Module 3 - Section 3 - Work and Power Flashcards
Work is done whenever ….
Energy is transferred
What’s another way of saying energy transferred ?
Work done
What does ‘work’ mean ? and what’s it measured in ?
It means the amount of energy transferred from one form to another when a force causes a movement of some sort - its measured in joules ( J )
What happens when you move something ?
explain in detail
Usually you need a force to move something as you need to overcome another force - for e.g. if you are lifting a box you need to give the box a force ( resultant force ) so that it can overcome the force of gravity so work is done against gravity
The thing being moved then gets kinetic energy while its moving
Then the kinetic energy gets transferred to another energy store once the object stops moving - for e.g. the boxes kinetic energy is transferred to the gravitational potential energy store
What happens when a chair gets pushed on a level floor ?
energy changes etc
When a chair gets pushed it gains kinetic energy and it does work against friction - then when the chair stops moving - the energy gets transferred from kinetic energy to heat because of the friction
What happens when a spring gets stretched ( energy changes etc ) ?
When you stretch a spring you put kinetic energy into the spring as the spring moves and this means work is done against the stiffness of the spring and then the energy gets transferred to the elastic potential energy of the spring
What’s the equation for work done ?
Work done = Force causing motion x Distance moved
W = Fx
Why is work done not necessarily regarded as the total energy ?
This is because work done is the energy transferred from one form to another and not the total energy because for e.g. if a book has been moved from the lowest shelf to the top shelf - the gravitational potential energy will increase but the book still had some potential energy to begin with - this means that the work done is just the increase in potential energy not the total potential energy
Remember that distance has to be converted to …
metres
What does the equation assume ?
the equation assumes that the direction of the force is the same as the direction of movement
Whats the definition of the joule ?
The definition of the joule is derived from the equation W = F x d
One joule is the work done when a force of 1 newton moves an object through a distance of 1 metre
What do you do if the direction on the force isn’t the same as the direction of movement ?
If the force direction isn’t the same as the direction of movement then you need to consider the horizontal and vertical components of the force
For example if there is a question where there is a sled and a person is pulling it by a rope - the direction of the force is the direction at which the person is pulling the rope at however the sled would be moving at another direction - just forward at no angle however the person is pulling the rope at an angle so therefore the direction of the force is different
So you resolve the force - the vertical component of the force isn’t causing any motion and so its not doing any work and therefore we don’t have to work it out - the only movement is in the horizontal direction so we resolve that vector - the only thing the vertical component is doing is balancing out some weight meaning that there’s a smaller reaction force
So you have to resolve the horizontal component to find out the work done - this is the only force you need to consider - so all you do is find the force out and then x it by the distance
What’s the equation if the force direction is not the same as the direction of motion ?
W = Fx Cos theta
So W = work done
F = force
Cos theta is the angle between the direction of the force and the direction of motion
What is power ? And what’s the equation ?
Power is the rate of doing work - so its the work done per second
In other words its the amount of energy transferred from one form to another form per second
The equation for power is:
P = W/t
Power measured in watts, work done measured in joules and time measured in seconds
OR for electrical appliances:
P = VI
Power = voltage x current
What does one watt defined as ?
The watt is defined as a rate of energy transfer equal to 1 joule per second
One watt is equal to one joule per second
