P3d(i) Work and Power You will find out about What work is How to calculate work The energy needed to do work

Work When a Force moves Work is done. Pulling, Pushing and Lifting are all examples of Work. Work depends on two things: The Size of the Force acting on the object (N) And The Distance the object has moved (m) The equation to calculate work is: Work Done = Force x Distance The unit is Joule (J) The Forces moved here are: The Barbell The Box The Weight of the Woman Example: Louise pushes a 7kg pram 1,500m. How much Work has she done? Work Done = Force x Distance Work done = 7kg x 1,500m Work Done = 10,500J REMEMBER: Energy is transferred when work is done. Your Work and your Energy are important. They are precious… like precious Jewels. The unit for Work AND Energy is the Joule!

Difference between Mass and Weight Mass is an amount of substance. It is measured in kilograms (kg). The mass tells you the amount of particles. Weight is the force of attraction on a mass due to gravity. It is measured in Newtons (N). Recall that Force = mass x acceleration When free-falling, acceleration is the same as the planet’s gravitational field strength (g). So g is substituted for a. On earth, g has a value of 10N/kg. As above the force of attraction on a mass due to GRAVITY is called weight (W) so the Force is the Weight. So W is substituted for F. F = m a F = m g W = m g Example: A man has a mass of 80kg. What is his weight? W = m g W = 80 x 10 W = 800N Misconception: A lady stands on her bathroom scales and says she weighs 45kg. This is NOT right. Her MASS is 45kg. Her Weight is actually 450N. So many people get this wrong which is why it can be confusing… For every 1kg, earth’s gravity pulls it inwards with 10N. So 100kg is pulled inwards by 1000N.

More about Work Work is only done if a Force moves an object. If the object does not move then no Work is being done. So you may think you are working hard when studying but because you are not moving a great distance in fact very little work is actually being done. It is the same for a kickboxer kicking a punch-bag – if the punch-bag does not move then no work is being done no matter how hard she looks like she is working! Work INCREASES when: The size of Force increases Or The distance the object moves increases (moved in the direction of the Force) A high-jumper does Work in raising their weight off the track. Wayne Rooney standing about and not moving… well no Work being done here!

Gravity on the Moon? If the Earth’s gravitational field strength is 10N/kg and the Moon’s is 1.6N/kg this means that the Moon’s gravitational pull is about 6 times less than Earth’s. That’s why the astronauts seemed to float when they walked on the Moon…

Questions 1.An Olympic weightlifter lifts 200kg above his head at a height of 1.8m. How much Work did he do? 2.Malcolm has a weight of 725N. What is his mass? 3.Emily does 12,000J of work pushing her car a distance of 5m. How big was the friction force she had to push against? 4.If a planet’s gravitational field strength is 16N/kg how much would a 60kg person weigh? 5.A bus has 650,000J of kinetic energy and it stops in 60m once the brakes have been pressed. What is the braking force?

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