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Physics Part I

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Class 10 Class 12
Two inclined frictionless tracks, one gradual and the other steep meet at A from where two stones are allowed to slide down from rest, one on each track (Fig. 6.16). Will the stones reach the bottom at the same time? Will they reach there with the same speed? Explain. Given θ1 = 30°, θ2 = 60°, and = 10 m, what are the speeds and times taken by the two stones?

The given question can be illustrated using the figure below: 


AB and AC are two smooth planes inclined to the horizontal at ∠θ1 and ∠θ2 respectively.

The height of both the planes is the same, therefore, both the stones will reach the bottom with same speed.

As P.E. at O = K.E. at A = K.E. at B 


     mgh = 1/2 mv12 = 1/2 mv2

∴                          v1 = v

As it is clear from fig. above, acceleration of the two blocks are

 a1 = g sin θ1 ,

 a2 = g sin θ

As θ2 > θ

∴       a2 > a

From v = u + at 

            = 0 + at 

rightwards double arrow      t = v/

As ∝ 1/a, and a2 > a

∴    t2 < t

That is, the second stone will take lesser time and reach the bottom earlier than the first stone.


Define watt.

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

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. 

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 .

What is watt?

SI unit of power is Watt. 

What are different units of energy?

The different units of energy are :

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