7.1	Work, Energy and Power

 

Energy:

• Energy is defined as the ability to do work

• Energy exists in many forms such as potential energy

• Energy can change during work and energy cannot be created or destroyed because energy is always constant

Work:

• Work is defined as the product of the force applied by the displacement in the direction of the force

• Formula: Work (J) = Force (N) x Displacement (m)

• SI units: joule, J and 1 Joule = 1 Newton

• The work done is equal to the quantity of energy transferred

Example question:

Mazlan uses a force of 30 N to push a box 5 m away. Calculate the work done by Mazlan.

Answer:

\(\begin{aligned} \text{Work (J)}&=\text{force (N)}\times\\&\quad \text{ displacement (m)}\\\\&=30\text{ N} \times 5\text{ m}\\\\&=150\text{ J} \end{aligned}\)

 Power:

• Power is defined as the rate of doing work

• Formula:

 \(\begin{aligned} \text{Power(W)} &=\dfrac{\text{ Work done(J)}} { \text{Time (s)}}\\\\                                &=\text{ Force (N)} \times\\&\quad \dfrac{\text{ displacement (m)}} {\text{Time (s)}} \end{aligned}\)

• SI units: watts, W, and 1 Watt = 1 Joule

• The rate of doing the work

Example question:

The engine of a car generates 1 800 W of power, the car travels 20 m in 30 seconds. Calculate the force produced by the engine of the car.

Answer:

\(\begin{aligned} &\text{Power (W)}&\\&=\dfrac{\text{force (N)}\times \text {displacement (m)}}{\text{Time (s)}}\\\\1\,800 \text{ W}&=\dfrac{\text{force (N)}\times 20 \text{ m}}{30\text{ s}}\\\\\therefore \text{force}&=\dfrac{1\,800\times 30}{20}\\\\&=2\,700\text{ N}. \end{aligned}\)

Question:

When Prakash pushes a box with a force of 20 N as ar as 10 m, work has been done by Prakash. 

a) Give a definition of work.

b) Determine the work done by Prakash

c) If Prakash uses 5 seconds to push the box, calculate the power generated by him.

Answer:

a)	Work is the product of force multiplied by displacement in the direction of force.
b)	\(\begin{aligned} \text{Work}&=\text{force}\times \text{displacement}\\\\&=20\text{ N}\times 10\text{ m}\\\\&=200\text{ J}. \end{aligned}\)
c)	\(\begin{aligned} \text{Power}&=\dfrac{\text{work}}{\text{time}}\\\\&=\dfrac{200\text{ J}}{5\text{ s}}\\\\&=40\text{ W}. \end{aligned}\)