# Energy, Work and Power

Objectives:

• To define energy, work and power
• To describe the relationship between work and energy
• To display an ability to calculate work done by a force
• To differentiate between work and power and correctly calculate power used.

Energy

Energy is the ability or capacity to do work. Energy is a scalar* quantity (a number) that we associatewith a system of objects, e.g., planets orbiting a sun, massesattached to springs, electrons bound to nuclei, etc. It turns out that energy possesses a fundamental characteristic which makes it very useful for solving problems. It is measured in joules

Forms of energy

chemical, nuclear, thermal, electrostatic, solar, mechanical, heat, etc

Types of energy

Kinetic energy Kis energy associated with the state of motion of an object. The faster an object moves, the greater its kinetic energy.

Kinetic energy = 1/2mv 2

Where m is the mass in kg and v is velocity in m/s

Work-Kinetic Energy Theorem

Change in the kinetic of a particle = net work done on the particle

Potential energy Urepresents stored energy, e.g., in a spring. It can be released later as kinetic energy.

Potential energy= mgh

Where m is the mass in kg, g is the acceleration due to gravity and h is the height distance

Work

Work Wis the energy transferred to or from an object by means of force acting on the object. It is the product of force and distance.  Energy transferred to the object is positive work, and energy transferred from the object is negative work.

Sample Problem 1

A 105g hockey puck is sliding across the ice. A player exerts a constant 4.5N force over a  distance of 0.15m. How much work does the player do on the puck?

Solution

Force =4.5N

Distance =0.15m

work=?

Work done =force x distance

= 4.5 x 0.15 =0.678joules

Power

Power is a measure of how much work is done in a period of time – the rate of doing work. The longer it takes to do work, the less power. Power is designated by the letter P. The equation for power is;          P =W/t.

The unit of measuring power is watt (W). One watt is 1J of energy transferred in one second. A glass of water weighs about 2N, and if you move it 5 m to your mouth, then you have done 1J of work. If it takes 1 second to move the glass then you are working at the rate of 1W. Since the watt is such a small unit, power is more often expressed in kilowatts (1000W).

Sample Problem 2

An electric motor lifts an elevator 9 m in 15.0s by exerting an upward force of 1.20 x 104N. What power does the motor produce in watts and kilowatts?

Solution

Force=1.20 x 104N

Distance=9m

Time=15 sec

Power=?

Using the definitions of work (W = Fd) and power (P = W/t) we can solve separately or

combine the equations to solve at one time (P = Fd/t)

P = Fd/t = (1.20 x 104N)(9.00m)/15.0s = 7200W or 7.2kW

Energy and Work

If you have ever moved, you know what work it is to pick up a box and move it to another location. Sliding it across the floor is not much better due to friction therefore; energy is expended or conserved when work is done.

Lesson activity:

1. Reflect on the definitions of energy, work and power
2. Give the standard units of  energy, work and power
3. Do you think a relationship exist between energy and work? If yes discuss this relationship