9th class 2nd lesson Laws of Motion

class-09

Questions&Answer

02.laws of motion.

1. Explain the reasons for the following.
a) When a carpet is beaten with a stick, dust comes out of it.
b) Luggage kept on the roof of a bus is tied with a rope.
c) A pace bowler in cricket runs in from a long distance before he bowls..

A. (a) When a carpet is beaten with a stick, the dust particles comes out of it due to static inertia.
Actually on beating, the carpet moved and the dust particles still remains their position due
to static inertia. So they comes out of it.
(b) If the luggage kept on the roof of a bus is not tied with a rope, due to dynamic inertia they
can fall down from the top of the bus when the bus stopped suddenly. So they were tied with a rope.
(c) A pace bowler in cricket runs in from a long distance before he bowls, to give sufficient dynamic inertia to the ball.

2. Two objects have masses 8kg and 25kg. Which one has more inertia? Why?

A. (i) The mass of an object is a measure of inertia.
(ii) If mass of object increases, the inertia of that object also increases.
(iii) So the object having 25 Kg. mass has more inertia than the 8 Kg. object.

3. What is the momentum of a 6.0 kg bowling ball with a velocity of 2.2 m/s?

A. Mass of the ball (m) = 6.0 Kg
Velocity of the ball (v) = 2.2 m/s
Momentum (P) = mv
= 6.0 x 2.2
= 13.2 Kg.m/s

4. Two people push a car for 3 s with a combined net force of 200 N. (a) Calculate the impulse provided to the car. (b) If the car has a mass of 1200 kg, what will be its change in velocity?

A. (a) Net force applied on car (F) = 200 N
Time (t) = 3 sec.
Impulse = F x t
= 200 x 3
= 600 N-s.
(b) Mass of the car (m) = 1200 Kg.
Net force applied (F) = 200 N
Time (t) = 3 sec
Impulse = Change in momentum
F x t = m (v-u)
200 x 3 = 1200 (v-u)
600 = 1200 (v-u)
v-u=600/1200=1/2 = 0.5 m/s
Change in velocity = 0.5 m/s

5. What force required to produce an acceleration of 3 m/s2 in an object of mass 0.7 Kg.?

A. Mass of the object (m) = 0.7 Kg.
Acceleration (a) = 3 m/s2
The required force (F) = ma
= 0.7 x 3
= 2.1 N

6. An object of mass 5 Kg is moving with a velocity of 10 ms-1. A force is applied so that in
20 s, it attains a velocity of 25 ms-1. What is the force applied on the object?

A. Mass of object (m) = 5 Kg.
Initial velocity (u) = 10 m/s
Time (t) = 20 s
Final velocity (v) = 25 m/s
= m(v-u/t)
= 5 x(25-10/20)
= 5 x(15/20)
=15/4
=3.75 N

7. Illustrate an example of each of the three laws of motion.

A. Newton’s first law of motion:

A body continues its state of rest or uniform motion unless a net
force acts on it.

Ex:

When the bus which is at rest begins to move suddenly, the person standing in the bus falls
backward. This happens because, the net force not acts on the person, so that he still remains
his state of motion.

Newton’s second law of motion:

The rate of change of momentum of a body is directly
proportional to the net force acting on it . And it takes place in the direction of net force.

Ex:

The fielder while catching a fast moving ball, pulls back his arms to experience the smaller
force on his hands. This is due to change in momentum takes a long time.

Newton’s third law of motion:

For every action, there should be equal and opposite reaction.

Ex:

When birds fly, they push the air downwards with wings and the air pushes back the bird in
upward direction with same force. This way the birds can fly.

8. If a fly colloids with the wind shield of a fast moving bus,
(a) is the impact force experienced , same for the fly and bus? Why?
(b) Is the same acceleration experienced by the fly and the bus? Why?

A. (a) The impact of force experienced by the bus and fly are same. (As per Newton’s third law of
motion force and anti forces are equal and in opposite direction.)
(b) The acceleration of the bus and fly are different. It depends upon various factors. In general
more mass gets less acceleration. So the acceleration of bus is less.

9. Divya observed a horse pulling a cart. She thought that cart also pulls the horse with
same force in opposite direction. As per third law of motion the cart should not move
forward. But her observation of moving cart raised some questions in her mind. Can you
guess what questions were raised in her mind?

A. The following questions may arise to divya :
(i) How much force applied by horse on the floor?
(ii) There are two forces. The force by horse on cart and the force by cart on horse. Are the
forces same ?
(iii) How the cart moves?
(iv) Why the cart apply force on the floor to pull cart?

10. How do you appreciate Galileo’s thought of “any moving body continues in the state
only until some external force acts on it.” Which is a contradiction to the Aristotle’s
belief of “any moving body naturally comes to rest.”

A. (i) Aristotle believed that any moving body comes to rest naturally. It may be true.
(ii) But Galileo thought that any moving body continues its state only until some external force br acts on it.
(iii) Also he proved his statement with the experiments of an inclined plane.
(iv) He stated that if no force acts on a body, it moves to infinite distance.
(v) This leads to developments in various concepts in dynamics.
(vi) So I appreciate Galileo’s thought.

11. If a car is traveling westwards with a constant speed of 20 m/s, what is the resultant
force acting on it?

A. The car is travelling with a constant speed. So no net force acts on it.

12. A man of mass 30 Kg uses a rope to climb which bears only 450N. What is the
maximum acceleration with which he can climb safely?

A. Mass of the man (m) = 30 Kg
Tension in the rope (T) = 450 N
Maximum acceleration (a) = ?
T = F
T = ma
450 = 30 x a
a = 450/30
a = 15 m/s²

13. A passenger in moving train tosses a coin which falls behind him. It means that the
motion of the train is ….

(a)Accelerated (b)Uniform

(c)Retarded (d)Circularmotion

A. Accelerated motion in particular conditions.
But in general the coin fall in the hand.