Pressure, Work, Energy and Power

Time for some more equations:

Pressure (Pa)
Force (N)
Area (m2)

Work = Force x Distance moved in the direction of the force

Work (J)
Force (N)
Distance moved (m)

Where Work Done = Energy Transferred
Note that 1 Joule = 1 Nm


Energy

Kinetic Energy = ½ x mass x velocity2

Kinetic Energy (J)
Mass (kg)
Velocity (ms-1)

Change in Gravitational Potential Energy = mass x gravitational field strength x change in height

Gravitational field strength = 9.81 NKg-1
Change in Gravitational Potential Energy (J)

Note that often questions rely on the principal that
Change Kinetic Energy = Change in Gravitational Potential Energy

 


Questions involving kinetic energy being transferred to gravitational potential energy

Example
A helicopter of mass 800kg at a height of 1500m loses power and begins to fall; assuming the speed of the helicopter at the top of its flight is zero what is its speed on impact. Assume air resistance is negligible and the gravitational field strength is 9.81.

Ep = mgh
Ep = 800 x 9.81 x 1500
Ep = 11772000 J

Ek = 11772000
11772000 = ½ x 800 x v2
v = 171.55 ms-1


Power

Power (W or Js-1)

Definition
Joule – The amount of work done when a force of one Newton moves a distance of one meter in the direction of the force.