Chapter 11 Work and Energy Solutions
Question - 11 : - A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
Answer - 11 : -
Power = Work/Time
P = W/T
Time = 10 s
Work done = Energy consumed by the lamp = 1000 J
Power = 1000/10 = 100 Js-1 =100 W
Question - 12 : - Define average power
Answer - 12 : - The average Power of an agent could also be outlined because the total work done by it within the total time taken.
Question - 13 : - Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.
(a) Suma is swimming in a pond.
(b)A donkey is carrying a load on its back.
(c) A wind-mill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food grains are getting dried in the sun.
(g) A sailboat is moving due to wind energy.
Answer - 13 : -
Work is finished whenever the given 2 conditions are satisfied:
(i) A force acts on the body.
(ii) There’s a displacement of the body by the applying of force in or opposite to the direction of force.
(a) Whereas swimming, Suma applies a force to push the water backwards. Therefore, Suma swims within the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, work is finished by Seema whereas swimming.
(b) Whereas carrying a load, the donkey should apply a force within the upward direction. But, displacement of the load is within the forward direction. Since, displacement is perpendicular to force, the work done is zero.
(c) A wind mill works against the gravity to elevate water. Hence, work is finished by the wind mill in lifting water from the well.
(d) During this case, there’s no displacement of the leaves of the plant. Therefore, the work done is zero.
(e) An engine applies force to tug the train. This permits the train to maneuver within the direction of force. Therefore, there’s a displacement within the train in the same direction. Hence, work is finished by the engine on the train.
(f) Food grains don’t move within the presence of alternative energy. Hence, the work done is zero during the method of food grains obtaining dried within the Sun.
(g)Wind energy applies a force on the sailing ship to push it within the forward direction. Therefore, there is a displacement within the boat in the direction of force. Hence, work is finished by wind on the boat.
Question - 14 : - 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 - 14 : -
Work done by the force of gravity on an object depends solely on vertical displacement. Vertical displacement is given by the distinction within the initial and final positions/heights of the thing that is zero.
Gravity-related work is expressed as,
W= m g h
Where,
h= Vertical displacement = zero
W = m g × zero = 0 J
Consequently, the work done on the given object by gravity is zero joule.
Question - 15 : - A battery lights a bulb. Describe the energy changes involved in the process.
Answer - 15 : -
When a bulb is connected to a battery, then the energy of the battery is transferred into voltage. Once the bulb receives this voltage, then it converts it into light-weight and warmth energy. Hence, the transformation of energy within the given situation may be shown as:
Chemical Energy → Electrical Energy → Light Energy + Heat Energy.
E k = K.E. of the thing moving with a rate, v
K.E. once the thing was moving with a rate five m s-1
1 (E k) 5 = two × twenty × (5)
= 250 J
Kinetic energy once the thing was moving with a rate two m s-1 one
(E k) 2 = two x twenty × (2)
= 40 J
Question - 16 : - Certain force acting on a 20 kg mass changes itsvelocity from 5 m s-1 to 2 m s-1. Calculate the work done by the force
Answer - 16 : -
Given data:
Initial velocity u=5 ms–1
Mass of the body = 20kg
Final velocity v = 2 ms–1
The initial kinetic energy
Ei = (1/2) mu2 =(1/2) x 20 x (5 ms–1)2 =250kgms-2
= 250Nm = 250J
Final kinetic energy Ef = (1/2) mv2 =(1/2) x 20 x (2 ms–1)2 =40kgms-2 =40 Nm =40J
Therefore,
Work done = Change in kinetic energy
Work done = Ef –Ei
Work done =40J – 250J
Work done = -210J
Where negative sign indicates that force acts contrary to motiondirection.
Question - 17 : - A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, hat is the work done on the object by the gravitational force? Explain your answer.
Answer - 17 : -
Work done by gravity depends solely on the vertical displacement of the body. It doesn’t rely on the trail of the body. Therefore, work done by gravity is given by the expression,
W= m g h
Where,
Vertical displacement, h = 0
∴ W= mg × zero = 0
Therefore the work done on the body by gravity is therefore zero.
Question - 18 : - The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Answer - 18 : - No, the method doesn’t violate the law of conservation of energy. This is because once the body falls from a height, then its mechanical energy changes into kinetic energy increasingly. A decrease within the mechanical energy is capable a rise in the kinetic energy of the body. Throughout the method, total energy of the body remains conserved. Therefore, the law of conservation of energy isn’t desecrated.
Question - 19 : - What are the various energy transformations that occur when you are riding a bicycle?
Answer - 19 : -
During riding a bicycle, the muscular energy of the rider is regenerate into heat and mechanical energy.
Kinetic energy provides rate to the bicycle and warmth energy heats our body.
Muscular energy mechanical energy + heat
Question - 20 : - Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
Answer - 20 : - When we push a large rock, there’s no transfer of muscular energy to the stationary rock. Also, there’s no loss of energy as a result of muscular energy is transferred into energy, which causes our body to become hot.