Question:
When do we say that work is done?
Answer:
Work is said to be done when the following conditions are met:
(i) a force acts on the body.
(ii) A displacment of the body occurs by virtue of applied force in the direction of applied force.
Work and Energy
Q 1.
Name a machine that transforms muscular energy into useful mechanical work.
Q 2.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 3.
What do the machines need for their working ?
Q 4.
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 5.
How does the wind move the blades of a wind-mill ?
Q 6.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 7.
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 8.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 9.
When is work done by a force zero ?
Q 10.
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 11.
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 12.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 13.
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 14.
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 15.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 16.
What is the form of energy possessed by a running car?
Q 17.
Can a body have energy without having momentum ?
Q 18.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q 19.
The velocity of a body moving in a straight line is increased by applying a constant force F,for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.
Q 20.
Name two forms of mechanical energy.
Q 21.
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 22.
Define average power.
Q 23.
A coolie carries a load of 50 kg on his head and walks on a level road upto 100 m.What is the work done by him ?
Q 24.
A boy of mass 55 kg runs up a flight of 40 stairs,each measuring 0.15 m.Calculate the work done by the boy
Q 25.
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 26.
Name the device which converts electrical energy into mechanical energy.
Q 27.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 28.
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 29.
Fill in the blanks :
Work done against a force is stored as _____.
Q 30.
Is a person doing any work by holding a suitcase ?
Q 31.
Is energy a vector quantity ?
Q 32.
When an arrow is shot from a bow,it has kinetic energy.From where does it get the kinetic energy ?
Q 33.
Name a few forms of energy and their transformation into each other.
Q 34.
In physics,energy is an indirectly observed quantity that is often understood as the ability of a physical system to do work on other physical systems.However,this must be understood as an overly simplified definition as we shall come to know in higher classes that not all energy can perform work.
(a) How did the concept of energy emerge ?
(b) How much electric energy is generated in a typical lightning strike ? What are the forms to which this energy gets converted ?
(c) What is the kinetic energy of a passenger seated in a moving plane :(i) with respect to the plane (ii) with respect to the Earth.
Q 35.
The recent horrific storm surge flooding New Jersey and New York in US by Hurricane Sandy was almost prefectly predicted well in advance,but was more extreme than the average person might expect from a minimal hurricane.There is a metric that quantifies the energy of a storm based on how far out tropical-storm force winds extend from the centre,known as Integrated Kinetic Energy (IKE).
(i) What is Sandy’s ranking in terms of IKE among all the hurricanes witnessed so far?
(ii) What is the IKE of Sandy?
(iii) How was Sandy different from Hurricane Katrina ?
(iv) Which human value did Sandy generate among the people round the globe?
Q 37.
Fill in the blanks :
Work done is negative when the angle between force and displacement is ____ (acute/obtuse)
Q 38.
Fill in the blanks :
Work done against gravity in lifting a mass is stored as ______. Numerically, it is equal to m ×_ ×__.
Q 39.
Fill in the blanks :
Work done by gravity on a falling body is available as ______ (kinetic/potential) energy on reaching the ground.
Q 40.
A roller coaster is shown in figure, at points i.e. ➊ and ➋, comparatively at what point (i) potential energy is maximum (ii) kinetic energy is maximum
Q 41.
A heavy stone is lowered to the ground with the help of rope. Is the work done by the applied force positive or negative?
Q 42.
From where the falling raindrops gains kinetic energy?
Q 43.
Name the type of energy possessed by a raised hammer?
Q 45.
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 46.
How much work is done on a body of mass 1kg whirling on a circular path of radius 5m?
Q 47.
A bullet of mass 20 g is found to pass two points 30 m apart in 4 s ? Assuming the -speed to be constant,find its kinetic energy.
Q 48.
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 49.
What is the kinetic energy of an object ?
Q 50.
Define 1 W of power.
