Physics

Work and Energy

Question:

Four men lift a 250 kg box to a height of 1 m and hold it without raising or lowering it (a) How much work is done by the men in lifting the box ?(b) How much work do they do in just holding it ?(c) Why do they get tired while holding it ? (g = 10m/s2)

Answer:

Mass of the box, m = 250kg
Height through which it is lifted,h= 1 m
Work done in lifting the box = increase in the PE of the box = mgh = (250) (10) (1) = 2500 N
(b) When the men just hold the box,S=0 and as such W = F s = 0
(c) To hold the box,the men have still to apply a force equal to the weight of the box in the opposite direction.To apply this force,men have to exert muscular effort which is the cause of their tiredness.
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Work and Energy

Q 1.

Fill in the blanks :
Work is a ______ (scalar/vector) quantity.

Q 2.

Fill in the blanks :
Work done on an object is _____ if displacement is zero.

Q 3.

An electric bulb of 60 W is used for 6 h per day.Calculate the ‘units’ of energy consumed in one day by the bulb

Q 4.

A man exerts a force of 200 N in pulling a cart at a constant speed of 16 m/s.Calculate the power spent by the man.

Q 5.

A force of 10 N moves a body with a constant speed of 2 m/s. Calculate the power of the body.

Q 6.

Calculate potential energyof a person having 60 kg mass on the summit of Mt. Everest. Height of Mt. Everest is 8848 m from sea level. (g = 9.8 m/s2)

Q 7.

A bullet of mass 0.03 kg moving with a speed of 400 m/s penetrates 12 cm into fixed block of wood.Calculate the average force exerted by the wood on the bullet.

Q 8.

The work done by the heart is I J per beat.Calculate the power of the heart if it beats 72 times/min.

Q 9.

Define 1 J of work.

Q 10.

Give an example where potential energy is acquired by a body due to change of its shape.

Q 11.

What do you mean by the term potential energy ? Give some examples of bodies possessing potential energy ?

Q 12.

Fill in the blanks :
Work done is defined as the product of _____ and ______ in the direction of force.

Q 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’.
  • Suma is swimming in a pond.
  • A donkey is carrying a load on its back.
  • A wind-mill is lifting water from a well.
  • A green plant is carrying out photosynthesis.
  • An engine is pulling a train.
  • Food grains are getting dried in the sun.
  • A sailboat is moving due to wind energy.

Q 14.

A momentum of the body is increased by 20%. What is the percentage increase in its kinetic energy?

Q 15.

A boy of mass 20 kg climbs up a ladder in 20 seconds. If the height of a ladder is 10 metres, calculate amount of power used.(Take g = 10 m/s2)

Q 16.

Calculate the power of an engine required to lift 105 kg of coal per hour from a mine 360 m deep.

Q 17.

An electric heater is rated 1500 W.How much energy does it use in 10 hours ?

Q 18.

What are the units of work and energy ?

Q 19.

What do you understand by the term energy ? Give its scientific conception.

Q 20.

Name a few forms of energy and their transformation into each other.

Q 21.

Calculate the amount of work done in drawing a bucket of water weighing 10kg from a well of depth 20m.

Q 22.

Can a body have energy without possessing momentum?

Q 23.

Fill in the blanks :
SI Unit of work is _______.

Q 24.

Fill in the blanks :
Work done is negative when the angle between force and displacement is ____ (acute/obtuse)

Q 25.

Fill in the blanks :
A force acting on a body shows a displacement in the direction normal to the direction of the force. It is said _______ done on the body.

Q 26.

A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Q 27.

What is the kinetic energy of an object?

Q 28.

Does the work done on a load depend on the time taken to raise the load?

Q 29.

Is work a scalar or vector quantity?

 
 

Q 30.

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, what is the work done on the object by the gravitational force? Explain your answer.

Q 31.

The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Q 32.

Give examples for the following scenarios of energy transformations. You may choose the answer(s) from the following list of words

solar cell, turbine, mixer, fan, steam engine, automobiles, green plants, nuclear reactor, electric bulb, torch, electric bulb, microphone, loudspeaker, geyser, heater, car battery, inverter, LPG

 

SNo.    Energy Transformation
From ⟶ To
     Give Example
1. Mechanical ⟶ Electrical          _________
2. Electrical ⟶ Mechanical      _________
3. Electrical ⟶ Heat      _________
4. Heat ⟶ Mechanical      _________
5. Electrical ⟶ Light      _________
6. Electrical ⟶ Sound      _________
 
7. Chemical ⟶ Heat      _________
8. Electrical ⟶ Chemical      _________
9. Sound ⟶ Electrical      _________
10. Chemical ⟶ Electrical      _________
11. Light ⟶ Electric      _________
12. Chemical ⟶ Mechanical      _________
13. Nuclear ⟶ Electrical      _________
14. Light ⟶ Chemical      _________

Q 33.

Can there be displacement of an object in the absence of any force acting on it?

Q 34.

Define power.

Q 35.

A motor pump does same amount of water by lifting underground water in shorter time than a hand pump. What do you infer from this statement?

Q 36.

State the relation between commercial unit of energy and joules.

Q 37.

A 30g bullet initially travelling at 500 m/s penetrates 12 cm into a wooden block. How much average force does it exerts?

Q 38.

A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground.Calculate the work done by him on the luggage.

Q 39.

A boy of mass 50 kg runs up a staircase of 45 steps in 9 s.If the height of each step is 15 cm,find his power.Take g = 10 m/s2.

Q 40.

A man does 200 J of work in 10 s and a boy does 100 J of work in 4 s.(a) Who is delivering more power ?
(b) Find the ratio of the power delivered by the man to that delivered by the boy.

Q 41.

What is the kinetic energy of an object ?

Q 42.

What is power ?

Q 43.

Define 1 W of power.

Q 44.

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.

Q 45.

A battery lights a bulb.Describe the energy changes involved in the process

Q 46.

A certain household has consumed 250 units of energy during a month.How much energy is this in joules ?

Q 47.

How is work done by a force measured ?

Q 48.

When is work done by a force positive ?

Q 49.

Is energy a vector quantity ?

Q 50.

When an arrow is shot from a bow,it has kinetic energy.From where does it get the kinetic energy ?