1. When do we say that work is done?
Ans. When a force changes the position or produces motion in a body, we can say work is done.
2. Give the formula for calculating work done. What is the S.I unit of work?
Ans. Work done = Force (N) x Displacement (m) and S.I unit is Joule ( Nm)
Ans. Work is zero when there is no displacement after applying force on a body.
No, it has no direction, it has only magnitude so it is a scalar quantity
4. Define 1 joule of work.
Ans. One joule of work is done when a force of 1 N displaces the object by 1 m.
Ans. Expression for work done ; Work = Force x Distance
Ans. No work is done along the direction of motion if the force is perpendicular to the direction, work done = F x S Cos ∅
If ∅ = 90, then Cos 90 = 0
Therefore Work done = 0
Ans. The work done by the force when the displacement is made an angle with force W = F x S Cos ∅
Ans. Sum of potential energy and Kinetic energy is called mechanical energy.
Mechanical energy = P.E + K.E
Ans. - It is possessed by a body of its position. Example: A book kept on a table , when a stone kept at a high, has potential energy. Potential energy is denoted by P.E.
P.E = mgh, Where m is mass of the object, g is the acceleration due to gravity and h is the height of the the object from the ground.
Ans. Elastic energy is the stored energy of an elastic object which is used to get original shapes, when objects are stretched, compressed, or deformed.
Ans. 1 horsepower equals to 746 watt.
Ans. The commercial unit of energy is kilowatt hour (kWh).
One kilowatt hour is the amount of electrical energy consumed when an electrical appliance with a power rating of 1 kilowatt is used for 1 hour.
1 kWh is equal to 3.6 × 10^6 J of energy.
1 kilowatt hour of electrical energy is known as 1 unit.
Ans. Energy can neither be created nor destroyed, it can only be converted from one form to the other.
Example: Electric energy to light energy, electric energy to sound energy, light energy to electric-
Ans. Relation between work and energy - When a force changes the position or produces motion in a body, we can say work is done and ability to do work is called energy.
Ans. We know that potential energy = mgh, Therefore potential energy is directly proportional to the height. So the potential energy of a body becomes double when its height is doubled.
Ans. We know that Kinetic energy = 1/2 mv^2
If the speed of a body is halved, then K.E = 1/2 m (v/2)^2= 1/2 x mv^2 x 1/4 = 1/8 mv^2.
K.E becomes one eighth.
Ans. A running car possess mechanical energy due to kinetic energy and potential energy.
Ans. Commercial units of electrical energy in Joules 3.6 × 10^6 J of energy.
II. Short Answer Type Question:
1. Work and energy are related to each other. Explain the given statement.
Ans. Work is done when force is applied to move an object.
For example: when we kick the paddle of a bicycle, we exert an external force , causing it to move. This force is the transformation of energy and we can say that the energy is the capacity to do work.
Energy can be transferred in the form of force to an object to help it move, and this phenomenon is known as work or work done.
In this way work and energy are related to each other.
2. If the angle between displacement and force acting on a particles is ∅. For which value of ∅ is the work done the minimum and maximum?
Ans. When the angle between displacement and force acting on a particles is 90 degree, the work done is minimum or zero.
Reason: Work done = F x S cos∅, cos 90 = 0, so work done is 0.
When the angle between displacement and force acting on a particles is 0 degree, the work done is maximum.
Reason: Work done = F x S cos∅, cos 0= 1
[ Cos 0 > Cos 90, cos 60, cos 45.
3. Moon is experiencing a gravitationalforce due to earth and is revolving around the earth in a circular orbit. How much work is done by the moon?
Ans. Work is done by the moon is zero.
Reason: We know that work done = F x S cos∅.
Here the ∅ is 90 degree because displacement S is along the tangential direction to the circular orbit which is perpendicular to the direction of gravitational force, so the average work done is 0.
4. What is the work done against gravity when a body is moved horizontally along a frictionless surface?
Ans. When a body is move horizontally along a frictionless surface, the force of gravity acts perpendicular to the direction of motion, therefore ∅ is 90 degree and so the work done against gravity is zero.
5. By what factor does the kinetic energy of a particle increase if the speed is increased by a factor of 3?
Ans. We know that ,
Kinetic energy = 1/2 mv^2.
Therefore, Kinetic energy is directly proportional to the square of speed or velocity, so if the speed is increased by a factor of 3 and mass is constant, the kinetic energy of a particle increases 9 times.
6. A 40 kg girl is running along a circular path of radius 1 m with a uniform speed. How much work is done by the girl in completing one circle?
Ans. Work done by the girl is 0, because after completing one circle displacement is zero and we know that work done is equal to the product of displacement and force. So there is no work done
6. A 2 m high person is holding a 25 kg trunk on his head and is standing at a roadway bus terminus. How much work is done?
Ans. Work done by the person is zero, because he is standing at a roadways bus terminal and displacement is zero.
As we know that work done = F x S. Where F is force and S is displacement, work done is equal to
7. A horse of mass 210 kg and a dog of mass 25 kg are running at the same speed. Which of the two possesses more kinetic energy?
Ans. Horse possesses more kinetic energy.
Reason: We know that Kinetic energy is directly proportional to the speed and mass. Here speeds are cosntant and mass of horse is greater than the mass of dog. So horse have more kinetic energy tha dog.
8. How many joules make one kilowatt-hour?
Ans. One kilowatt-hour is equal to 3.6 x 10^6 J
Reason : 1 kilowatt-hour = 1 kilowatt x 1 hour = 1000 W x 3600 sec = 3.6 x 10^6 J
9. When displacement is in a direction opposite to the direction of force applied, what is the type of work done?
Ans. When displacement is in a direction opposite to the direction of force applied, type of work done is negative.
Reason: Work done = Force x Displacement ( in the direction of force)
If displacement is in opposite direction, then displacement is negative.
Therefore Work done = Force x ( - Displacement) = - work done.
10. If the heart works 60 Joule in one minute, what is its power?
Ans. If the heart works 60 Joule in one minute:
Here, Work = 60 J. and Time = 1 min = 60 sec.
We know that Power = Work/Time = 60J/60s = 1 watt.
11. Define power and 1 watt of power. Write the S.I unit of power.
Ans. The rate of doing work is called power.
Power = Work done/Time taken.
Its SI unit is watt.
One watt when the rate of consumption of energy is 1 J/s.
12. Calculate the work done when a force of 15 N moves a body by 5 m in its direction.
Ans. Given
Force F = 15N
Distance covered by body = 5 m
We have
Work done = Force x Distance
Therefore, work done = 15N x 5m= 75Nm or 75 J
- At what speed a body of mass 1 kg will have a kinetic energy of 1 J?
- What would be the amount of work is done on an object by a force if the displacement of the object is zero?
- How much work is done by a weight lifter when he held a weight of 80 kg on his shoulder for two minutes?
- A car and a truck are moving with the same velocity of 60 km/hr. Who has more kinetic energy?
- List two conditions that need to be satisfied for the work to be done on an object.
- A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
- What is the work done by the earth is moving around the Sun?
III. Long Answer Type Questions:
- The potential energy of a free-falling body decreases progressively. Does this violate the law of conservation of energy?
- Find the expression for the gravitational potential energy of a body of mass m at a height h.
- Find the expression for the kinetic energy of a body of mass m moving at a speed v.
- What will be the kinetic energy of a body when its mass is made four times and velocity is double? Light and heavy objects have the same momentum. Find out the ratio of their kinetic energies. Which one has larger kinetic energy?
- State the expression for the kinetic energy of a particle of mass m moving at a speed v.A rocket is moving up with a velocity v if the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?
- Write the formula or expression for the change in kinetic energy of a body of mass which experiences a change in its velocity from u m/s to v m/s when force is applied on it.
- Define kinetic energy. A horse of mass 200 kg and a dog of mass 20 kg are running at the same speed. Who h of the two possesses more kinetic energy?
(i) What is the S.I unit of kinetic energy? What is the kinetic energy of a body of mass moving with a velocity of 4 m/s?
(ii) A freely falling body eventually stops on the ground. What happens to its kinetic energy on the ground? - The kinetic energy of an object of mass ('m')moving with a velocity of 5 m/s is 25 J . Calculate its kinetic energy when its velocity is double.
- Write the form of energy possessed by the object in the following situation
(i) a coconut falling from the tree.
(ii) an object raised to a certain height.
(iii) blowing wind.
(iv) a child driving a bicycle. - Given below are a few situations, study them and state in which of the given cases work is said to be done. Give reasons for your answer.
(i) A person pushing hard a huge rock does not move
(ii) A bullock pulling a cart up to 1 km on road.
(iii) A girl pulling a trolley for about 2 m distance.
(iv) A person standing with a heavy bag on his head. - An object was thrown at a certain angle from 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.
(i) What is the work done by the force of gravity on the object?
(ii) How is work done by a force measured? A porter lifts luggage of 20 kg from the ground and puts in on his head 1.7 m above the ground. Find the work done by the porter on the luggage? - A box about 10 kg is placed at a point A on a horizontal surface. It is raised to a point B which is just at a distance of 2 m above A. Find the work done against by the force of gravity on the box. Justify your answer. ( g=10 m/s).
- An archer stretched a bow to release an arrow to hit the target at a distance of 10 m. Explain who does the work. In which form is the energy possessed by the bow and the arrow?
- A body of mass 2 kg is thrown up with a speed of 25 m/s. Find the maximum potential energy.
- What is the work to be done to increase the velocity of a car from 30 km /hr to 40 km/hr, if the mass of the car is 1500 kg?
- A bag of wheat 60 kg. Find the height to which it is lifted so that its potential energy is 3000 J. ( g= 10 m/s).
- When will be work positive, negative or zero? Explain with examples.
- Write the names of four different forms of energy and give an example where energy is transformed from one form to another?
- A 5 kg ball is thrown upward with a speed of 10 m/s.
(i) Calculate the maximum height attained by it.
(ii) Find the potential energy when it reaches the highest point. Cc
