Given below are the Class 9 Science Work And energy Worksheet
a. Concepts questions
b. Calculation problems
c. Multiple choice questions
d. Long answer questions
e. Fill in the blank's
Question 1. What do you understand by transformation of energy? Explain with the help of suitable example? Answer
Transformation of energy is the conversion or changing of energy from one form to another. Example
a.the ball is dropped from edge of cliff. Here the Potential energy of the ball is converted into Kinetic energy
b. In electric motor, electrical energy is converted into mechanical energy
Question 2. List two essential conditions for work to be done? Answer
An object having capability to do work is said to possess energy.
Energy has the same unit as that of work
Question 4. Define work? Write an expression of work in terms of force and displacement. State SI unit of work done? Answer
Work done on an object is defined as the magnitude of the force multiplied by the distance moved by the object in the
direction of the applied force.
$W=F \times d$
The SI unit of work is joule:
$1 \; joule = 1 \; newton \times 1 \; metre$
Question 5. Compare the kinetic energies of two objects of masses 10kg anf 50kg respectively but having same momentum? Answer
Let $v_1$ and $v_2$ be the velocities of 10 kg and 50 kg mass
As the momentum is same
$10v_1 = 50v_2$
or
$\frac {v_1}{v_2} = 5$
Now
$K_1= \frac {1}{2} \times 10 \times v_1^2$
$K_2= \frac {1}{2} \times 50 \times v_2^2$
$K_1 : K_2 = \frac {1}{5} \times \frac {v_1^2}{v_2^2} = 5$
Question 6. What is a work done on a body moving in a circular path? Answer
Zero as Force acting is perpendicular to displacement
Question 7. Identify the state the type of energy transformation in the following cases:
a.Riding a bicycle
b.Burning of cracker Answer
a. The muscular energy of the cyclist is converted into rotational energy of the pedals of the bicycle which is transferred to its wheels. The kinetic energy of the rotation of the wheels is converted into the kinetic energy of the bicycle.
b. Chemical energy to heat, light, and sound energy.
Question 8. Define potential energy? Write an expression for potential energy. Write the SI unit of potential energy? Answer
The energy possessed by a body due to its change in position or shape is called the potential energy.
The gravitational potential energy of an object of mass, m raised through a height, h from the earth�s surface is given by
$PE= mgh$
SI unit of Potential energy is Joule
Question 9. What is work done by the force of gravity on a satellite moving round the earth? Justify the answer? Answer
work done by the force of gravity on a satellite moving round the earth is zero as force of gravity acts at right angle(90°) to the direction of the motion of the satellite.So there is no displacement in the direction of force
Question 10. Define kinetic energy? Write an expression for kinetic energy? Write the SI unit of kinetic energy? Answer
An object in motion possesses what is known as the kinetic energy of the object. An object of mass, m moving with velocity v has a kinetic energy given by
$K= \frac {1}{2} mv^2$
SI unit is Joule
Question 11. Define 1 Watt of power? A lamp consumes 1000J of electrical energy in 10s. Calculate its power? Answer
1 watt is the power of an agent, which does work at the rate of 1 joule per second
1 W= 1 Joule/s
Power = Energy /time = 1000/10 =100 W
Question 12. Name the two types of mechanical energy? Give two examples when an object possesses both types of energies? Answer
Potential energy and Kinetic energy.
a. A body in free fall possess both the Kinetic energy and potential energy
Question 13. Calculate the amount of work required to stop a car of 1000kg moving with a speed of 72km/h? Answer
Question 14. Does the gravitational potential energy of a body depend on the path along which the body is moved, while going from one point to another? Illustrate your answer with the help of suitable example? Answer
Gravitational potential energy does not depend on the path along which the body is moved, while going from one point to another
Consider the below figure
A block is moved from position A to B by two path
a. AB : Work done againts gravity = mgH
b. ACDB :
Workdone on moving the block A to C = zero ( As gravity is perpendicular to motion)
Workdone on moving from A to D = mgH
Workdone on moving the block D to B = zero ( As gravity is perpendicular to motion)
Therefore, total workdone=mgH
Hence Gravitational potential energy does not depend on the path along which the body is moved, while going from one point to another
Question 15. An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line? What is the work done by the force of gravity on the object? Answer
Since the initial and final point lies on the same horizontal line, work done by the force of gravity on the object will be zero
Question 16. State the kind of energy transformation that takes place in the following devices:
a. Solar battery
b.Electric motor
c.Microphone
d.Thermal power plant
e.Hydroelectric power plant Answer
a. solar energy to electric energy
b. electrical energy to mechanical energy
c. Electric energy to sound energy
d. Chemical energy to heat energy and then heat energy to mechanical energy and which is then finally converted into electrical energy
e. Potential energy to kinetic energy and then electrical energy
Question 17. Can a body have energy without momentum? Explain? Answer
Yes. Body can still have energy in the form Potential energy
Question 18. Can a body have momentum without having energy? Explain? Answer
No. Since mechanical energy is zero, there is no potential energy and no kinetic energy. Kinetic energy being zero, velocity is zero. Hence, there will be no momentum.
Question 19. Define potential energy? Give SI unit? State two factors on which the magnitude of potential energy depends. Give one example each for the change in potential energy due to:
a.position
b.configuration of the body? Answer
The energy possessed by a body due to its change in position or shape is called the potential energy. SI unit is Joule
The gravitational potential energy of an object of mass, m raised through a height h from the earth�s surface is given by m g h. So Potential energy depends on mass and height
a. When height is changed, potential energy is changed
b. if mass is changed,potential energy is changed
Question 20. Calculate the kinetic energy of a car of mass 750kg moving with a velocity of 54km/. Find the new kinetic energy of a car if passenger of mass 50kg sit in the car? Answer
When passenger of mass 50kg sit in the car, Total mass becomes =750 +50=800 kg
$K= \frac {1}{2} \times 800 \times (15)^2= 90,000 J$
Question 21. Establish a relationship between momentum and kinetic energy of a body? Answer
$p=mv$ or $v= \frac {p}{m}$ -(1)
$K=\frac {1}{2} mv^2$ -(2)
Substituting the value v from (1) in (2)
$K= \frac {1}{2} m (\frac {p}{m})^2$
$K=\frac {p^2}{2m}$
Question 22. Calculate the kinetic energy of a car of mass 500kg moving with a velocity of 18Km/h. Find the new kinetic energy if two passengers of mass 50 kg each sit in the car? Answer
When two passenger of mass 50kg sit in the car, Total mass becomes =500 +2*50=600 kg
$K= \frac {1}{2} \times 600 \times (5)^2= 7500 J$
Question 23. Derive an expression for the kinetic energy of an object?
Question 24. A man whose mass is 50 kg climbs up 30 steps of a stair in 30 s. If each step is 20 cm high, calculate the power used in climbing the stairs. [Given g = 10 m/s2] Answer
Height climbed =30 * 20 =600 cm =6 m
Weight =50*10=500 N
