Pressure, Work, Energy and Power

Time for some more equations:

Pressure =  Force
                    Area                                                                                

Pressure (Pa)
Force (N)
Area (m²)

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

Work (J)

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
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 negligible what is its speed on impact. Assume air resistance is negligible and the gravitational field strength is 9.81.

E_p = mgΔh
E_p = 800 x 9.81 x 1500
E_p = 11772000 J

E_k = 11772000
11772000 = ½ x 800 x v²
v = 171.55 ms^-1

 

Power

 Power = Work Done
               Time Taken                                                              

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.