Laws of Motion
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Question : 1
Total: 40
[For simplicity in numerical calculations, take g = 10 m s– 2
Give the magnitude and direction of the net force acting on
(a) a drop of rain falling down with a constant speed,
(b) a cork of mass 10 g floating on water,
(c) a kite skillfully held stationary in the sky,
(d) a car moving with a constant velocity of 30 km h– 1
(e) a high-speed electron in space far from all material objects, and free of electric and magnetic fields.
Give the magnitude and direction of the net force acting on
(a) a drop of rain falling down with a constant speed,
(b) a cork of mass 10 g floating on water,
(c) a kite skillfully held stationary in the sky,
(d) a car moving with a constant velocity of 30 km h
(e) a high-speed electron in space far from all material objects, and free of electric and magnetic fields.
Solution:
(a) Since the drop of rain is falling downward with a constant speed,so the acceleration of the rain drop is zero. Hence according to Newton’sfirst law of motion, net force on the drop is zero i.e. a = 0,
∴ From F = ma, F = 0.
(b) As the cork is floating in water, so the weight of the cork is balancedby upthrust (equal to weight of water displaced). Therefore net force onthe cork is zero.
(c) Since the kite is held stationary, so its acceleration is zero. Henceaccording to Newton’s first law of motion, the net force acting on the kiteis zero.
(d) Since the car is moving with a constant velocity, so its accelerationis zero. Hence according to Newton’s first law of motion, the net forceacting on the car is zero.
(e) As there are no electric and magnetic fields and no material(gravitating) objects around, so no force (gravitational/electric/magnetic)is acting on the electron, so net force on it is zero.
∴ From F = ma, F = 0.
(b) As the cork is floating in water, so the weight of the cork is balancedby upthrust (equal to weight of water displaced). Therefore net force onthe cork is zero.
(c) Since the kite is held stationary, so its acceleration is zero. Henceaccording to Newton’s first law of motion, the net force acting on the kiteis zero.
(d) Since the car is moving with a constant velocity, so its accelerationis zero. Hence according to Newton’s first law of motion, the net forceacting on the car is zero.
(e) As there are no electric and magnetic fields and no material(gravitating) objects around, so no force (gravitational/electric/magnetic)is acting on the electron, so net force on it is zero.
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