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Forces

• Forces are pushes and pulls. They can change the size or shape of an object, change the speed of an object, change the direction of a moving object.
• Forces are measured by Force-meters (also called Newton Meters or spring balances).
• The size of a force is measured in Newtons (N)
• When two objects rub together, friction will slow them down.
• Air resistance can be reduced by streamlining.
• Weight is a force. It is the pull of gravity by the Earth.
• When an object is put in water, the water pushes up on it. This force is called upthrust.
• When an object floats, the upthrust is equal to the weight of the object. The two forces are balanced.
• Sea-water is more dense than fresh water so a boat floats higher in sea-water.
• Levers and pulleys are simple machines. A long spanner is easier to turn than a short spanner.
• A lever or pulley is used to lift a load when you apply an effort force.
• A lever can be a force-magnifier (like a spanner) or a distance-magnifier (like the arm).
• To start (or stop) a heavy object moving needs a big force.
• Forces are shown in diagrams by arrows, the bigger the force, the longer the arrow.
• An object can have more than one force acting on it. If the forces are not balanced there is a resultant force. It is the resultant force that makes the object move.
• Newtons Law: If the forces on an object are balanced then if it is at rest, then it stays at rest. If it is moving it keeps moving at a steady speed in a straight line.

• Forces are usually a push or a pull.
• Forces are measured using a newtonmeter and the unit of force is Newtons.
• Forces work in pairs - as you push down on a table the table pushes up with equal force.
• When there is no change in velocity or shape of an object then the forces are balanced e.g. book on table
• When there is a change in velocity or shape of an object then the forces are not balanced e.g. person falling through ice.
• An example of a force is gravitational force. To work out the weight of an object (with gravity working on it) you use the following formula:
• Weight (N) = mass (kg) x gravitational force (N/kg)
  

A force is either a push, a pull or a twist.

Forces are measured in this module and we learn all about the different types of force. Forces can do 3 things:-

1. they can change the size or shape of an object
2. they can change the speed of an object - make it go faster or slower - or start or stop
3. they can change the direction of a moving object

There are lots of forces to consider here. Weight is a force (mass x gravity), there is magnetic force and we look also at electric force - but only in passing! Forces are measured in Newtons. Friction is a force that tries to slow objects down. It happens when they rub together but drag or air resistance is a friction force too. We can reduce friction by lubricating the moving parts with oil or making a car more streamlined - altering the shape to be smoother.

Here are 4 types of forces:

• air resistance - drag: when an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed.

• contact force: happens when two objects are pushed together. They exert equal and opposite forces on each other. The contact force from the ground pushes up on your feet as you push down to walk forwards.

• friction: the force which resists movement between two surfaces which are in contact.

• gravity: the force which pulls objects towards the Earth. We call the pull of gravity on an object its weight. The Earth pulls with a force of about 10 newtons on every kilogram of mass.

Remember

Forces are often represented by arrows (=). The bigger the arrow the bigger the force. Forces are measured in newtons (N).

• The mass of an object tells us how much stuff there is there. We measure mass in kilograms (kg). The mass stays the same wherever you are.
• The weight of an object is a force . It is the pull of gravity on an object. We measure weight in newtons (N). The weight of an object acts towards the centre of the Earth.
• On the Earth gravity pulls with a force of 10 newtons for every kilogram (10 N/kg). We call this the gravitational field strength.

Look at this picture of a car and the forces acting upon it. Then read this typical exam question about mass and gravity.

The picture shows a car in a scrapyard. The mass of the car is 750 kg.

Question 1

What is the weight of the car?

The Solution

You should have worked out the weight of the car this way:

Question 2

What force does the arrow W represent?

The Answer

The arrow W is to show the direction of the weight on the car.

Question 3

What force does the arrow C represent?

The Answer

The arrow C represents the direction of the force from the crane to lift the car.

Question 4

What is the minimum force needed to lift the car?

The Answer

The minimum force is 7500 N, to overcome the pull of gravity.

Question 5

Suggest why the force from the crane engine will need to be bigger than your answer to (d).

The Answer

There will be some friction in the cables and pulleys, so the force will have to be bigger.

Do not forget that:

• Weight is the force of an object due to the pull of gravity.
• Weight is measured in newtons.
• When you draw an arrow for a force, label the arrow to show what force it is.
• Gravity is the force of attraction between any two masses. We only notice the force if one of the objects is very big - like the Earth, Moon or Sun.
• On the Earth gravity pulls with a force of 10 newtons for every kilogram (10 N/kg). We call this the gravitational field strength.
• The pull of gravity on the Moon is less, because the mass of the Moon is less. The gravitational field strength of the moon is 1.6 N/kg.
• Gravity keeps the planets in orbit around the Sun and the moon and artificial satellites in orbit around the Earth.
• W = mg means Weight (N) = mass (kg) x gravitational field strength (N/kg)

 Floating and Sinking

When placed in water some objects float and others sink. Have you ever wondered why? It is to do with the DENSITY of the object. This is mass/volume. If an object is more dense than water it will sink and if it is less dense than water it will float!

Levers

These are simple machines because they can make work easier by multiplying the force you are using.

Forces can be made to move things by turning the object around a pivot or fulcrum. It is just like a seesaw; with a downward force pushing on one side, an object on the other side will move. The downward force of the effort causes the load, on the other side, to move. When the two forces are equal there is no movement and the seesaw is balanced or in equilibrium.

Friction

An object travelling at speed has a force called friction acting on it to slow its motion. It is important to remember that friction isn't all bad.

Friction between the tyres and the road enables a vehicle to move because the tyres grip the road. The brakes on a car or bicycle work because of the friction between the brake pad and wheel rim. We can walk because of the grip between the soles of our shoes and the pavement.

In air or water friction is called drag or air/water resistance. Energy is needed to overcome these frictional forces and reduces the maximum speed we can reach. To reduce friction, the shape of the object can be streamlined by using curved surfaces and rounded corners.

An aircraft or object flying in a straight line at constant speed does so because all the forces are equal. The lift of the wings is equal to the weight or downward force caused by gravity. The thrust force of the engine is equal to the air resistance or drag caused by the body shape.

Summary

A force is a push, a pull or a twist.

When there is more than one force on an object:

• If all the forces are in balance, the object will stay as it is - stationary or moving - at a steady speed in a straight line.
• If the forces are unbalanced, it will:
• change speed
• change shape
• change direction.

• Read the scale on a force-meter.
• Use a force-meter to measure forces.
• Plan an investigation to measure the strength of people's finger muscles.
• Make a list of where friction is needed and where friction is not wanted (for example on a bicycle).
• Plan an investigation to see what makes a shoe more or less easy to slide.
• Make a table of 'floaters' and 'sinkers'.
• Measure the weight of an object and measure the upthrust on it when it is put into water.
• Design a boat that will support as much weight as possible.
• Label the pivot, the effort force and the load force for some common machines.
• Investigate a pulley system.
• Plan another investigation on a car rolling down a slope, to see how the distance traveled depends on the height it started from.
• Build a rubber band 'racer' and test it.