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Matter In Our Surroundings
Question
Wired Faculty App

When is the work done by a force said to be
(i) positive and (ii) negative?

Answer

(i) Positive work: when the force acting on the object is in the direction of displacement, then work done by a force is positive. 

Positive work done is given by, 

                   W = Fs cos 0° = Fs
Examples:

(i) When a body falls freely under gravity (θ = 0°), the work done by the force of gravity is positive.

(ii) When a horse pulls a cart, the applied force and displacement are in the same direction, the work done by the horse is positive.

(ii) Negative work: when the force acts in a direction opposite to the direction of displacement, work done is negative. 

Negative work done is given by, 

            W = Fs cos 180°
                 = Fs (-1)
                 = - Fs

Examples: 

(i) When a body is lifted up, the work done by the gravitational force is negative. Gravitational force is acting downwards and weight is lifted up. 

(ii) When brakes are applied to a moving vehicle, the work done by the braking force is negative. The braking force acts in the backward direction while the displacement of the vehicle is in the forward direction.

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Some More Questions From Matter In Our Surroundings Chapter

Convert the following temperatures to Kelvin scale.

35°C

Give reason for the following observations.

Naphthalene balls disappear with time without leaving any solid.

Give reason for the following observations.

We can get the smell of perfume sitting several metres away.

What is the physical state of water at

a) 25°C
b)00C
c)1000C

What is the physical state of water at
0°C

What is the physical state of water at
100°C

Give two reasons to justify
Water at room temperature is a liquid.

Give two reasons to justify
An iron almirah is a solid at room temperature.

Why is ice at 273K more effective in cooling than water at the same temperature ?

What produces more severe burns, boiling water or steam ?