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 — 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.