1. Aim: How do we explain the concept of power?

2. Definition of Power
Power is the rate at which work is done. What are the units with which it is measured? Watts Is power a vector or a scalar quantity? Scalar

3. Power
P = W/t = dW/dt= Fd/t= Fv

4. Horsepower
1hp = 746 W

5. Power Examples
1) An older model car accelerates from 0 to speed v in 10 s. A newer, more powerful sports car accelerates from 0 to 2v in the same time. What is the ratio of powers for the two cars if they have the same mass?

7. Elevator

8. Power Examples
2) A 1,000 kg elevator carries a load of 800 kg. A constant friction force of 4000 N retards its motion upward.
What is the minimum power delivered the motor to lift the elevator at a constant speed of 3 m/s?
What power must the motor deliver at any instant if it is designed to provide an upward acceleration of 1 m/s2?
a)64.8 kW b) 2.34 x 104 v

10. 3. A 700 N Marine in basic training climbs a 10
3. A 700 N Marine in basic training climbs a 10.0 m vertical rope at a constant speed in 8.00s. What is his power output?
875 W

11. A motor of what power is required to perform the task?
4. A skier of mass 70.0 kg is pulled up a slope by a motor driven cable.
How much work is required to pull him a distance of 60.0 m up a 30 degree slope (assumed frictionless) at a constant speed of 2.00 m/s?
A motor of what power is required to perform the task?
a)20.6kJ b)686 W

12. Power Problem 4