9th Sci. Exercise, chapter 2. Work and Energy 

1. Fill in the Blanks

• The unit of work in the CGS system is ..... .
• 1 joule = ..... ergs.
• When the angle between applied force and displacement is 90°, the work done is .... .
• The capacity of a body to perform work is called its .... .
• 1 horsepower = ... Watts.
• 1 J = 1 N × .... . 
• A commercial unit of electrical energy is 1 kWh = ...... J.
• 1 ... = 1 dyne × 1 cm. 

2. Answer in One Sentence

• What is potential energy?
• State the law of conservation of energy.
• Define power.
• Define 1 Joule
• When is work said to be negative?

3. True or False (Rewrite false statements correctly)

• The kinetic energy of an object depends only on its mass.
• Work done by gravitational force on a satellite moving in a circular orbit around the earth is zero.
• In the CGS system, the unit of power is the watt.
• 1 unit of domestic electricity is equal to 1 kWh.
•  The units of work and energy are the same.

4. Numerical Problems (Practice Examples)

• A machine performs 1200 J of work in 30 seconds. Calculate its power.
• A metal block of mass 10 kg is raised to a height of 8 m. Calculate the potential energy stored in it. (g = 9.8 m/s²)
• A body of mass 500 g is moving with a velocity of 4 m/s. What is its kinetic energy?
• An electric heater of 1000 W is used for 4 hours daily. How many units of electricity does it consume in 30 days?

5. Detailed Answers

• Derive the expression for the kinetic energy of an object of mass m moving with velocity v.
• Differentiate between positive, negative, and zero work with suitable examples.
• Draw a figure of the transformation of energy

6. Write down difference between Kinetic energy and potential energy.

7. Why is the work done on an object moving with uniform circular motion zero?