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Work, Energy and Power

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Physics Part I

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Physics

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12

Consider a drop of rain water having mass 1 g falling from a height of 1 km. It hits the ground with a speed of 50 m/s. Take g constant with a value 10 m/s2. The work done by the (i) gravitational force and the (ii) resistive force of air is

  • (i) – 10 J (ii) –8.25 J

  • (i) 1.25 J (ii) –8.25 J

  • (i) 100 J (ii) 8.75 J

  • (i) 10 J (ii) –8.75 J


D.

(i) 10 J (ii) –8.75 J

wg+ wa= Kf– Ki
mgh space plus space straight w subscript straight a space equals space 1 half mv squared space minus 0
10 to the power of negative 3 end exponent space straight x space 10 space space straight x space 10 cubed space plus space straight w subscript straight a space equals space 1 half space straight x space 10 to the power of negative 3 end exponent space straight x space left parenthesis 50 right parenthesis squared

wa= –8.75 J i.e. work done due to air resistance and work done due to gravity = 10 J

wg+ wa= Kf– Ki
mgh space plus space straight w subscript straight a space equals space 1 half mv squared space minus 0
10 to the power of negative 3 end exponent space straight x space 10 space space straight x space 10 cubed space plus space straight w subscript straight a space equals space 1 half space straight x space 10 to the power of negative 3 end exponent space straight x space left parenthesis 50 right parenthesis squared

wa= –8.75 J i.e. work done due to air resistance and work done due to gravity = 10 J

4191 Views

Is work a scalar or a vector quantity?

Work is dot product of two vectors. i.e. F with rightwards arrow on top times S with rightwards arrow on top.

And dot product is a scalar quantity. Therefore, work is a scalar quantity. 
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Define watt.

Power is said to be one watt if one joule of work is done in one second. 
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What is watt?

SI unit of power is Watt. 
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Define the unit joule.

Work done is said to be one joule if one newton of  force displaces the body through a distance of one meter in the direction of applied  force .
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What are different units of energy?

The different units of energy are :

(i) Joule            
(ii) Erg
(iii) eV            
(iv) KWh
(v) Calorie.

804 Views