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Using Forces

• Forces are pushes and pulls.
• Forces are measured in newtons (N).
• Forces are measured using a force-meter, also called newton-meters or spring balances.
• An object can have more than one force acting on it. If the forces are nor balanced there is a resultant force. It is the resultant force that moves the object.
• If 2 forces acting on an object are equal and opposite we say they are balanced forces. In this case there is no resultant force.
• Weight is a force. It is the pull of the Earth's  gravity.
• When two object rub together a force called friction slows them down.
• When an object is put in water the water pushes up, this force is called 'upthrust'.
• Newton's Law: If the forces on an object are balanced then if it still, it stays still, if it is moving it continues at a steady speed in a straight line.
• Structures are usually stronger if they are built of triangular shapes.
• When a bridge bends some parts are under compression and others are under tension.
• The bigger the weight on a simple beam bridge the more it will bend.
• The larger the force on a spring the more it stretches. Equal weights on a spring make it stretch by equal amounts, until the spring deforms.
• Average speed = distance traveled/time taken
• Speed is measured in metres per second or kilometres per hour.
• If an object speeds up it is accelerating. If it slows down it is decelerating.

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:
• start to move in the direction of the force
• speed up (accelerate)
• slow down (decelerate)
• change direction.

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).

Compression & Tension

We also look in this module at construction and the difference in strength of different shapes -particularly for building bridges. A triangular shape is the best! When a bridge is being squashed from both sides we say it is in compression and when it is being pulled from both ends we say it is in tension.

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

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.
• Design and build a structure from straws to be as strong and as tall as possible.
• Plan an investigation to find out which shape gives the strongest bridge.
• Design and build a bridge from paper and straws.
• Plan an investigation to find out what affects the sagging of a beam bridge.
• Plan an investigation to see hoe the length of a spring depends on the weight hanging on it.
• Calculate the speed of an object.