What can you remember from P3 in Y11? Definition Definition Formula Formula Derived Units Derived Units Actual units Actual units

1.To understand how to successfully complete mechanical power problems 2.To apply these skills to the slightly more involved questions at AS Book Reference : Pages

Some further thoughts on work done from last lesson.... Two people each shifting boxes up a flight of stairs... One at a full sprint, while the other takes all day. Currently, through our view of work done, we would calculate the work done by each to be the same Clearly this conflicts with our everyday definition of “working hard”

Definition : Power is defined as the rate of energy transfer Formula : Power =  Energy  Time And when energy is transferred by a force doing work... Power = Work done time taken to do that work Units: Derived : Js -1 = called Watts (capital W)

Worked Example: A person with a mass of 48kg (480N) climbs a flight of stairs with a height of 10m in 12s Power = Work done time taken to do that work Power = 480N x 10m 12s Power = 400W

Looking at the power Equation again: Power = Work done time taken to do that work Power = Force x distance moved etc... t However, d/t is a very familiar concept..... Power = Force x velocity

Engines produce motive power. A powered vehicle can be found in different scenarios: 1.Moving at constant speed & height 2.Moving & gaining speed 3.Moving & gaining height No reason why we couldn’t mix and match!

Moving at constant speed & height All of the resistive forces, (friction, air resistance etc) are equal and opposite to the motive force. The work done by the engine is lost to the surroundings (heat, sound etc) Power Engine = Force Resistive x velocity

Moving & gaining speed The motive force from the engine exceeds the resistive forces. We have an unbalanced force and so we accelerate The work done by the engine is the sum of the energy lost to surrounding and the gain in kinetic energy due to the increase in speed Power Engine = Force Resistive x velocity + K.E gain per sec Note K.E. = ½mv 2 coming soon.....

Moving & gaining height If we are driving up an incline we are gaining height.... If we are gaining height we are gaining gravitational potential energy (GPE) The work done by the engine is the sum of the energy lost to the surroundings and the gain in gravitational potential energy Power Engine = Force Resistive x velocity + GPE gain per sec Note G.P.E. = mgh coming soon.....

Show that a juggernaut lorry with an output power of 264kW moving at a constant speed of 70 mph (31 m/s). What are the resistive forces experienced? Power Engine = Force Resistive x velocity Force Resistive = Power Engine / velocity Force Resistive = 264,000 / 31 Force Resistive = 8516N

Power is the rate of energy transfer and can be calculated using: Power = Work done time taken to do that work When powered motion is involved we can use: Power = Force x velocity This can be applied to scenarios with either constant level velocity or where there are gains in kinetic energy and/or potential energy due to increases in velocity and height respectively.