Lesson Aims: Friction Where do they need to put the sledge to to get it to start moving? Why doesn’t sledge move on the flatter part of the slope? Demonstrate: Book on a board/table – increase angle until it slides.

Friction and drag During today’s lesson you will be able: Lesson Aims: During today’s lesson you will be able: To recognise the effect that friction as a force has on motion. To be able to describe the origin of friction on a micro scale. To describe some uses and disadvantages of friction.

Lesson Aims: Homework Create a poster to show and explain an advantage and a disadvantage of friction e.g. Friction Advantage Disadvantage

Experiment to measure the force of friction on different surfaces. Lesson Aims: Experiment to measure the force of friction on different surfaces. Method: Using the Newton-meter measure the force needed to make the 1kg mass just start to move on each of the different surfaces. Repeat the test 3 times on each surface to find an average force of friction. Results: Surface tested Force needed to move 1kg weight Average Force of Friction 1st try 2nd try 3rd try

Which surface had the highest/lowest force of friction? Lesson Aims: Conclusion Which surface had the highest/lowest force of friction? Why do you think that is? Evaluation Look at your results for each surface, were they reproducible? (i.e. did you get similar results each time. Do you think your results are reliable? Why?

Lesson Aims: Why do we get friction?

Lesson Aims: Friction Even though some objects look smooth, microscopically they’re very rough & jagged. As the surfaces move they catch, grind and drag against each other. This leads to the force of friction which opposes movement. The force of friction also leads to the transfer of energy to heat. Book Desk

Friction Friction always tries to slow moving objects down – it opposes motion. Friction is created whenever two touching objects or surfaces move past each other. Friction also occurs when things move through air. This is called air resistance or drag. NOTE: The size of the frictional force equals the applied force unless the applied force is bigger than the maximum value of the frictional force. If this is the case then the frictional force remains at the maximum possible value.

Lesson Aims: Drag Drag is the force of friction created by an object moving in a fluid (liquid or air). It also opposes the direction of motion. Drag is most noticeable when things move at high speeds . Why do car designers try to reduce drag when they design a car? (use the word streamlined in your answer) Drag Thrust

Air resistance and drag Air resistance is a type of friction caused when objects move through the air. Cars are designed so that they are streamlined. The flow of air around the body is made as smooth as possible so that air resistance is minimized. Air resistance depends on: the size of the car; the shape of the car; the speed of the car. 400 N 300 N

Other sources of friction in cars One of the most important sources of friction in cars is that between the tyres and the road. When the car brakes, the maximum possible amount of friction is desirable so that the car does not skid. The friction between the tyres and road is affected by the: inflation pressure of the tyres; road surface; surface condition caused by the weather (rain, ice, etc).

Effects of frictional forces

Lesson Aims: Uses of Friction Without friction we wouldn’t be able to walk, sit in a chair or drive safely. Everything would keep sliding and falling over. We need friction between our feet and the ground or we would not be able to stand up.

Friction can be a problem Lesson Aims: Friction can be a problem If you want to push something heavy, friction makes it harder to move. Friction produces heat. Causing heat to build up is not always a good thing, especially inside a car engine. We try to reduce the amount of friction in a car engine by adding oil.

Friction in a bicycle Friction can be both good and bad in a bicycle. Lesson Aims: Friction in a bicycle Friction can be both good and bad in a bicycle. Create a table to compare where it is useful in a bike, and where it is a problem. Friction in a bicycle Good Bad Keeps tyres on road Resists your movement on the road. Makes brakes slow you down Resists your movement in the air. Keeps you sat on the saddle and your feet on the pedals Resists movement of all turning parts – wheels/ pedals/ chain – that is why we have ball bearings/grease

Sources of friction