Chapter 11 Work and Energy Solutions
Question - 21 : - A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Answer - 21 : -
1 unit of energy is up to one B.T.U. (kWh).
1 unit = one kWh
1 kWh = 3.6 x 106 J
Therefore, 250 units of energy = 250 × 3.6 × 106
= 9× 108 J.
Question - 22 : - An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Answer - 22 : -
With the help of the expression, energy consumed by an electric heater will be obtained,
P = T
Where,
Power rating of the heater,
P = 500 W = 1.5 power unit Time that the heater has operated,
T = ten h Work done = Energy consumed by the heater
Therefore, energy consumed = Power × Time
= 1.5 × 10 = 15 kWh
Hence, the energy consumed by the heater in 10h is 15 kWh.
Question - 23 : - Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
Answer - 23 : - Consider the case of oscillation pendulum.
When an apparatus moves from its mean position P to either of its extreme positions A or B, it rises through a height h on top of the mean level P. At this time, the K.E. of the bob changes fully into P.E. The K.E. becomes zero, and also the bob possesses solely P.E. Because it moves towards purpose P, its P.E. decreases increasingly. Consequently, the K.E. will increase. Because the bob reaches purpose P, its P.E. becomes zero and also the bob possesses solely K.E. This method is perennial as long because the apparatus oscillates.
The bob doesn’t oscillate forever. It involves rest as a result of air resistance resists its motion. The apparatus loses its K.E. to beat this friction and stops once a while. The law of conservation of energy isn’t desecrated as a result of the energy lost by the apparatus to beat friction is gained by its surroundings. Hence, the overall energy of the apparatus and also the encompassing system stay preserved.
Question - 24 : - An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?
Answer - 24 : -
Question - 25 : - Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Answer - 25 : -
Given data:
The mass of the body = 1500kg
Velocity v = 60km/hr
The work required to stop the car = kinetic energy change of thecar
Question - 26 : - In each of thefollowing a force, F is acting on an object of mass, m. The direction ofdisplacement is from west to east shown by the longer arrow. Observe thediagrams carefully and state whether the work done by the force is negative,positive or zero.
Answer - 26 : -
Case I
In this case, the direction of force functioning on the block isperpendicular to the displacement. Therefore, work done by force on the blockare going to be zero.
Case II
In this case, the direction of force functioning on the block iswithin the direction of displacement. Therefore, work done by force on theblock are going to be positive.
Case III
In this case, the direction of force on the block is contrary tothe direction of displacement. Therefore, work done by force on the block aregoing to be negative.
Question - 27 : - Soni says that theacceleration in an object could be zero even when several forces are acting onit. Do you agree with her? Why?
Answer - 27 : -
Acceleration in associate object might be zero even once manyforces are working on it. This happens once all the forces get rid of oneanother i.e., the online force working on the thing is zero. For a uniformlymoving object, the online force working on the thing is zero. Hence, theacceleration of the thing is zero. Hence, Soni is correct.
Question - 28 : - Find the energy in kW h consumed in 10 hours by fourdevices of power 500 W each.
Answer - 28 : -
Energy consumed by an electrical device will be obtained withthe assistance of the expression for power,
P = T
Where,
Power rating of the device,
P = five hundred W = 0.50 power unit Time that the device runs,
T= ten h Work d consumed by the device thus, energy c × Time
= 0.50 × 10 = 5 kWh
Hence, the energy consumed by four equal rating devices in
10 h = 4 × 5 kWh = 20 kWh = 20 Units.
Question - 29 : - A freely fallingobject eventually stops on reaching the ground. What happens to its kineticenergy?
Answer - 29 : -
When the object falls freely towards the bottom, its mechanicalenergy decreases and K.E. will increase, because the object touches the bottom,all its mechanical energy gets reborn into K.E. Because the object hits thelaborious ground, all its K.E. gets reborn into heat and sound energy. It mayalso deform the bottom relying upon the character of the ground and thereforethe quantity of K.E. possessed by the thing.