**Work **is done when a force moves the object in the direction of the force and is given by the product of the force and the distance moved in the direction.

- W = Fs , Where W= Work, F =
**Constant**Force (N), S = Distance moved in the direction of the force (m) - SI Unit for work is joules (J)
- One joule is defined as the work done when a force of 1N moves an object through a distance of 1m in the direction of the force

**NO** work is done when

- the object does not move
- the direction of the force and the direction in which the point of application moves are perpendicular to one another

**Note:**

This means that if you are carrying a stack of books and walking, you are NOT doing any work on the books! This is because the force exerted is not in the same direction as the motion!

**Energy **of a system is defined as its capacity to do work

- SI unit : joules (J)
- Scalar Quantity

Energy can be converted from one form to another. It can also be transferred from one body to another through work done and/or heat exchanges.

**The Principle of conservation of energy states that energy cannot be created nor destroyed in any process.**

- Total amount of energy of a closed system remains
**constant.** - E.g. A television converts electrical energy(electricity) into light, sound and thermal energies.

**Kinetic Energy**, $E_{k}$ is the energy a body possessed by virtue of its motion.

- $E_{k} = \frac{1}{2} m v^{2}$
- where m = Mass, v = Velocity

**Gravitational Potential Energy** is defined as the amount of work done in order to raise the body to the height *h* from a reference level.

- G.P.E.= mgh, where m = mass, g = acceleration due to gravity, h = height

**Power** is defined as the rate of work done or energy converted with respect to time.

- $P = \frac{W}{t}$ OR $P = \frac{E}{t}$, where W = work, t = time, E = Energy
- SI Unit for power is watt (W), scalar quantity
- Another useful equation for power: P = Fv, where F = force, v = velocity (Simple derivation below)

**Efficiency** of a system is given by

- Efficiency = (Useful energy output/total energy input) X 100%

**Simple derivation of P = Fv**

$P = \frac{W}{t}$

$P = \frac{F \times d}{t}$

$P = Fv$, where $v = \frac{d}{t}$

Mark Chiu scribbled

No work is done when object is not accurate for the definition. Object can be moving in constant velocity (net force=0) while work done is 0 as well.

Ashiq Hussain scribbled

this is nice conversation. i need more detailed derivation for this P=FV function.

regards

James scribbled

I dont get it. Why arent you doing any work when you are walking? I thought you are going against friction and air resistance?

Mini Physics scribbled

I slightly modified the sentence above:

From:

This means that if you are carrying a stack of books and walking, you are NOT doing any work! This is because the force exerted is not in the same direction as the motion!

To:

This means that if you are carrying a stack of books and walking, you are NOT doing any work on the books! This is because the force exerted is not in the same direction as the motion!

Will this be better?

Note:Usually, scenarios given are idealized. We take air resistance and friction to be negligible for the computation of work. Furthermore, the given scenario assumes that the books are staying at exactly same height throughout the walk. It is obvious that this cannot be true for a real life scenario.James scribbled

Oh alright thanks.

Aidan scribbled

Should it not be “This is because the force exerted is not in the same direction as the motion.” rather than “This is because the force exerted is not in the same direction as the force.”?

Mini Physics scribbled

You are right. I’ve corrected the relevant statements.

Thanks for pointing it out. 🙂