CCEA Unit 1: Physics 1

P1.2 Energy  Work
1.2.7 recall and use the equation; work = force x distance and that work done equals the amount of energy transferred In Physics, work has a very precisely defined meaning. Mechanical work is done whenever anything is moved against a force or resistance i.e., there must be movement, and the movement must be against some resistance. WORK DONE = FORCE x DISTANCE The unit of work is the Joule (J) and is the work done when a force of 1 Newton (N) moves through a distance of 1 metre (m). E.g. If you have to pull with a force of 50N to move a box steadily 3m in the direction of the force. The work done is 50N x 3m = 150Nm = 150J i.e. JOULES = NEWTONS X METRES Energy is the ability to do work. The amount of work that is done tells us how much energy has been converted from one form to another. WORK DONE = ENERGY CHANGED Large amounts of work may be measured in kilojoules or even megajoules. i.e. 1000J = 1kJ 2000000J = 2MJ Examples 1. Calculate the work done by John when he pushes a brokendown car with a force of 200N, for 1km. 2. A man lifts a brick of mass 5kg from the floor to a shelf 2 metres high. How much work is done? P1.2 Energy  Power We can understand the power of a machine by considering a crane lifting cargo onto a ship. Suppose the crane is lifting a weight of 50000N through a height of 10m onto a ship. We can easily calculate the work done by the crane in lifting the cargo: It is all very well to know this, but the crane would be useless if it took several hours to lift the load onto the ship. In order to appreciate the usefulness of the crane, we need to know how quickly it can do the work of lifting the cargo. Suppose the crane lifts the cargo in 5 seconds: Work done by the crane per second = The work done by the crane per second is known as its power. Power is the rate of working. POWER = WORK DONE /TIME TAKEN POWER = ENERGY CONVERTED/TIME TAKEN Power can be expressed in joules per second, but we usually use the word Watt (W) in place of joules per second. Examples
