Science-Forces REVISION USE! Contains Music
What are forces A force can be a push or a pull. For example, when you push open a door you have to apply a force to the door. You also have to apply a force to pull open a drawer. You cannot see a force but often you can see what it does. Forces can change the speed of something, the direction it is moving in or its shape. For example, an elastic band gets longer if you pull it.
Measuring forces Forces can be measured using a force meter. Force meters contain a spring connected to a metal hook. The spring stretches when a force is applied to the hook. The bigger the force applied, the longer the spring stretches and the bigger the reading. The unit of force is called the newton, and it has the symbol N. So 100 N is a bigger force than 5 N. A force meter is used to measure forces.
Weight, Mass and Gravity People often confuse mass and weight. Remember that weight is a force, and is measured in newton. Mass is measured in kilograms (kg).
Mass The mass of an object is the amount of matter or "stuff" it contains. The more matter an object contains, the greater its mass. An elephant contains more matter than a mouse, so it has a greater mass. Mass is measured inkilograms, kg, or grams, g. A 100 kg object has a greater mass than a 5 kg object. Remember an object's mass stays the same wherever it is.
Gravity All objects have a force that attracts them towards each other. This is called gravity. Even you attract other objects to you because of gravity, but you have too little mass for the force to be very strong. Gravitational force increases when: • the masses are bigger • the objects are closer Gravity only becomes noticeable when there is a really massive object like a moon, planet or star. We are pulled down towards the ground because of gravity. The gravitational force pulls in the direction towards the centre of the Earth. "Down" is towards the centre of the Earth, wherever you are on the planet
Weight Weight is a force caused by gravity. The weight of an object is the gravitational force between the object and the Earth. The more mass the object has the greater its weight will be. Weight is a force, so it's measured in newton. On the surface of the Earth an object with a mass of 1 kg has a weight of about 10 N. Remember that mass is measured in kilograms, kg, and weight is measured in newton,N. The mass of an object stays the same wherever it is, but its weight can change. This happens if the object goes somewhere where gravity is stronger, or weaker, such as the Moon. The Moon has less mass than the Earth, so its gravity is less than the Earth's gravity. This means that objects weigh less on the Moon than they do on the Earth. The Moon's gravity is one sixth of the Earth's gravity. A 120 kg astronaut weighs 1200 N on Earth. On the Moon they would weigh only 200 N. The astronaut's mass is 120kg wherever they are. The weight of an object changes if the strength of gravity changes Remember: a mass of 1kg has a weight of about 10N on Earth.
Pressure You may get told off if you swing around on one leg of a chair instead of sitting properly. Apart from the risk that you will damage the chair or hurt yourself, the chair leg can damage the floor. This is because it puts too much pressure on the floor. Working out pressure To work out pressure, we need to know two things: the force or weight applied the area over which the force or weight works. This is the equation for working out pressure: pressure = force ÷ area
Question A force of 20 N acted over an area of 2 m2 (two square metres). What is the pressure?
Answer force ÷ area = pressure 20 ÷ 2 = 10 N/m2
Pascals Notice that the unit of pressure here is N/m2 (newtons per square metre). Sometimes you will see another unit being used. This is called the pascal, Pa. 1 Pascal = 1 N/m2, so in the example above the pressure is 10 Pa.
Using pressure Drawing pins Drawing pins make good use of pressure Drawing pins have a large round end for you to push. The round end has a large area, so it applies a low pressure to your thumb. The sharp end has a very small area. The same pushing force produces a high pressure there, so it pushes into the notice board. Chair legs If you swing round on one leg of a chair, you put four times as much pressure on one point of the floor as you do if you sit properly. This is because four chair legs spread the pressure over four times more area than one chair leg can.
Force Diagrams We can show the forces acting on an object using a force diagram. In a force diagram, each force is shown as a force arrow. An arrow shows: the size of the force (the longer the arrow, the bigger the force) the direction in which the force acts. The arrow is usually labelled with the name of the force and its size in newtons. Text books often show a force with a thick coloured arrow, but it is best if you just use a pencil and ruler to draw an arrow with a single line.
Balanced Forces When two forces acting on an object are equal in size but act in opposite directions, we say that they are balanced forces. If the forces on an object are balanced (or if there are no forces acting on it) this is what happens: an object that is not moving stays still an object that is moving continues to move at the same speed and in the same direction So notice that an object can be moving even if there are no forces acting on it.
Hanging objects The forces on this hanging crate are equal in size but act in opposite directions. The weight pulls down and the tension in the rope pulls up. The forces on this hanging crate are balanced.
Floating in water Objects float in water when their weight is balanced by the up thrust from the water. The object will sink until the weight of the water it pushes out of the way is the same as the weight of the object. A boat floats because its weight is balanced by the up thrust from the water.
Standing on the ground When an object rests on a surface such as the ground, its weight is balanced by the reaction force from the ground. The ground pushes up against the object. The reaction force is what you feel in your feet as you stand still. Without this balancing force you would sink into the ground. The weight of a book lying on a table is balanced by the reaction force from the table top.
Unbalanced Forces When two forces acting on an object are not equal in size, we say that they are unbalancedforces. If the forces on an object are unbalanced this is what happens: • an object that is not moving starts to move • an object that is moving changes speed or direction Resultant forces The size of the overall force acting on an object is called the resultant force. If the forces are balanced, this is zero. In the example above, the resultant force is the difference between the two forces, which is 100 - 60 = 40 N. Unbalanced forces make the truck speed up.
Helpful And Unhelpful Frictional Forces Whenever an object moves against another object, it feels frictional forces. These forces act in the opposite direction to the movement. Friction makes it harder for things to move. Helpful frictional forces Friction can be useful: friction between our shoes and the floor stop us from slipping friction between tyres and the road stop cars from skidding friction between the brakes and wheel help bikes and cars slow down Frictional forces are much smaller on smooth surfaces than on rough surfaces, which is why we slide on ice. Unhelpful frictional forces Friction can also be unhelpful. If you don't lubricate your bike regularly with oil, the friction in the chain and axles increases. Your bike will be noisy and difficult to pedal. When there is a lot of friction between moving parts, energy is lost to the surroundings as heat. Think of what happens when you rub your hands together quickly. The friction warms them up.
Air Resistance and Streamlining Air resistance Bikes, cars and other vehicles experience air resistance as they move. Air resistance is caused by the frictional forces of the air against the vehicle. The faster the vehicle moves, the bigger the air resistance becomes. The top speed of a vehicle is reached when the force from the cyclist or engine is balanced by air resistance. Streamlining A streamlined racing cyclist Racing cyclists crouch down low on their bikes to reduce the air resistance on them. This helps them to cycle faster. They also wear streamlined helmets. These have special, smooth shapes that allow the air to flow over the cyclist more easily. Modern cars are also streamlined. Their smooth shapes make the air resistance smaller, which allows them to travel further on the same amount of fuel. A streamlined racing cyclist
Force Moments Forces can make objects turn if there is a pivot. Think of a playground see-saw. The pivot is the thing in the middle of it. When no-one is on the see-saw it is level, but it tips up if someone gets onto one end. Turning forces around a pivot are called moments. It is possible to balance the see-saw again if someone else gets onto the other end and sits in the correct place. This is because the turning forces are balanced. We say the moments are equal and opposite.
Working Out Moments To work out a moment, we need to know two things: the distance from the pivot that the force is applied. the size of the force applied This is the equation for working out a moment: moment = force × distance Example Imagine that a force of 10 N acted on a see-saw 2 m from the pivot. This is how we would work out the moment: force × distance = moment 10 × 2 = 20 Nm Notice that the unit of moment is Nm (newton metre). Don't get confused with a 'newton meter', which is another name for a forcemeter. Here is an example of balanced moments. 10N at 2m from the pivot is balancing 20N at 1m from the pivot. The objects create moments of 20Nm that are equal and opposite, so the see-saw is balanced.
Using Moments A see-saw will balance if the moments on each side of the pivot are equal. This is why you might have to adjust your position on a see-saw if you are a different weight from the person on the other end. If a nut is difficult to undo with a short spanner, a longer spanner will help. This is because there will be a bigger moment on the nut, when the same force is applied further from the pivot. Using the same principle you can increase the moment applied by a lever or a crowbar, and this can help you move heavy objects more easily.