Draft copy. Friction and motion. Friction: pros and cons

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1 As you have learned, moving objects often slow down because there is a force acting on them. The force is acting in the opposite direction to the way the objects are moving. This force is called friction. Friction: pros and cons Friction is a force that tends to stop two things from sliding over each other. Sometimes friction is useful: Car tyres can push against the surface of the road, so the car can move forward. Brake blocks can squeeze against a bicycle wheel rim, so that the bike slows down. Shoes can grip the floor, so that you don t slip when you try to walk. However, sometimes friction is a nuisance: It slows moving things down, and extra force is needed to keep them moving. It can heat things up and can damage the moving surfaces. Friction can be a nuisance! Engine: Friction generates thermal energy and wears engine parts out Friction and motion Brakes, tyres, roads and friction Driving a car or motorbike safely would be impossible without friction. Especially important is the friction between the tyres and the road. When the driver or rider puts on the brakes, the wheels stop turning Friction can be useful! Steering wheel and driving gloves: These make it easier to hold on and steer the car. Brake pads and discs: Friction between them will slow the car down. Tyres and ground: Friction means: the engine can push car forward; grip means the car will stick to the ground and will stop the car when braking; and the car can be made to change direction.

2 and the vehicle will slow down as long as the tyres can grip the road. The grip between the tyre and the road depends on friction. This friction can be affected by: The type of road surface. Some surfaces are specially roughened to provide good friction and safe braking. Whether the road is wet or not. Water acts as a lubricant (see How to reduce friction, later in this chapter) between the tyre and the road and makes braking more difficult. The condition of the tyres. Worn tyres are smooth and so friction with the road is reduced, especially in wet conditions. This diagram shows how braking is affected by the condition of the road. Assume a driver of a car travelling at 80 km per hour (50 mph) wants to stop his car. The car travels this distance while he thinks about braking. 16 m 35 m 16 m Dry Wet The car travels this distance with the brakes on before it stops. 70 m On wet roads: Total stopping distance = 86 metres Speed and braking Even when a car has perfect tyres and brakes and the road is dry, stopping a car can be very difficult. The faster a car is travelling, the longer it will take for it to stop. This is because the car will travel further while the driver reacts to the situation, and also because the friction must act for longer to stop the car moving forward. On dry roads: Total stopping distance = 51 metres Friction: pros and cons 63

3 4 Friction and motion The UK Highway Code shows the distances required for braking at different speeds. At 60 mph (96.6 km per hour) under perfect conditions, stopping the car still takes nearly 20 times the length of the car. 20 mph 6 m 6 m = 12 m (40 ft) 3 car lengths 30 mph 9 m 14 m = 23 m (75 ft) 6 car lengths 40 mph = 36 m (120 ft) 9 car lengths m 24 m 50 mph = 53 m (175 ft) 13 car lengths 15 m 38 m 60 mph = 73 m (240 ft) 18 car lengths 18 m 55 m 70 mph = 96 m (315 ft) 24 car lengths Key: 21 m 75 m Standard car length = 4 m Thinking time Speed affects braking distance Braking distance How to reduce friction and stop damage to surfaces Friction is caused by tiny bumps between surfaces. These bumps are just like tiny pieces of sandpaper and stop the surfaces from moving. They also damage the surface by scraping pieces of material away and by heating the surfaces. The heating effect is caused by the particles in the surface being forced to vibrate. As they vibrate, they will have more kinetic energy and will get hotter. The heat can be enough to melt one or both of the surfaces and can stop the materials from carrying out their job properly. Friction, and the damage it can cause, can be reduced by: Smoothing off the surfaces. A smooth surface has less friction than a rough one. Adding a substance that keeps the surfaces slightly apart. This kind of substance is called a lubricant. Good examples are grease or oil. Friction is always more between rough surfaces. So, reduce friction by Smoothing out the surfaces; Adding oil(a lubricant) to keep the rough surfaces apart.

4 Air resistance and thermal energy This motorbike would meet a lot of air resistance. py The type of friction described in the example of the space shuffle on the previous page is called air resistance (also called drag). This is a kind of friction between a moving object and the air. Drag acts on you as you cycle along the road. The amount of drag can be reduced by making the object more streamlined. A streamlined object is shaped so that the air can flow smoothly past it. This reduces the air resistance so that the object can move more quickly through the air. Movement through water Air resistance is a kind of friction co When an object moves through the air, it will make the particles of the air move about. This can be very serious. The space shuttle, for example, generated an enormous amount of thermal energy when it re-entered the Earth s atmosphere. Some of this thermal energy was transferred back to the spacecraft, making it glow white hot. The shuttle would have been severely damaged by this massive amount of thermal energy if it did not have protection from the special tiles that covered it. This heating effect, together with damage to the tile covering (which happened on take-off), caused the disastrous breakup of the space shuttle Columbia in 2003, resulting in the deaths of all seven crew members. ra ft This motorbike is streamlined to reduce wind resistance. The same amount of force will make this one go faster than the one above. D Surface of spaceship Air Vibrating particles generate thermal energy. The particles on the surface of the shuttle are fixed in place, so no thermal energy is generated by vibration. The space shuttle (NASA ) was covered in thermal-resistant tiles made of silica. These protected it from the thermal energy generated by friction as it re-entered the atmosphere from space. The tiles became heated until they were white hot (see at the nose at upper right). Movement through water As well as air, water also provides resistance to the motion of objects and living things through it _Ch04.indd 65 29/04/15 3:45 pm

5 4 Friction and motion The streamlined shapes that help to reduce air resistance also help in movement through water. The evolution (see Biology, Chapter 9) of living organisms that live in water has provided many excellent examples of streamlining. Some of these are so effective that humans have copied them. Submarine Shark: Streamlining reduces drag in water. Because friction is a force, it too can be measured using a force meter. The way that this is done is shown in the diagram. Measurements of friction between different kinds of surface must be done as a fair test. Mass can be added: This means the trolley and the test material don t slide so easily with the result that the force meter can not measure the pull. Test material: This can be wrapped around the trolley. Good materials to test are carpet, nylon and sandpaper. Surface: This can be a smooth piece of wood or metal. Formica (like a kitchen work surface) is also suitable, as is glass. Torpedo Wooden trolley: A hook means it can be pulled by the force meter. Remember if you want reliable results: Force meter: As you pull on the force meter, the trolley will begin to slide. You can read the force needed to make the trolley move. The force on the scale will be measured in newtons (N). make it a fair test only change one factor (variable) at a time repeat the experiment and calculate an average of your results. 66

6 Investigation: Measuring friction The aim of this experiment is to investigate friction. Your group will be provided with a friction sledge or block of wood (as shown on the previous page), some weights, a force meter and a set of surfaces (materials) that you can attach to the bottom of the sledge. The surfaces are polythene, J cloth, sandpaper, carpet, denim cloth (or any other materials that your teacher has provided you with). Your task is to investigate how the surface on the bottom of the sledge affects the force that is needed to pull the sledge across the lab bench. 1 Write down the independent (input) variable, the dependent (outcome) variable. 2 Write down any other things that could affect the outcome. These are the control variables. Before you begin to take readings from the apparatus you must do some preliminary work to find out what values of controlled variables to use. See how to attach the surfaces to the sledge and find out which surface is the most slippery and which is the least slippery. Now find out what is a suitable weight to put on your sledge. You want the slipperiest surface to need enough force for you to be able to measure it. Check that you are using the best forcemeter for the job. You will need to be able to measure the force that is needed to pull the sledge across the bench at a steady speed. Practice pulling the sledge steadily. Create a suitable table using the headings below, and record the value of the force used. Surface Force needed to pull sledge steadily in newtons Reading 1 Reading 2 Average Now test the least slippery surface under the same conditions. You may need to use a different forcemeter. Record the force. Test each of the other surfaces in turn. Make sure that you are always using the forcemeter that is most appropriate for the task and record the value of the force in the table. Repeat your tests on each surface making sure you record your results. Then work out the average value of the force needed. 3 Represent your results in a bar chart. 4 (a) Which was the most difficult surface for the sledge to move on? (b) Represent the surfaces in a drawing to explain your findings. Exercise 4.1: Friction 1 State the names of two friction forces that would slow down a bicycle. 2 State two things that always happen when friction takes place. 3 Which of the following statements does not describe air resistance? (a) It is also called drag. (c) It helps streamlined cars to move quickly. (b) It is a kind of friction. (d) It produces thermal energy. Working Scientifically Movement through water 67