Laws of Motion | NCERT Solutions | Q 1 to 20
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Q5.1 00:00:37 Q5.2 00:08:10 Q5.3 00:13:27
Q5.4 00:21:25 Q5.5 00:27:52 Q5.6 00:32:56
Q5.7 00:37:46 Q5.8 00:42:19 Q5.9 00:47:30
Q5.10 00:52:14 Q5.11 01:04:43 Q5.12 01:16:17
Q5.13 01:22:00 Q5.14 01:34:52 Q5.15 01:43:26
Q5.16 01:54:26 Q5.17 02:01:21 Q5.18 02:06:07
Q5.19 02:11:56 Q5.20 02:16:23
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Laws of Motion | NCERT Solutions | 5.1 to 5.20 |
5.1 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 on a rough road,
(e) a high-speed electron in space far from all material objects, and free of electric and magnetic fields.
5.2 A pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble,
(a) during its upward motion,
(b) during its downward motion,
(c) at the highest point where it is momentarily at rest.
Do your answers change if the pebble was thrown at an angle of 45° with the horizontal direction?
Ignore air resistance.
5.3 Give the magnitude and direction of the net force acting on a stone of mass 0.1 kg,
(a) just after it is dropped from the window of a stationary train,
(b) just after it is dropped from the window of a train running at a constant velocity of 36 km/h,
(c) just after it is dropped from the window of a train accelerating with 1 ms-2,
(d) lying on the floor of a train which is accelerating with 1 ms-2, the stone being at rest relative to the train. (Neglect air resistance throughout).
5.4 One end of a string of length 𝑙 is connected to a particle of mass m and the other to a small peg on a smooth horizontal table. If the particle moves in a circle with speed 𝑣 the net force on the particle (directed towards the centre) is:
(i) 𝑇 (ii) 𝑇−(𝑚𝑣^2)/𝑙 (iii) 𝑇+(𝑚𝑣^2)/𝑙 (iv) 0
T is the tension in the string. [Choose the correct alternative].
5.5 A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 ms-1. How long does the body take to stop?
5.6 A constant force acting on a body of mass 3.0 kg changes its speed from 2.0 ms-1 to 3.5 ms-1 in 25 s. The direction of the motion of the body remains unchanged. What is the magnitude and direction of the force?
5.7 A body of mass 5 kg is acted upon by two perpendicular forces 8 N and 6 N. Give the magnitude and direction of the acceleration of the body.
5.8 The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg.
5.9 A rocket with a lift-off mass 20,000 kg is blasted upwards with an initial acceleration of 5.0 ms-2. Calculate the initial thrust (force) of the blast.
5.10 A body of mass 0.40 kg moving initially with a constant speed of 10 m s-1 to the north is subject to a constant force of 8.0 N directed towards the south for 30 s. Take the instant the force is applied to be t = 0, the position of the body at that time to be 𝑥 = 0, and predict its position at t = -5 s, 25 s, 100 s.
5.11 A truck starts from rest and accelerates uniformly at 2.0 m s-2. At t = 10 s, a stone is dropped by a person standing on the top of the truck (6 m high from the ground). What are the (a) velocity, and (b) acceleration of the stone at t = 11s ? (Neglect air resistance.)
5.12 A bob of mass 0.1 kg hung from the ceiling of a room by a string 2 m long is set into oscillation. The speed of the bob at its mean position is 1 m s-1. What is the trajectory of the bob if the string is cut when the bob is (a) at one of its extreme positions, (b) at its mean position.
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