WORK AND ENERGY

Q. 1Identify, Work is a scalar or vector quantity.Ans.Work is a vector quantity as it depends on the force applied and the direction in which it is applied.

 Q. 2.State the factors on which work done depends.Ans.Work done by a force depends on:1. Magnitude of the force2. The distance through which the body moves in the direction of force

Q. 3.Define work.Ans.Whenever a certain amount of force is able to move a body , work is said to be done. Mathematically, it is the product of the force exerted on the body and the distance move by the body in the direction of force. Work = Force exerted x Distance (traveled in the direction of force) W = F x s Q. 4.Define the SI unit of Work.Ans.The SI unit of work is Joule(J) Work = Force x Distance1 Joule = 1 Newton x 1 Metre Therefore , we define 1 Joule as the amount of work done when a force of one Newton moves a body through a distance of 1 Metre in the direction of force applied. Q. 5.Calculate the amount of work done in lifting a body of mass 3 Kg through a distance of 11 metres.Ans.Work = Force x DistanceForce = Mass x Acceleration due to gravity          = 3 x 9.8          = 29.4 Newtons Work = 29.4 x 11          = 323.4 Joules

 Q. 6.Suppose a man is trying to push a wall. But the wall doesn't move. What is the amount of work done?Ans.If a man is applying force on a wall and tries to push it then the amount of work done by the man will be 0 (zero) as the wall doesn’t move. That is the amount of force applied is not able to move the object. Mathematically ,W = F x sSince the wall doesn’t move s = 0Hence ,W = F x 0 = 0Work done = 0 Joules

 Q. 7.Which formula do we use to calculate the work done when a body is moved against gravity ?Ans.When the work is done against gravity then the amount of work done is equal to the weight of the body times the vertical distance through which it is lifted. Work done = Weight of body x vertical distance Against gravity W = mg x h W = mghWhere,W - Workm - Massg - acceleration due to gravityh - vertical distance

 Q. 8.Calculate the force exerted by a body when the work done is 2500 J through a distance of 50 metres.Ans.

 Q. 9.Calculate the mass of a man who has to do 1500 joules of work in climbing a stair case 5 metres high (g = 9.8 m/s2)Ans.

 

  Q. 10.A car with a speed of 25m/s weighing 900 Kg stops at a distance of 40 metres. Calculate the force exerted and the and work done by the brakes.Ans.

Q. 11.Give examples of atmospheric phenomena that have large kinetic energy?

Ans.Hurricanes, tornadoes and microburst.  Q.12.What is the mass of television located on a shelf that is 1.5 metres above the floor and that has a gravitational potential energy of 153.6 joules?

Ans.Potential Energy = m x g x h153.6 = m x 9.8 x 1.5

m = 153.6/ 14.7 Kgm = 10.45 Kg Q.13.Total mass of a jet skier along with Jet Ski is 230 Kg. He is traveling in the south direction with a kinetic energy of 40,250 J. Find the velocity?

 Q.14.A baseball weighing 173 grams is thrown with a velocity of 21 m/s. What is the kinetic energy of the baseball?

K.E. = 38.15 J Q.15.Explain the principle behind the wind up toys.Ans.Wind up toys have a spring inside it which gets wind up with the help of winding keys. So when we wind-up the spring of a toy car by using a winding key, then some work is done by us. As a result, the spring gets coiled up more tightly. The work done in winding the spring gets stored in the wound up spring in the form of elastic potential energy (i.e potential energy due to change in shape) When we release the winding key the spring tries to attain its normal shape. During this process the potential energy stored in it get converted in to kinetic energy. This turns the wheels of the toy car.  Q.16.What kind of energy does a flying bird or a flying airplane has during their course of action?Ans.A flying bird or a flying airplane has kinetic energy as well as potential energy during their course of action. The flying bird and airplane have kinetic energy because they are moving. And they have potential energy at any point of time in their flight due to their height above ground at that particular point of time. Thus a body can have both potential and kinetic energy at the same time. 

 Q. 17.A roller coaster car weighing 525 kg is moving with a speed of 20.5 m/s. Find its kinetic energy? What would be the new kinetic energy if roller coaster's speed is halved?Ans.

 

Ifthe velocity of a body is halved, its kinetic energy becomes one fourth as kinetic energy is directly proportional to velocity squared. So the new kinetic energy would be 1/4 X 1.10 x105 JoulesOR

 Q. 18.A cat and a mouse are running with the same speed. If the weight of the cat is 20 times that of mouse, what is the ratio of their kinetic energy?

Ans.Let the speed of cat and mouse = v m/s as their speeds are same.Since, the cat weighs 20 times that of mouse, therefore,If the weight of mouse = mThen, the weight of cat = 20 mNow, kinetic energy of cat = 1/2 x 20 m x v2 = 10 mv2                            

Kinetic energy of mouse = 1/2 x m x v2 = 1/2 mv2

Kinetic Energy of cat/ Kinetic Energy of mouse = 10/(1/2) = 20/1Hence,KE of cat : KE of mouse  is 20:1 Q. 19.A 35 kg panther sitting on a branch of tree has a potential energy of 1056 J. How far above ground is the tree branch?

Ans.Potential Energy = m x g x h1056 J = 35 kg x 9.8 m/sec2 x hh = 1056 / 343 mh = 3.08 m Q. 20.Calculate the kinetic energy of a body of mass 0.1 kg and momentum 20 kg m s-1.Ans.

Q. 21.Home electricity bills come in units called Kilowatt-hour. How many joules is equal to 1 kilowatt hour. Why kilowatt hours are used instead of joules in the bill?Ans.1 Kilowatt hour = 36, 00,000 Joules = 3.6 x 106 JOne kilowatt hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour. Kilowatt hour is used for measuring electricity consumed as joule represent a very small quantity of energy and therefore it is inconvenient to use joules where large quantities of energy is involved.

  Q.22.What energy transfer takes place while using solar cells to operate solar powered water heater?Ans.Solar cells in solar panels convert solar energy to electric energy which could be used directly or could also be stored in the cells. Then this electric energy is used to heat up electric rod in heater to heat water.

Energy transfer: solar energy> electric energy>heat energy.

 Q.23.When we burst a cracker, is it correct to say that the chemical energy of crackers get used up or destroyed as we are left with just ashes after bursting cracker?Ans.Its wrong to say that energy gets used up or destroyed . According to law of conservation of energy, energy can neither be created nor destroyed it just converts from one form to other.

On bursting a cracker the chemical energy of crackers transform into heat and light energy.

 Q.24.Which appliance consumes more energy a 100 watt electric bulb or a 150 watt electric iron in a minute?Ans.Power = Energy consumed / Time takenSo Energy consumed = power x time

1)      Energy consumed by bulb= 100 w x 60s = 6000 J2)      Energy consumed by electric iron = 150w x 60s = 9000 J

 Thus, an electric appliance with greater power consumes more energy. Q.25.A microwave uses 450 KJ of electric energy in 5 minutes. Calculate its power rating.Ans.Electric energy consumed, E = 450 KJ

= 450 x 1000 J = 450000 J

Time taken, t = 5 minutes = 5 x 60 sec = 300 sec

Power, P = E/t

= 450000/300 = 1500 watts

 Q.26.In a water log swig (a boat shape water log is made to slide from a height) if the potential energy of 60 kg water log boat was 5880 joules. Find the speed with which water log just touches the ground?Ans.

Kinetic energy when just touching the ground would be equal to the initial potential energy (law of conservation of energy states total mechanical energy of system remains same)

½ mv² = 5880 J½ x 60 x v² = 5880v² = (5880 x 2)/ 60 = 196v = 14 m/s

 Q.27.How does refrigerator uses electric energy? A) change kinetic energy of food particles. B) removes thermal energy from inside the refrigerator. C) changes heat into cold. D) produces cold.Ans.The point of a refrigerator is to lower the temperature of the contents.  This means you must remove energy from the contents. Hence refrigerator uses electric energy to removes thermal energy from inside the refrigerator. Q.28.Explain how wind mills are used to generate electricity?Ans.In wind mill kinetic energy of wind is used to move the blades of  fan of wind mill which in turn are connected to coils of an a.c. generator. The kinetic energy of wind  move the fan of mill which in turn moves the coil of generator thus converting kinetic energy to  electric energy. Thus in the above examples there is only transformation of energy from one form to another so that the total energy of the system remains constant  Q.29.Ram uses a 100 watt bulb for 10 hrs, a 150 watt fan for 10 hrs and 200 watt computer for 4 hours. Calculate his monthly usage of electricity. If per unit charge is Rs 5, what would be his monthly electricity bill?Ans.Power of   bulb = 100/1000= 0.10 KwPower of fan = 150/1000 = 0.15 Kw

Power of computer = 250/1000= 0.25 KwEnergy consumed = power x timeEnergy consumed by bulb = 0.10 x 10 = 1 kw-hrEnergy consumed by fan = 0.15 x 10 =1.5 kw-hrEnergy consumed by computer = 0.25 x 4 = 1 kw-hrRam’s daily electricity units consumed = 1 + 1.5 + 1 = 3.5 kw-hrSo his monthly consumption would be 3.5 x  30 = 105 kw-hrGiven 1kw- hr or one unit of electricity is for Rs 5Ram’s monthly bill would be 105 x 5 = Rs 525

 Q.30.A box weighing 25 kg is dropped from a height of 4 m. Find the potential energy, kinetic energy and their sum at height 4 m, 3 m, 2 m, and 1 m above the ground.Ans.As per the Law of conservation of energy the total energy at any height during the free fall of the box would be constant , as energy can neither be created nor destroyed. Use this statement to find kinetic energy of box at different heights 

Height at which object is located

Potential energyEp= mgh

Kinetic energy(Ek =½ mv²)

Ep + Ek

4 25 x 9.8 x 4 = 980 0 9803 25 x 9.8 x 3 = 735 245 9802 25 x 9.8 x 2 = 490 490 9801 25 x 9.8 x 1 = 245 735 980