P2a (part 2)

Página 1

Work Done

When a force moves an object through a distance, energy is transferred and work is done

when something moves something else is providing energy to do so

The thing putting in energy needs s supply of energy (e.g electricity)

It does the work by moving an object

Energy is transferred

Whether energy is usefully transferred or wasted work is still done

Gravitational Potential Energy

measured in joules (j)

On Earth always have gravity acting on us

When we are above the Earth’s surface we have potential (stored) energy

gravitational potential energy

The amount of gravitational potential energy an object on Earth has depends on its mass and height above the ground

On earth gravity is 10 N/kg

Kinetic Energy

Energy of movement

All moving objects have it

Kinetic energy of an object has depends on its mass and speed

Measured in joules

Kinetic Energy Transferred

is work done

moving car has lots of kinetic energy

To slow down this energy needs t be converted to something else

To stop a car kinetic energy is transferred into heat energy as friction

Falling objects convert kinetic energy to potential energy

Te further it falls the faster it goes

Kinetic energy gained = potential energy lost

Power

A measure of how quickly work is being done

and so how quickly energy is being transferred.

Forces and Elasticity

Work done to an elastic object is stored as elastic potential energy

If you apply force you might cause an object to stretch and change shape

After the force has been removed the object can return to it's original shape

If it can do this it is an elastic object

Work is done to an elastic object to make it change shape

Energy is not lost, but stored as elastic potential energy

This potential energy is converted to kinetic when the force is removed

e.g. a spring bouncing back into position

Hooke's Law

When an elastic object - such as a spring - is stretched, the increased length is called its extension. The extension of an elastic object is directly proportional to the force applied to it:

F = k × e

F is the force in newtons, N

k is the 'spring constant' in newtons per metre, N/m

e is the extension in metres, m

This equation works as long as the elastic limit (the limit of proportionality) is not exceeded

If a spring is stretched too much, for example, it will not return to its original length when the load is removed

The spring constant The spring constant k is different for different objects and materials

It is found by carrying out an experiment

For example, the unloaded length of a spring is measured

Different numbers of slotted masses are added to the spring and its new length measured each time

The extension is the new length minus the unloaded length

Assuming the limit of proportionality (elastic limit) is not exceeded, a graph of force against extension produces a straight line that passes through the origin

The gradient of the line is the spring constant, k. The greater the value of k, the stiffer the spring

Calculating power Here is the equation which relates power to work done (or energy transferred) and time:

P is the power in watts, W

E is the work done (or energy transferred) in joules, J

t is the time taken in seconds, s.

For example, an electric drill transfers 3000 J in 15 s. What is its power?

Power = 3000 ÷ 15 = 200 W

Momentum

A moving object has momentum

The tendency of the object to keep moving in the same direction

It is difficult to change the direction of movement of an object with a lot of momentum

Measured in kg m/s

Direction

momentum doesn't just depend on the object’s mass and speed

Velocity is speed in a particular direction, so the momentum of an object also depends on the direction of travel

This means that the momentum of an object can change if:

The object speeds up or slows down

The object changes direction

Work and potential Energy

Kinetic Energy

Elasticity and Power

Momentum