it will slow down and eventually stop due to friction Rubbing your hands together causes friction - causing your hands to heat up Friction can occur in many ways, including the following…

Friction Friction occurs between solid surfaces which are gripping / sliding past each other (e.g. a tyre on the road / marble down a ramp) Resistance (drag) from the air or liquid - as you move air or liquid particles collide into you (this is why a parachute slows you down and to go very fast cars need to become streamlined) Friction increases as speed increases - more speed = more air particles colliding into you Friction between two surfaces Friction increases as speed does Air resistance - example of friction

All bad? Friction, such as air resistance slows objects down But it is not always bad - you and your partner now need to write down some examples of friction when it is being bad (for us), and when it is being good

Friction - bad Air resistance slows vehicles down, so the engine needs to propel them (it must work harder at higher speeds) Friction makes it difficult to swim through water quickly Friction causes wear and heating - machines which have parts which touch / slide over each other produce lots of heat and wear - meaning they need lubricants

Friction - good Air resistance slows vehicles down, which can be very useful for vehicles such as the space shuttle which use the Earth’s atmosphere to slow it down, allowing it to land Parachutes utilise air resistance to slow people down, when falling to Earth If there wasn’t any friction we couldn’t slow our bikes down - brakes utilise friction to our advantage We wouldn’t be able to move without friction - just as low friction surfaces such as ice as hard to walk on, no friction would be impossible! Friction holds nuts and bolts together etc…

Question Why is it harder to run through water, than it is to run through the air? Answer this question using your new knowledge of friction If you finish this, can you think of ways we could get through the water more quickly (using scientific ideas)

Friction and Force Is friction affected by the force applied to an object? What’s your prediction? Design an investigation to find out

Friction and Surface area Is friction affected by the surface area of an object? What’s your prediction? Design an investigation to find out