Topic 6 Work, energy and power: chapter 5

Question 1

define work done.

Answer 1

energy transferred to or from an object via the application of a force along a displacement

Question 2

is work done when you apply a force on an object but the object doesn’t go under any displacement?

Answer 2

no work is done when there is no displacement.
example:
- holding a weight above your head.
- pushing a car but it doesn't budge
- moon orbiting earth

Question 3

what are the two factors that affect the work done?

Answer 3

• size of the force
• displacement

these both factors are directly proportional to work done.

Question 4

what is the formula of work done?

Answer 4

w = f * s where w = work done(N m)
f = force
s = the distance moved in the direction
of force

Question 5

what is energy transferred?

Answer 5

work done(N m) = energy transferred(J)
therefore,
1 N m = 1  J

Question 6

what is 1 joule?

Answer 6

1 joule is the amount of work done when a force of 1 N moves a distance of 1 m in the direction of force

OR

1 joule is the amount of energy transferred when a force of 1 N moves a distance of 1 m in the direction of the force

Question 7

what happens when an object moves at an angle to force(f)?

Answer 7

when an object move at an angle to the force(f), there is only a component of f acting in the direction of displacement(s).

see fig 5.6 on page no. 73

work done = (f cos(@))*s

Question 8

how is work done by a gas?

Answer 8

if a gas expands and the walls of the container are pushed outwards, then work is done by the gas on its surroundings.
example:
- car engine
- steam engine

Question 9

how to find work done by a gas.

Answer 9

work done = force * displacement 
pressure = force / area 
force = pressure * area 
 therefore, 
work done = (pressure * area) * displacement 

change in volume = area * displacement

therefore,
work done = pressure * change in volume

Question 10

what is the assumption we make when we use the formula W = P * change in V? and when is it true?

Answer 10

we assume that as the gas expands, the pressure remains constant.
this is true when the gas is expanding against the pressure of atmosphere, which changes only very slowly.

Question 11

how does an object gain g.p.e?

Answer 11

when you do work, energy is transferred from you to the object. you lose energy and the object gains energy.

when you do work resulting in vertical displacement of the object, the object gains gravitational potential energy.

Question 12

define g.p.e

Answer 12

it is the energy stored in an object due to its position above earth’s surface

Question 13

how can we calculate the change in g.p.e?

Answer 13

change in g.p.e = weight * change in height

= m * g * h

Question 14

when can we use the formula g.p.e = mgh ?

Answer 14

only for relatively small changes in height

Question 15

what are the other forms of potential energy?

Answer 15

• electric potential energy

- elastic potential energy

Question 16

what is the relationship between kinetic energy and the speed of an object?

Answer 16

the faster an object is moving, the greater its kinetic energy

Question 17

how does an object gain kinetic energy?

Answer 17

a force can make an object accelerate, i.e, move an object. therefore, work is done and energy is transferred. this energy is converted into kinetic energy to move the object, i.e, an object gains kinetic energy when work is done by the force applied and the object accelerates as a result.

work done by the force to make an object accelerate 
                                     =
energy transferred 
                      = 
kinetic energy

Question 18

how to find the kinetic energy of an object?

Answer 18

kinetic energy = 1/2 * mass * speed^2

Question 19

what is a point to remember when calculating kinetic energy involving change in speed?

Answer 19

you cant calculate the change in the kinetic energy by squaring the change in speed.
that is,
k.e. = 1/2 * mass * (v-u)^2 is wrong.

rather you need to do it in steps:
initial k.e = 1/2 * mass * initial speed^2
final k.e. = 1/2 * mass * final speed^2

change in k.e. = final k.e. - initial k.e.

Question 20

what is the relationship between g.p.e and k.e. of a falling object when no energy is lost to the surroundings ?

Answer 20

decrease in g.p.e = increase in k.e

and vice versa

Question 21

how to find the speed of a falling object?

Answer 21

the speed of a falling object remains same regardless of its mass

this is because both k.e. and g.p.e depends on mass.

that is,

k.e. = g.p.e 
1/2*mass*speed^2 = mass*gravitational field 
                                   strength*height 
1/2*m*v^2 = m*g*h
we can cancel out the mass.
therfore,
1/2*v^2 = g*h
v^2 = 2*g*h

therefore, final speed of a falling object depends on gravitational field strength and height.

Question 22

what is the relationship between the speed and mass of a falling object?

Answer 22

the speed of a falling object remains same regardless of its mass

Question 23

how to find the efficiency of a device or a system?

Answer 23

efficiency = (useful output energy / total input energy)*

100%

Question 24

what is the principle of conservation of energy?

Answer 24

energy can not be created or destroyed. it can only be converted from one form to another.

Question 25

how can the principle of conservation be represented using a diagram?

Answer 25

it can be represented by the snakey diagram.

fig 5.16- pg no. 79

Question 26

what do we assume when we use the principle of conservation of energy?

Answer 26

energy changes within a closed system. therefore, the total energy of the system remains constant.

Question 27

define power

Answer 27

the rate at which work is done by an object

Question 28

what is the formula of power?

Answer 28

power(W or Js^-1) = work done(j) / time taken(s)

Question 29

a tip to remember when dealing with practical problem of efficiency, energy and power

Answer 29

when dealing with practical examples, remember that the system’s efficiency can not be 100% ,
hence, the input power must be more than the system’s output power.

Question 30

how can we find the power of a moving object?

Answer 30

p = W / t = (F*d)/t
v = d/t 
therefore,
p = F * v 
that is,
power = force * velocity

Question 31

what is the relationship between force applied, power and velocity of a moving object?

Answer 31

p = F * v

therefore,
the greater the force, the greater the velocity, the greater the power.

Question 32

what is the average power of a human?

Answer 32

energy required = 10 MJ per day

power = 116 W

Question 33

is human body an efficient system?

Answer 33

no, human body isn’t an efficient system.
the inefficiency of our muscles explain why we get hot when we exert energy.
to lift a heavy load and increase it’s energy by
1000 J (output energy), we need 5-10 times the amount of energy (input energy).