Work done by a body is
- If a force displaces the object in its direction, then the work done is positive
The example of this kind of work done is motion of ball falling towards ground where displacement of ball is in the direction of force of gravity.
- If the force and the displacement are in opposite directions, then the work is said to be negative. For example if a ball is thrown in upwards direction, its displacement would be in upwards direction but the force due to earth’s gravity is in the downward direction.
So here in this case gravity is doing negative work when you throw the ball upwards. Hence the work done by gravitational force is negative. Mathematically when displacement is opposite to the force work done is given by
- Negative work just means that the force and the displacement act in opposite directions.
Case of zero work done
- If the directions of force and the displacement are perpendicular to each other, the work done by the force on the object is zero.
For example, when we push hard against a wall, the force we are exerting on the wall does no work, because in this case the displacement of the wall is d = 0. However, in this process, our muscles are using our internal energy and as a result we get tired.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Energy consumed by the household = 250 units
1 unit of energy = 1 kWh = 3.6 x 106 J
∴ 250 units of energy = 250 x 3.6 x 106
= 9 x 108 J.
Calculate the work done in moving a body of mass 50 Kg through a height of 5 m.(g = 10m/s2)
here given, mass = 50 kg
Height = 5 m , g = 10 ms-1
We know, w = mgh
⇒ w = 50 * 10 * 5
⇒ w = 2500 J
Scientifically, work is said to be done in which of the following cases
Manish pulls the trolley for a certain distance:- force is applied and there is displacement also , so work is done.
Calculate the distance between the man and the box when the work done to move the box was 100 J with a force of 15 N.
As given, W = 100 J, F = 15 N
We know, W = Fs
⇒ s = W/ F
⇒ s = 100 / 15
⇒ s = 6.6 m
A body of mass 5 kg is moving with an acceleration of 5 ms-2 gets a displacement of 2m, the work done by the force is:
We know, F = ma
F = 5 * 5
F = 25
We, also know;
W = F. s
= 25 * 2
W = 50 J
The energy possessed by an oscillating pendulum of a clock is
The energy possessed by an oscillating pendulum of a clock is called mechanicalenergy. It is a combination of kinetic energy and potential energy. Kinetic energy is the one that is due to the motion of an object whereas potential energy due to the virtue of an objects position.
To lift 700 kg of water, through a height of 10 m we use a pump. The work done in operating the pump is;
Take g = 10 m/s2.
Work done = mgh
= 700 kg x 10m/s x 10m = 70000 joule
A coolie was standing with 2 suitcases on his head and 2 bags in his hand. The luggage exerted a force of 159 N on him. What is the amount of work done by him?
Work done is zero, since there is no displacement of the coolie.
The winners of long jump each weighing 56 Kg, 40 Kg and 45 Kg stand on the podium. What is the work done by them on the podium while they receive the medals?
The winners of long jump each weighing 56kg,40kg and 45 kg stand on podium.Work done by them is 0 Ioule because there is no displacement and if the displacement is 0 then work done will be also 0.