Question:
What is a conservative forces? Give an example.
Answer:
Conservative force is the one which does a work moving an object from one point to another but is independent of path. Generally the work done is reversible.
e.g. gravitational force is a conservative force. When an object rises up to a certain height (h), its potential energy increases by a value of mgh. This increase in potential energy is independent of path being followed i.e. when object is lifted up or pushed up through a slant. When the object falls back to same ground level, its potential energy also decreases to initial value.
Work and Energy
Q 1.
Name a machine that transforms muscular energy into useful mechanical work.
Q 2.
Two girls,each of weight 400 N,climb up a rope through a height of 8 m.We name one of the girls A and the other B.Girl A takes 20 s while B takes 50 s to accomplish this task.What is the power expended by each girl ?
Q 3.
What do the machines need for their working ?
Q 4.
How does the wind move the blades of a wind-mill ?
Q 5.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 6.
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?
Q 7.
When is work done by a force zero ?
Q 8.
The mass of a ball A is double the mass of another ball B.The ball A moves at half the speed of the ball B.Calculate the ratio of the kinetic energy of A to the kinetic energy of B.
Q 9.
What is the form of energy possessed by a running car?
Q 10.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 11.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 12.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 13.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 14.
Define potential energy.Derive an expression for the potential energy of a body of mass m,at a height h above the surface of the Earth.
Q 15.
An object of mass 15 kg is moving with a uniform velocity of 4 m/s.What is the kinetic energy possessed by the object ?
Q 16.
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 =
.
Q 17.
A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Q 18.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q 19.
A force of 5 N is acting on an object.The object is displaced through 2 m in the direction of the force.[in Fig.].If the force acts all through the displacement,find the work done by the force.
Q 20.
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 21.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 22.
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 23.
Define average power.
Q 24.
A battery lights a bulb.Describe the energy changes involved in the process
Q 25.
Name two forms of mechanical energy.
Q 26.
Many people debate about whether there is a God,or what God is.Is God an individual personality,sitting up in heaven listening to and granting our requests,or is God simply all the energy that exists in the Universe,or is God simply nature or the laws of nature ?
Q 27.
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 28.
Name the type of energy possessed by a raised hammer?
Q 29.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 30.
When is work done by a force negative ?
Q 31.
What are the units of work and energy ?
Q 32.
Why do some engines require fuels like petrol and diesel ?
Q 33.
What is meant by the term potential energy ? Give its two examples.
Q 34.
How does an object possessing energy perform work ? What is the unit of energy ?
Q 35.
Name a few forms of energy and their transformation into each other.
Q 36.
Can a body have energy without having momentum ?
Q 37.
The global climate has always fluctuated.Millions of years ago,some parts of the world that are now quite warm, were covered with ice,and over recent centuries,average temperatures have risen and fallen in cycles.What is new,however,is that current and future climate change will be caused not just by natural events but also by activities of human beings.Suggest three simple ways to help save the climate of our planet.
Q 38.
A box is pulled across a floor by applying a force of 100N at an angle of 60 degrees above the horizontal. How much work is done by the applying force in pulling the object to a distance of 8m?
Q 39.
Fill in the blanks :
If the mass is doubled while its speed remains the same, the kinetic energy will be _____ (be same/double/four times) of the initial kinetic energy.
Q 40.
A person throws a stone of mass 3kg. It crosses the top of a wall of 2m high at a speed of 4m/s. What is the total mechanical energy possessed by the stone when it crosses the wall?(Take g = 10 m/s)
Q 41.
Identify the kind of energy possessed by a running athlete.
Q 42.
An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Q 43.
State the relation between commercial unit of energy and joules.
Q 44.
A force of 7 N acts on an object.The displacement is,say 8 m,in the direction of the force,[in Fig.].Let us take it that the force acts on the object throughout the displacement What is the work done in this case ?
Q 45.
Define 1 J of work.
Q 46.
What is the kinetic energy of an object ?
Q 47.
Define 1 W of power.
Q 48.
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 objjgft by the gravitational force ? Explain your answer.
Q 49.
An electric heater is rated 1500 W.How much energy does it use in 10 hours ?
Q 50.
Is work a scalar quantity or a vector quantity ?
