Pearson Edexcel International As A Level Physics Student Book 1 Answers

[Christa Voth]

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20 FORCES AND MOTION FORCES AND MOTION a SOME OTHER EXAMPLES OF FORCES If the man gets someone to help him push the car, the forward force is bigger. \"' Figure 2.4 More forces! Both of the forces pushing the car are acting in the same direction, so you can It is not always easy to spot forces acting on objects. The compass needle find the total forward force by adding the two forces together. If both people in Figure 2.4a, which is a magnet, is affected by the magnetic force between are pushing with a force of 300 N, then: it and the other magnet. Magnetic forces are used to make electric motors rotate, to hold fridge doors shut, and in many other situations. total forward force = 300 N + 300 N If you comb your hair, you sometimes find that some of your hair sticks to the = 600 N comb as shown in Figure 2.4b. This happens because of an electrostatic force between your hair and the comb. You can see a similar effect using a Van de This means we can just add all the forces together to find the resultant force. Graaf! generator, as shown in Figure 9.6 on page 87. As force is a vector quantity, we also need to think about the directions in which the forces are acting, and we do this by deciding which direction is the A parachute causes the parachutist to descend more slowly because positive(+) direction. In this case, we can think of the force from the people as an upward force acts on the parachute called air resistance or drag. Air positive and the force from friction as negative. The + and - signs just show resistance is like friction - it tries to oppose movement of objects through that the forces are acting in opposite directions. the air. Designers of cars, high-speed trains and other fast-moving objects try to reduce the effects of this force. Objects moving through liquids So, if the force from friction is 300 N: also experience a drag force - fast-moving animals that live in water have streamlined (smooth and efficient) shapes to reduce this force. unbalanced force = 300 N + 300 N - 300 N Hot air balloons are carried upwards in spite of the pull of gravity on =300 N them because of a force called upthrust. This is the upward push of the surrounding air on the balloon. An upthrust force also acts on objects in BALANCED AND UNBALANCED FORCES liquids. Figure 2.7 shows two situations in which forces are acting on an object. In More types of force, such as electric and nuclear forces, are mentioned in the tug of war contest the two teams are pulling on the rope in opposite other chapters of this book. The rest of this chapter will look at the effects of directions. For much of the time the rope doesn't move because the two forces. forces are balanced. This means that the forces are the same size but act in opposite directions along the line of the rope. There is no unbalanced force in MORE THAN ONE FORCE one direction or the other. When the forces acting on something are balanced, the object does not change the way it is moving. In this case if the rope is As we saw earlier, in most situations there will be more than just one force stationary, it remains stationary. Eventually, one of the teams will become tired acting on an object. Look at the man trying to push the car, shown in and its pull will be smaller than that of the other team. When the forces acting Figure 2.5. The two forces act along the same line, but in opposite directions. on the rope are unbalanced the rope will start to move in the direction of the This means that one is negative (because it acts in the opposite direction to greater force. There will be an unbalanced force in that direction. Unbalanced the other) and, if they are equal in size, they add up to zero and the car will not forces acting on an object cause it to change the way it is moving. The rope move. was stationary and the unbalanced forces acting on it caused it to accelerate. \"' Figure 2.7 Balanced forces and unbalanced The car in Figure 2.7 is designed to have an enormous acceleration from rest. forces As soon as it starts to move the forces that oppose motion - friction and drag - must be overcome. The thrust of the engine is, to start with, much greater -F friction opposing than the friction and drag forces. This means that the forces acting on the car the motion in the horizontal direction are unbalanced and the result is a change in the way that the car is moving - it accelerates! Once the friction forces balance the \"' Figure 2.5 The re ,ultanl force is zero because the two forces are balanced. thrust the car no longer accelerates - it moves at a steady speed. pushes on the car FRICTION by the men Friction is the force that causes moving objects to slow down and finally stop. friction opposing The kinetic energy of the moving object is transferred to heat as work is the motion done by the friction force. For the ice skater in Figure 2.8 the force of friction is very small so she is able to glide for long distances without having to do any \"' Figure 2.6 The total pushing force is the sum of the two individual forces. work. It is also the force that allows a car's wheels to grip the road and make it accelerate - very quickly in the case of the racing cars in Figure 2.8. Scientists have worked hard for many years to develop some materials that reduce friction and others that increase friction. Reducing friction means that machines work more efficiently (wasting less energy) and do not wear out so quickly. Increasing friction can help to make tyres that grip the road better and to make more effective brakes.

22 FORCES AND MOTION FORCES AND MOTION 4 Figure 2.8 The ice skater can glide because friction is low. The cars need friction to grip the road. CHANGING SHAPES Friction occurs when solid objects rub against other solid objects and also We have seen that forces can make things start to move, accelerate or when objects move through fluids (liquids and gases). Sprint cyclists and decelerate. The examples in Figure 2.1 1 show another effect that forces can Olympic swimmers now wear special materials to reduce the effects of fluid have - they can change the shape of an object. friction so they can achieve faster times in their races. Sometimes fluid friction Sometimes the change of shape is temporary, as in the suspension spring is very desirable - for example, when someone uses a parachute after jumping in the mountain bike (Figure 2.11 a). Sometimes the shape of the object is from a plane! permanently changed, like a crushed can or a car that has collided with another object. A temporary change of shape may provide a useful way of INVESTIGATING FRICTION The simple apparatus shown in Figure 2.9 can be used to discover some basic 4 Figure 2.11 Forces can cause changes in absorbing and storing energy, as in the spring in a clock (Figure 2.1 1b). A facts about friction. The weight force on the line running over the pulley pulls shape. permanent change may mean the failure of a structure like a bridge to support weight to vary normal the block horizontally along the track and friction acts on the block to oppose a load. Next we will look at temporary changes in the lengths of springs and ...--reaction force this force. The weight is increased until the block just starts to move; this HOOKE'S LAW elastic bands. happens when the pull of the weight force just overcomes the friction force. surfaces i The friction force between the block and the track has maximum value. TEMPORARY CHANGES OF SHAPE under test weights added here The apparatus can be used to test different factors that may affect the size of the If you apply a force to an elastic band, its shape changes - the band stretches until test block just friction force, such as the surfaces in contact - the bottom of the block and the and gets longer. All materials will stretch a little when you put them under begins to slide over surface of the track. If the track surface is replaced with a rough surface, like a tension (that is, pull them) or shorten when you compress or squash them. You sheet of sandpaper, the force required to overcome friction will be greater. can stretch a rubber band quite easily, but a huge force is needed to cause a test surface noticeable extension in a piece of steel of the same length. It is important to remember friction when you are investigating forces and motion. 4 Figure 2.9 This apparatus can be used to Friction affects almost every form of motion on Earth. However it is possible to Some materials, like glass, do not change shape easily and are brittle, breaking investigate friction. do experiments in the science laboratory in which the friction force on a moving rather than stretching noticeably. Elastic materials do not break easily and tend object is reduced to a very low value. Such an object can be set in motion with a to return to their original shape when the forces acting on them are removed, A 'catch box' filled with bubble wrap small push and it will continue to move at a constant speed even when the force is like the spring in Figure 2.11 b. Other materials, like putty and modelling clay, (or similar) under the suspended no longer acting on it. An experiment like this is shown in Figure 2.10. are not elastic but plastic, and they change shape when even quite small masses keeps hands and feet out of forces are applied to them. the 'drop zone'. You may also have seen scientists working in space demonstrating that We will look at elastic materials, like rubber, metal wires and metals formed object s keep moving in a straight line at constant speed, once set in motion. into springs, in the next part of this chapter. They do this in space because the object s are weightless and the force of air resistance acting on them is very small. SPRINGS AND WIRES EXTENSION WORK Springs are coiled lengths of certain types of metal, which can be stretched or compressed by applying a force to them. They are used in many different Objects in orbit, such as spacecraft, are described as 'weightless' because they do situations. Sometimes they are used to absorb raised bumps in the road as not appear to have weight. However the Earth's gravity is still acting on them, and on suspension springs in a car or bicycle. In beds and chairs they are used to the spacecraft. You can think of a spacecraft in orbit as 'falling around the Earth'. As make sleeping and sitting more comfortable. They are also used in door locks the objects inside the spacecraft are also falling around the Earth at the same rate, to hold them closed and to make doors close automatically. they do not seem to fall inside the spacecraft. To choose the right spring for a particular use, we must understand some d start t = Os 0 t = 0.5S important features of springs. A simple experiment with springs shows us that: 2m 2m Springs change length when a force acts on them and they return to their original length when the force is removed. t = 1.0s 0 t = 1.5s ['.J I This is true provided you do not stretch them too much. If springs are stretched 2m 2m beyond a certain point they do not spring back to their original length. 4 Figure 2.10 Alinear (straight line) air-track reduces friction dramatically. The glider moves equal Robert Hooke discovered another important property of springs. He used distances in equal time intervals. Its velocity is constant. simple apparatus like that shown in Figure 2.12. Hooke measured the increase in length (extension) produced by different load forces on springs. The graph he obtained by plotting force against extension was a straight line passing through the origin. This shows that the extension of the spring is proportional to the force. This relationship is known as Hooke's law.

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