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

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

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 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