Ch-06: Work and Energy

Ch-06: Work and Energy

This post contains Physics Chapter 6 Notes Class 9 and Short Questions for the students. Name of this Chapter is "Work and Energy". Here you will find some solved Short Questions with Answers. Learn these question answers and related stuff for exam preparation.

Q.1: Why fossil fuels are called the non-renewable form of energy?

Ans. The fossils fuels are one used to get energy and consumed completely. So there are not renewable.

Q.2: A stone of mass 500g strikes the ground with a velocity of 20 mS-¹. How much is the kinetic energy of the stone at the time it strikes the ground?

Ans.
m = 500 g = 0.5 kg
v = 20 ms-¹
Since                K.E = 1/2 mv²
K.E = 1/2 x 0.5 kg x (20 ms-¹)²
= 1/2  x 0.5 kg x 400 m² s-²
= 100 J

Q.3: Explain mass-energy equation.

Ans. Equation:  E = m c²
Here E is energy, m is mass of the object while c is the speed of light (3 x 10⁸ m/s)

Q.4: Write the names of any four forms of energy.

Ans.

1. Mechanical Energy
2. Heat Energy
3. Electrical Energy
4. Sound Energy

Q.5: Define potential energy and write its equation also.

Ans. The energy possessed by a body due to its position is known as its potential energy.

Equation: P.E = mg h

Q.6: What is the unit of work, define it?

Ans. SI unit of work is joule (J).

Q.7: Define kinetic energy also write its equation.

Ans. The energy possessed by a body due to its motion is called its kinetic energy.

Equation: K.E = 1/2 mv²

Q.8: Define Efficiency.

Ans. The efficiency of a system is the ratio of the required form of energy obtained from a system as output to the total energy given to it as input.

Q.9: What is the SI unit of Power? Define it.

Ans. SI unit of power is watt (W). The power of a body is one watt if it does work at the rate of 1 joule per second ( 1 Js-¹).

Q.10: Write down the equation of Einstein and the value of C.

Ans. Einstein predicted the inter-conversion of matter and energy E = mc². The value of c is the speed of light (3 x 10⁸ ms-¹⁰)

Q.11: Differentiate between sound energy and mechanical energy.

Ans.
Sound Energy: When you knock at the door, you produce sound. A sound is a form of energy. It is produced when a body vibrates.

Mechanical Energy: The energy possessed by a body both due to its motion or position is called mechanical energy.

Q.12: Define work and write its unit.

Ans. Work is done when a force acting on a body displaces it in the direction of a force.

Unit: The unit of work is Joul (J)

Q.13: Who predicted the mass-energy equation? Also, write its equation.

Ans. The relation between mass m and energy E is given by Einstein’s mass-energy equation.

Equation:  E = m c²

Q.14: Define the power and write its unit.

Ans. Power is defined as the rate of doing work.

Unit: Unit of power is watt (W).

Q.15: When does a force work?

Ans. When the force acting on a body displaces it. Then work is said to be done.

Q.16: Differentiate between electrical energy and light energy.

Ans.
Electrical Energy: Electricity is one of the widely used forms of energy. We use electrical energy from batteries etc.

Light Energy: Light is an important form of energy. Most of the light comes from the sun.

Q.17: The kinetic energy of a body of mass 2 kg is 25 J. Find its speed.

Ans. 
K.E = 25 J
m = 2 Kg
Formula      K.E = 1/2 mV²
25 J = 1/2 (2 Kg) V²
25 = V²
V = 5 mS-¹

Q.18: Write two uses of wind energy.

Ans.
(i) Wind power is used to turn wind turbines.
(ii) In the United States, some wind farms generate more than 1300 MW of electricity a day.

Q.19: What will be the torque If a force of 150 N is applied on a spanner of 10cm?

Ans. 
τ = F x L
τ = 100 x 0.1
τ = 10 Nm