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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? • 1kg has a force of 10N Force Quantities in every day life A Newton is a very small unit •a. 1N to lift an Apple •b. 12N to lift your textbook •c. 20N to open a can of Coke •d. 50N to squash an egg •e. 100N or more to push a supermarket trolley •f. Car brakes can exert a force of about 5000N 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 300N to push the lawn mower 20m how much works does he do? • Answer: Work (j) = Force (N) x Distance (m) Work (j) = 300N x 20m =6000 joules Questions • Who does more work a woman carrying a case with a force of 30N for a distance of 100m or a woman carrying a case with a force of 40N for a distance of 75m? 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 =1j/s • If something has a power of 20watts it can do 20j of work in 1 second • Why do you think a 100w bulb is brighter than a 60w 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 20m 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 500N runs up a flight of stairs 6m high in 20s what is her power? • A body builder lifts a weight of 6000N 1m above his head in 1s what is his power? Remember • To change centimeters to meters divide by 100 • Eg. 10cm = 0.1m • To change grams to kilograms divide by 1000 • Eg. 200g = 0.2kg • To change kg to Newtons multiply by 10 • Eg. 60kg = 600N 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 50g to a spring it will stretch a certain amount and if you add another 50g 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