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# Work, energy and power CBSE Class 9

Work, energy and power
CONTENT LIST

## Work

When the force applied on an object produces a displacement of the object in the direction of force applied then Work is said to be done.

Work done on an object is calculated by multiplying the magnitude of the force and the distance moved by the object in the direction of the applied force (displacement).

Work done = force × distance in direction of force (displacement)

W= F × s

Necessary Conditions for Work to be done are:

1. Force should act on the object.
2. Object must be move in direction of force.

Unit of work:

1 Joule = 1 Newton × 1 metre.

The SI unit of work is joule also called Newton metre (Nm).

1 Joule is defined as the amount of work done by force of 1 N when it produce a displacement of 1 m in object.

Work is scaler phycical quantity.

How to calculate the Work done

When the body moves a distance s at an angle θ to the direction of force.

The work done on a body is given by:(W)=F⋅scosθ

When θ=0° means both the force and the displacement are in the same direction, positive work is done.

W = Fscosθ=Fscos0°=Fs

When θ=90° means force acts in a direction perpendicular to the direction of displacement, the work done is negative.

W= Fscosθ=Fscos90°=Fs (0)=0

When θ=180° means force acts in a direction opposite to the direction of displacement, the work done is negative.

W= Fscosθ=Fscos180°=Fs (-1)=-Fs

## Energy

In physics, the capacity for doing work is called energy.

It is considered a quantitative property that can be transferred from an object to perform work.

Forms of Energy

Energy exists in many different forms
1. Light Energy
2. Chemical energy
3. Electrical Energy
4. Mechanical Energy
5. Thermal energy
6. Nuclear energy
7. Gravitational Energy

### Kinetic Energy

The energy possesses by an object due to its motion is known as kinetic energy.

If an object of mass (m) is moving with velocity (v) then kinetic energy (K.E) of object = 1/2.mv²

Kinetc energy can't not be negative.

### Potential energy

The energy possesses by an object due to its position is called potential energy.

If an object of mass (m) is placed at a height (h) from ground then potential energy (P.E) of object = mgh (g is the acceleration due to gravity)

### Potential Energy

Potential energy can be negative. For example, potential energy at any point above ground level is positive Because a negative amount of work is done to bring an object close to the Earth.

## Potential Energy at a height

When work is done for raising the height of an object, energy is transferred as a gain in the gravitational potential energy of the object. The gravitational potential energy Ep of an object of mass 'm' at a height 'h' is given by: Ep= mgh.

## Law of Conservation of Energy

The law of conservation of energy says that energy can neither be created nor destroyed - only converted from one form of energy to another.

This means that any system always has the same amount of energy, unless it's added from the outside.

## Power

The rate of doing work is called as power. In other words, rate at which energy is consumed. SI unit of power is watt.

Power=Workdone/Time

Power = force X speed (velocity);
P = F × v

Power is how fast energy is used. Car engines do work, which allow the car to move down the road. A faster car can do more work,due to higher the horsepower. Higher horsepower vehicles can accelerate the car very quickly.

The unit for pwer is measured in Joule per second. A joule per second isknown as a watt (or W), which is the unit for power.

## Commercial Unit of Energy

Kilowatt-hour (kWh) is a commercial unit of energy. A kilowatt-hour is equal to an amount of energy consumed by a machine working at a steady rate of one kilo watt for one hour.

## FAQs

Q1:Define work ?

Answer: Work is the displacement of an object when a force (push or pull) is applied to it.

Q2:What is SI unit of work?

Q3: Define 1  joule?

Answer: if a force of 1N displaces an object for displacement of 1m in direction of applied force then workdone by the force is 1 joule.

Q4: What is the kinetic energy of an object?

Answer: The energy possessed by a body by virtue of its motion is called kinetic energy

Q5: What is  Work done on a body moving in circular path .

Q6:What is computation formula  of work done by a constant force F acting on body and undergoes a displacement of d .

Q7:  When force acting on the body is perpendicular to the direction of the displacement of the body. What is the work done  ?

Q8:What is called ability to do work ?

Q9: What is called the energy possessed by a body due to its position ?

Q10:   A body of mass m moving with a speed v has kinetic energy, Ek. The body  if moves with speed 2v, what will have kinetic energy ?

Q11: A body of mass m moving with a speed v has kinetic energy Ek. A body of mass 2m moving with the same speed will have a kinetic energy  ?

Q12: A ball of mass 0.5 kg has100 J of kinetic energy. What is the velocity of the ball ?

Q13:   Two bodies of equal mass move with uniform velocities u and 4 u respectively. Find the ratio of their kinetic energies ?

Q14:   Kamya climbs up a staircase in 5 minutes, Suraiya takes only 3 minutes in going up the same staircase. The weight of Kamya is equal to the weight of Suraiya. Which of the two does more work? ?

Answer: Both has done equal work

Q15:   Kamya climbs up a staircase in 5 minutes, Suraiya takes only 3 minutes in going up the same staircase. The weight of Kamya is equal to the weight of Suraiya.Which of the two does  have more power ?

Answer: Suraiya has more power .

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