Force Work and Power Forces A force is

Forces • A force is anything which causes an object to change its velocity

Examples of Forces 1. Push – Changes the velocity of an object 2. Pull

Measuring Forces • To measure force we use a newton meter also called a

Force Quantities in every day life A Newton is a very small unit •

Forces always occur in pairs • For every force there is an equal and

Friction • Friction is the force that prevents easy movement between two objects in

What would you use to reduce friction in the chain of your bike? •

To investigate friction • Using a force sensor, a can of coke, some carpet

What happens if you skid across a carpet very fast? • You might notice

Work • Work is done when a force moves an object • We can

Example • This man uses a force of 300 N to push the lawn

Questions • Who does more work a woman carrying a case with a force

Power • Which of these machines would you prefer to use to cool you

• If you said the first one it is probably because you know

• Power is measured in Watts where 1 watt =1 j/s • If

Questions • A crane lifts a slab of concrete that weighs 60000 N from

• A girl whose weight is 500 N runs up a flight of

Remember • To change centimeters to meters divide by 100 • Eg. 10 cm

Hooke’s Law • Hooke’s Law states that the extension of a spring is proportional

• We can show this by experiment. • In Hooke’s law you should

• As soon as the line goes straight you know the spring has

Drawing Graphs • A graph should always have the following 5 point checklist to

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Force, Work and Power

Forces • A force is anything which causes an object to change its velocity • Forces are measured in Newtons • Can you think of some forces You could use to move a ball that is not moving?

Examples of Forces 1. Push – Changes the velocity of an object 2. Pull – Changes the velocity of an object 3. Weight – Moves objects towards the ground 4. Friction – Slows down moving objects 5. Electric – Causes charged particles to move 6. Magnetic – Causes magnetic materials to move

Measuring Forces • To measure force we use a newton meter also called a spring balance • Can you find the force of your pencil case? • 1 kg has a force of 10 N

Force Quantities in every day life A Newton is a very small unit • a. 1 N to lift an Apple • b. 12 N to lift your textbook • c. 20 N to open a can of Coke • d. 50 N to squash an egg • e. 100 N or more to push a supermarket trolley • f. Car brakes can exert a force of about 5000 N on wheels to stop the car. 5

Forces always occur in pairs • For every force there is an equal and opposite force

Friction • Friction is the force that prevents easy movement between two objects in contact • High friction : Sand paper, Car Tyres and rough soled shoes • Low Friction: wet soap, ice and glass

What would you use to reduce friction in the chain of your bike? • If you said oil you are correct! • Oil is an example of a LUBRICANT • A LUBRICANT is anything that reduces friction • Can you think of any others?

To investigate friction • Using a force sensor, a can of coke, some carpet and some oil how can we show the effect of friction and lubricants?

What happens if you skid across a carpet very fast? • You might notice that you can get a burning feeling this is because friction causes heat • Name 4 advantages and 4 disadvantages of friction • Why do you think swimmers, skiers and cyclists all shave their legs?

Work • Work is done when a force moves an object • We can find the amount of work done by multiplying the force used by the distance moved Work (j) = Force (N) x Distance (m) Work and energy are measured in joules

Example • This man uses a force of 300 N to push the lawn mower 20 m how much works does he do? • Answer: Work (j) = Force (N) x Distance (m) Work (j) = 300 N x 20 m =6000 joules

Questions • Who does more work a woman carrying a case with a force of 30 N for a distance of 100 m or a woman carrying a case with a force of 40 N for a distance of 75 m?

Power • Which of these machines would you prefer to use to cool you down on a hot day?

• If you said the first one it is probably because you know it would cool you down more because it is more powerful than the handheld fan • Power is the rate at which work is done • The more powerful an object the faster it can do work

• Power is measured in Watts where 1 watt =1 j/s • If something has a power of 20 watts it can do 20 j of work in 1 second • Why do you think a 100 w bulb is brighter than a 60 w bulb? • We can work out power by dividing work done by the time taken to do the work Power (w) = Work done (j) Time (s)

Questions • A crane lifts a slab of concrete that weighs 60000 N from the ground up to a platform 20 m high in 30 s • (a) How much work does the crane do? • (b) What is the power of the crane?

• A girl whose weight is 500 N runs up a flight of stairs 6 m high in 20 s what is her power? • A body builder lifts a weight of 6000 N 1 m above his head in 1 s what is his power?

Remember • To change centimeters to meters divide by 100 • Eg. 10 cm = 0. 1 m • To change grams to kilograms divide by 1000 • Eg. 200 g = 0. 2 kg • To change kg to Newtons multiply by 10 • Eg. 60 kg = 600 N

Hooke’s Law • Hooke’s Law states that the extension of a spring is proportional to the force applied to it • This means that for example if you add 50 g to a spring it will stretch a certain amount and if you add another 50 g it will stretch by the same amount

• We can show this by experiment. • In Hooke’s law you should always get a straight line graph • A straight line graph tells you that the two things on the graph are proportional

• As soon as the line goes straight you know the spring has become overstretched • This should not happen in your experiment

Drawing Graphs • A graph should always have the following 5 point checklist to get full marks 1. A title saying what the graph shows 2. A labelled vertical axis and a labelled horizontal axis 3. Units written next to the label 4. Correct points on the axes 5. Points with lines joining them