FORCE

FORCEBy

Miss

Bui

cke

What we must know from the syllabus

OP4 Appreciate the concept of force, recall that the newton is the unit of force, and describe forces and their effects. OP5 Investigate examples of friction and the effect of lubrication. OP6 Investigate the relationship between the extension of a spring and the applied force.

Can you think of a force?Pushing

Pulling

lifting

Throwing

squeezing

What is happening in the picture? What force is being demonstrated?

What is a force?

A force is anything which causes an object to move or change its velocity.

The word force is used quite often in everyday conversations. Even though we use the word force regularly, it can be difficult to explain its exact meaning. In physics, we are very precise with the meaning of words and we must use the proper definitions when answering questions.

A famous scientist, Isacc Newton, wrote a very exact meaning of force:• A force can cause a stationery object to move.

• A force can cause a moving object to move more quickly, to move more slowly or to change the direction in which it is moving.

Unit of force

The unit used to measure force is the newton and the symbol is N.

Different types of force and their effects.

1.Force of gravity

2.Magnetic force

1. 4.3. Electric force

4. Force of friction2. 3.

1. The force of gravity

If a pen rolls over the edge of a table, it falls to the ground. This is because the earth pulls objects towards it. The pull of the earth is due to the force of gravity.

Gravity is pulling the apple to the ground

The story of Newton and the apple.The scientist and physicist Sir Isaac Newton made one of the greatest discoveries while sitting down under an apple tree when an apple fell on his head. Besides giving him a headache, it made Newton think about why the apple fell down straight to earth, and later realised that it didn't exactly “fall”, but rather it was drawn to the Earth's center. This is because the earth pulls objects towards it. The pull of the earth is due to the force of gravity.

2. Magnetic forceWhen a magnet is brought near a nail, you will notice that the magnet will attract the nail. The force that moved the nail is the magnetic force of the magnet.

If you bring two magnets near each other, they may attract or repel. The force of gravity always attracts objects towards the ground, while the magnetic force may attract or repel.

3. Electric force

When you rub a plastic pen against the sleeve of your jumper, you can lift small pieces of paper with the pen. The pen has become electrically charged and the electric force can attract the small pieces of paper.

4. Force of friction

How are brakes able to slow down a bicycle?

When you pull the brake handle the brake pad pushes against the wheel of the bicycle. The friction between the wheel and the brake pad causes the bicycle to move more Slowly.

Friction is a force which prevents easy movement between two objects in contact. The rougher the two surfaces in contact, the greater the friction.

Definition of friction

All surfaces are rough even though they do not seem to be.If you could see a close-up photograph of any surfaces,it would appear rough and jagged. The rough surface is what causes the force of friction.

How Does friction cause a matchstick to light?

The red part of the match on the end of the stick along with the red strip on the side of the box have rough surfaces which, when rubbed together quickly, create friction. The friction causes heat, which ignites the tip of the match.

When a block of wood is placed on a table the rough surfaceof the block is caught in the rough surface of the table.

When we push the block, some of your blushing force will move the block and some of your pushing force will be needed to overcome the friction between the table and the block. The amount of your pushing force needed to overcomethe friction will depend on how rough the surface in contact are.

What happens as a block of wood is pushed along a table?

Advantages of friction:

• Friction is needed to stop a bike. Friction between the brake pad and the wheel is what stops the bike.

• When driving a car around a corner friction is needed. If the tyres on a car are worn there is a risk that a lack of friction could cause the car to skid. An accident while turning a corner is more likely on a wet day because there is less friction between the tyres and the road.

• Friction allows grip, so a person can walk, climb and run. When we walk there is friction between our shoes and the ground, which stops us from slipping.

If you step onto an oily patch on the ground, you willmost likely slip. The oil patch reduces the friction betweenyour shoes and the ground.

Disadvantages of friction

Friction can also be a nuisance.

• Friction causes wearing of moving surfaces in contact eg. wear and tear of tyres and footwear.

• Getting a blister. The blister is caused by skin rubbing against the shoe.

• Heating effectWhen a space shuttle returns to earth there is a lot of friction between the surface of the shuttle and the gases in the atmosphere. This could cause the shuttle to overheat,with disastrous consequences.

• Friction wastes fuelWhen a car is moving there is friction between the air and the surface of the car. This friction is overcome by the force of the engine. However, this means that some of the fuel is neededto overcome friction. Therefore wasting fuel.

Reducing the force of friction

• To reduce friction between two surfaces you could try to make the surface smoother.

• There is no perfectly smooth surface so we oftenUse special materials to reduce friction

• These materials are called lubricants. Common examples are oil, liquid polish and soap.

Did you ever slip in the bathroom over a bar of soap??

There are many moving parts in a car engine. Friction would lead to wearing. Friction would also generateheat which would damage the engine.Oil and grease are used as lubricants in car engines.

The bones at the joint in the human body are surroundedby a fluid that acts as a lubricant. This greatly reduces friction.

Experiment: to investigate friction and the effects of lubrication

Equipment: Spring balance, four identical blocks of wood, sheet of sandpaper, piece of carpet, oil.

Method: • place one block of wood on the table attached to a spring balance• Pull the block along the table and record the force in neutons• Take the second block and attach a piece of sandpaper.• pull across the table and record the force in neutons• Take the third piece of wood and attach the carpet to it• Pull across the table and record the force• And finally apply oil to the last piece of wood• Pull across the table and record the force in neutons.

Result:The pulling force was different in each case.

Conclusion:The reading on the spring balance will be different for the four blocks.This shows that the force of friction depends on the type of surfaces that are in contact

Gravity as a force: weight

It is important to understand the difference between the words mass and weight. In everyday usage, these words often have the same meaning. In physics, they have Very different meanings.

weight mass

The weight of an object is the pull of the earth on it.

The mass of an object is the amount of matter in it.

Weight is a force Mass in measured in grams or kilograms

Weight is measured in newtons Mass does not have a direction

Weight has a direction, i.e a force that acts towards the earth

The mass of an object remains the same

Weight gets smaller as you rise upwards from ground level.

CalculationsThere is a simple formula to convert mass to weight.

Weight (in newtons) = mass ( in kilograms ) x 10

Sample question 1What is the weight of an object of mass 6.5kg?

Weight = mass x 10Weight = 6.5kg x 10Weight = 65 N

Sample question 2Calculate the weight of an object of mass 200g

Remember to change the unit of mass to kilogramsWeight = mass x 10Weight = .2kg x 10Weight 2 N

Sample question 3What is the mass of a person whose weight is 620 N ?

Weight = mass x 10Weight/10 = mass620/10 = mass62 kg

The effect of a force on a spring

The scientist Robert Hooke discovered the relationshipBetween the force applied to an elastic body (spring)and the extension of the elastic body (spring) causedby the force. This discovery is known as Hooke’s Law

Definition of Hooke’s lawThe extension of an elastic body (a spring) is directly proportional to the force causing the extension.

This means that if the force on the string doubles so will the extension; if the force triples then the extension will also triple, etc.

• When a spring is being extended as in the Hooke’s law experiment, a problem may arise

• This problem happens if the force used to extend the spring is too large.

• If the force is too big, the spring will be over-stretched and lose its elasticity.

• Hooke’s Law will be no more valid. We say that the spring has exceeded its elastic limit

Experiment:

To investigate the relationship between the extension of a spring and the force that is stretching it.

1. Use a newton-meter to stretch a spring and note the extension and the force used.

2. Repeat for lot of different forces.3. Plot a graph of force used against extension.

Result: Your graph should be a straight line.

Conclusion: because the points gave a straight line through the origin, it can be concluded that: the extension of an elastic body is directly proportional to the force causing the extension.

Graph showing extensions plotted against weight (force)