00:00
Read the passage, then answer the questions.
Friction is a contact force caused by two surfaces moving over each other. Surfaces that look and feel smooth are not really smooth at all. If you look at wood, glass or plastic under a microscope you see peaks and hollows.
When two surfaces slide past one another these tiny peaks catch on each other, and this is what produces friction. Friction always acts in the opposite direction to the movement, so it makes objects harder to move and slows moving objects down. Rough surfaces produce more friction than smooth surfaces. Friction also transfers energy to the thermal energy store of the surfaces, which is why rubbing your hands together warms them up, and why brake blocks wear away.
Friction is often useful. You need friction between your shoes and the ground in order to walk, and the brakes on a bicycle or a car only work because of friction. At other times friction is a nuisance: it wears away tyres and moving parts, and it wastes energy.
You can reduce friction by lubricating a surface with oil or grease so the surfaces slide past each other more easily, or by polishing a surface so it is smoother. You can increase friction by using a rougher surface, such as the tread on a tyre or the studs on a boot.
Read the passage and study Fig 1.3 and Fig 1.4, then answer the questions.
Any object moving through air or water has a resistance force acting on it that slows it down. This is called a drag force. Air resistance and water resistance are both drag forces. They happen because the object has to push the particles of air or water out of the way as it moves.
Drag always acts in the opposite direction to the motion, in the same way that friction does. The faster an object moves, the greater the drag force on it. The larger the surface area facing the direction of travel, the greater the drag force as well.
Engineers reduce drag by making vehicles streamlined. A streamlined shape is smooth and tapered so that air flows around it easily instead of being churned up behind it. Racing cyclists crouch low and wear smooth helmets, and lorries have curved deflectors on their roofs, all for the same reason. A peregrine falcon tucks its wings in when it dives, which makes it more streamlined so it can reach a much higher speed.
Sometimes a large drag force is wanted. A parachute has a very large surface area so that the drag force on it is big enough to slow a falling skydiver down safely.
Read the passage and study the graph, then answer the questions.
We use Newton's laws to describe how forces change motion. Newton's first law says the speed of an object stays the same unless a resultant force acts on it. Newton's second law says the larger the resultant force, the larger the acceleration. Newton's third law says if object A pushes on object B, then B pushes back on A with an equal force in the opposite direction.
When a skydiver jumps from an aeroplane, the only force acting at first is their weight, so there is a large resultant force downwards and they accelerate. As they speed up, the drag force on them increases. The resultant force downwards gets smaller, so the acceleration gets smaller too.
Eventually the drag force becomes equal to the weight. The forces are now balanced, the resultant force is zero and the acceleration is zero, so the skydiver falls at a steady speed. This maximum steady speed is called the terminal velocity.
When the parachute opens, the surface area increases sharply and so does the drag force. There is now a resultant force upwards, so the skydiver decelerates. As they slow down the drag force falls again until it equals the weight, and they reach a second, much slower terminal velocity, slow enough to land safely.
Read the passage, then answer the questions.
When you apply a force to an object you can change its shape. Changing the shape of an object is called deformation. You can compress an object, which means squashing it so it gets shorter, or you can stretch it, which means making it longer without breaking it.
The particles in a solid are held together by strong bonds. These bonds behave like tiny springs. When you stand on the floor, your weight pushes the particles closer together and the bonds are compressed. Squashed bonds push back, and this outward push is what we call the reaction force. It is the reason you do not fall through the chair you are sitting on.
When an object is stretched instead, the bonds are pulled further apart and they pull back. This pulling force in a stretched object is called tension. A bungee cord stretches as the jumper falls, and the tension in the cord eventually pulls the jumper back upwards.
Both the reaction force and tension are contact forces: they exist only while the two objects are touching.
Read the passage and study Fig 2.2 and Fig 2.3, then answer the questions.
When a force is applied to a spring, the spring stretches. The increase in length is called the extension. To find the extension you subtract the original length of the spring from its new length.
If you hang one mass on a spring you get an extension. If you hang two identical masses on the same spring you double the force, and the extension doubles as well. This relationship is called Hooke's law: the extension of a spring is directly proportional to the force applied to it.
If you plot a graph of force against extension for a spring obeying Hooke's law you get a straight line that passes through the origin. A straight line through the origin is what directly proportional looks like on a graph.
If you keep adding force, you eventually reach a point where the spring is stretched so far that it no longer returns to its original length when the load is removed. This point is called the elastic limit. Past the elastic limit the graph is no longer a straight line, and the spring is permanently deformed.
Read the passage, then work through each worked example and the questions that follow it.
Hooke's law can be written as an equation. The force applied to a spring is equal to the spring constant multiplied by the extension.
The spring constant, k, tells you how stiff a spring is. It is the force needed to extend the spring by one metre, so it is measured in newtons per metre (N/m). A spring with a high spring constant is stiff: it needs a large force to stretch it. A spring with a low spring constant is easy to stretch.
F is measured in newtons (N), k in newtons per metre (N/m) and e in metres (m).
Questions: set out your working using VESSU
Questions: set out your working using VESSU
Forces are not always given to you in newtons. A large force may be written in kilonewtons and a very small one in millinewtons. Before you substitute a force into F = k e you must always convert it into newtons first.
Complete the multiplier column, then use it in the questions below.
| Prefix | Symbol | Unit | Multiply the number by |
|---|---|---|---|
| mega | M | MN | 1 000 000 |
| kilo | k | kN | 1000 |
| milli | m | mN | 0.001 |
| micro | µ | µN | 0.000 001 |
Questions: convert each force into newtons first, then use VESSU
Read the apparatus list and method, study Fig 2.4, then complete the risk assessment and answer the questions.
Apparatus: clamp stand and boss, spring, 100 g slotted masses and hanger, pointer, metre rule, safety goggles, cushioned tray
Risk assessment: complete the two right-hand columns before you start.
| Hazard | Risk: what could happen | Control: how you will reduce the risk |
|---|---|---|
| Falling masses | They land on a foot, or bounce off the bench | Stand a cushioned tray under the masses; keep feet clear |
| An over-stretched spring | It flies off the clamp and hits someone in the eye | Wear safety goggles; never add more than five masses |
| A top-heavy clamp stand | The stand topples and falls off the bench | Weight or clamp the base; work away from the bench edge |
Set up the apparatus as shown in Fig 3.1 and follow the method carefully.
Apparatus: clamp stand and boss, spring, 100 g slotted masses and hanger, pointer, metre rule, safety goggles, cushioned tray
Record your measurements in the table as you go. Fill in every column.
A good results table is drawn before you start collecting data. Every column needs a heading and a unit, and the independent variable goes in the first column. Write down every reading as you take it rather than trying to remember it.
The mean is the average of your three readings. The extension is the mean reading minus your starting reading, so the extension in the top row will always be zero.
| Force (N) | Reading 1 (cm) | Reading 2 (cm) | Reading 3 (cm) | Mean (cm) | Extension (cm) |
|---|---|---|---|---|---|
| 0 | |||||
| 1 | |||||
| 2 | |||||
| 3 | |||||
| 4 | |||||
| 5 |
If one of your three readings is very different from the other two, it is an anomalous result, so leave it out when you calculate the mean.
Use the checklist to make sure your graph is drawn properly. Tick each box once you have checked it, then swap booklets with a partner and check theirs.
Use your graph to answer the questions and write your conclusion.
A conclusion says what your results show, and uses your data to back it up. If your line of best fit is straight and passes through the origin, the extension is directly proportional to the force and the spring is obeying Hooke's law.
An evaluation is different: it comments on how good the results are. Think about whether the points lie close to the line, whether there were any anomalous results, and what you would change if you did the practical again.
Read the passage and study the worked example, then answer the questions.
Every time you open a door, tighten a nut or use a pair of scissors you are using a turning force. A turning force acts at a distance from a fixed point called the pivot. The turning effect of a force is called its moment.
The size of a moment depends on two things: how big the force is, and how far from the pivot it is applied. The distance used is always the perpendicular distance to the pivot: the shortest distance, measured at right angles to the line of the force.
Force is measured in newtons (N), distance in metres (m) and a moment in newton metres (N m).
A spanner works as a force multiplier. Applying the same force further from the pivot gives a bigger moment, so a long spanner turns a stiff bolt that a short one cannot. This is also why a door handle is fitted at the edge of the door rather than next to the hinge.
Read the passage and study the worked example, then answer the questions.
When you sit on one end of a seesaw and a friend sits on the other, your weight produces a moment in one direction and theirs produces a moment in the other. One of these is a clockwise moment and the other is an anticlockwise moment.
The principle of moments states that an object is balanced when the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about that point. When this is true the object is in equilibrium and will not turn.
This is why a lighter person can balance a heavier one on a seesaw by sitting further from the pivot. The smaller force multiplied by the larger distance can give the same moment.
Read the passage and study Fig 4.3, then answer the questions.
The whole weight of an object seems to act through a single point. That point is called the centre of gravity, sometimes called the centre of mass.
Whether an object topples depends on where the line of its weight falls. If the line of weight from the centre of gravity falls inside the base of the object, the moment brings the object back down and it is stable. If you tip the object so far that the line of weight falls outside the base, the moment now turns the object further over and it topples.
This is why objects with a wide base and a low centre of gravity are the most stable. A racing car is built low and wide so it does not roll over on corners, and a bus carries its heavy engine low down for the same reason. A tightrope walker carries a long pole because it lowers their centre of gravity and makes it easier to keep the line of weight above the rope.
Read the passage and study Fig 5.1, then answer the questions.
A fluid is a substance with no fixed shape that can flow. Both gases and liquids are fluids. The particles in a gas move in random directions and at a range of different speeds.
We know this because of Brownian motion. In 1827 Robert Brown noticed that pollen grains in water moved about in a random, jerky way. Smoke particles move in the same way in air. The smoke particles are far bigger than the air particles, but the air particles move much faster, so each collision knocks the smoke particle off course.
Every time a gas particle collides with a surface it exerts a small force on that surface. There are billions of collisions every second, and the total force spread over the area of the surface is what we call gas pressure. Because the particles move in every direction, gas pressure acts in all directions, which is why a balloon inflates evenly on all sides rather than growing in one direction only.
When you blow up a balloon you add more air particles to the inside. More particles means more collisions with the walls each second, so a greater force on the walls and a higher pressure. The higher pressure inside pushes the rubber outwards and the balloon expands.
Read the passage and study Fig 5.2, then answer the questions.
The pressure of a fixed amount of gas can be changed in two ways: by changing the volume of its container, or by changing its temperature.
If you squash a gas into a smaller volume, the same number of particles now has less space. The particles reach the walls more often, so there are more collisions every second. This gives a greater force on the walls and therefore a higher pressure. If you increase the volume instead, the collisions become less frequent and the pressure falls.
If you heat a gas, the particles gain energy and move faster. They hit the walls more often and hit them harder, so again the force on the walls increases and so does the pressure. Cooling a gas has the opposite effect: the particles slow down, there are fewer and gentler collisions, and the pressure falls. This is why a balloon shrinks when you take it outside on a cold day.
Apparatus: 60 ml plastic syringe with a sealed nozzle, safety goggles
Read the passage and study Fig 5.3, then answer the questions.
You are surrounded by a fluid: the air. The air presses on your body all the time, and this is called atmospheric pressure. You do not feel it because the gases and liquids inside your body push outwards with the same pressure, so the forces are balanced.
Atmospheric pressure is caused by the weight of the air above you. Near the ground there is a very tall column of air above your head, so the pressure is at its greatest. As you climb a mountain there is less air above you, so the weight of that air is smaller and the atmospheric pressure is lower.
Gravity also pulls the air molecules towards the Earth, so the air is more dense near the ground and thinner high up. This is why mountaineers find it harder to breathe at altitude: each breath contains fewer oxygen molecules, which is why climbers on very high peaks often carry oxygen. It is also why aircraft cabins have to be pressurised.
Read the passage and study Fig 6.1 and Fig 6.2, then answer the questions.
Water is a fluid. When you swim underwater the water exerts a pressure on you. The water molecules push on each other and on every surface they touch, so liquid pressure acts in all directions, just as gas pressure does.
The pressure at any point in a liquid depends on the weight of the liquid above that point. The deeper you go, the more liquid there is above you and the greater the pressure becomes. This is exactly the same idea as atmospheric pressure, which depends on the weight of the air above you.
If you make three holes in a bottle of water, the water spurts out fastest from the lowest hole. The lowest hole has the greatest weight of water above it, so the pressure there is greatest and the water is pushed out hardest.
Engineers have to allow for this. The wall of a dam is built much thicker at its base than at the top, because the water pressure at the bottom of the reservoir is far greater and the wall must be strong enough to withstand it. Deep-sea submersibles are built with thick curved hulls for the same reason: a polystyrene cup taken to a depth of 1500 m is crushed to a fraction of its size.
Liquids also differ from gases in one important way. You can squash a gas into a smaller volume because its particles are far apart. You cannot squash a liquid, because its particles are already touching. We say liquids are incompressible.
Read the passage and study Fig 6.3, then answer the questions.
Because the pressure in a liquid increases with depth, the pressure on the bottom of a submerged object is greater than the pressure on its top. The upward push on the bottom is therefore larger than the downward push on the top, and the difference between them gives a resultant force upwards. This upward force is called upthrust.
Every object placed in a liquid experiences an upthrust. Whether it floats or sinks depends on how the upthrust compares with the weight. If the upthrust is equal to the weight, the forces are balanced and the object floats. If the weight is larger than the upthrust, there is a resultant force downwards and the object sinks.
This explains something that seems impossible at first. A small lump of steel sinks, but a cruise ship made of steel floats. The ship's hull is hollow and shaped so that it pushes a very large volume of water aside. The upthrust from that large volume of water is big enough to balance the huge weight of the ship.
Read the passage and study the worked example, then answer the questions.
Whether an object floats can also be explained using density. Density is a measure of how much mass is packed into a given volume. A material with tightly packed particles has a high density; a material with particles spread out has a low density.
Mass is measured in kilograms (kg), volume in metres cubed (m³) and density in kilograms per metre cubed (kg/m³).
The density of water is 1000 kg/m³. Anything with a density lower than this will float on water, and anything with a density higher than this will sink. Oil floats on water because its density is about 800 kg/m³. The average density of a cruise ship, counting all the air inside its hull, is lower than that of water, which is another way of explaining why it floats.