Question 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 m s 2,

(d) lying on the floor of a train which is accelerating with 1 m s 2, the stone being at rest relative to the train. Neglect air resistance throughout.

(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 m s 2,

(d) lying on the floor of a train which is accelerating with 1 m s 2, the stone being at rest relative to the train. Neglect air resistance throughout.

Answer

**(a)**1 N; vertically downward

Mass of the stone, *m* = 0.1 kg

Acceleration of the stone, *a* = g = 10 m/s^{2}

As per

*F* =* **ma = m*g

= 0.1 × 10 = 1 N

Acceleration due to gravity always acts in the downward direction.

**(b)**1 N; vertically downward

The train is moving with a constant velocity. Hence, its acceleration is zero in the direction of its motion, i.e., in the horizontal direction. Hence, no force is acting on the stone in the horizontal direction.

The net force acting on the stone is because of acceleration due to gravity and it always acts vertically downward. The magnitude of this force is 1 N.

**(c)**1 N; vertically downward

It is given that the train is accelerating at the rate of 1 m/s^{2}.

Therefore, the net force acting on the stone, *F*' = *ma* = 0.1 × 1 = 0.1 N

This force is acting in the horizontal direction. Now, when the stone is dropped, the horizontal force *F*,' stops acting on the stone. This is because of the fact that the force acting on a body at an instant depends on the situation at that instant and not on earlier situations.

Therefore, the net force acting on the stone is given only by acceleration due to gravity.

*F* = *mg* = 1 N

This force acts vertically downward.

**(d)**0.1 N; in the direction of motion of the train

The weight of the stone is balanced by the normal reaction of the floor. The only acceleration is provided by the horizontal motion of the train.

Acceleration of the train, *a* = 0.1 m/s^{2}

The net force acting on the stone will be in the direction of motion of the train. Its magnitude is given by:

*F* = *ma*

= 0.1 × 1 = 0.1 N

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State the number of significant figures in the following:

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^{2}(b) 2.64 x 10

^{24}kg(c) 0.2370 g cm

^{-3}(d) 6.320 J

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^{-2}(f) 0.0006032 m

^{2} - Q:- A physical quantity P is related to four observables a, b, c and d as follows :
The percentage errors of measurement in a, b, c and d are 1%, 3%, 4% and 2%, respectively. What is the percentage error in the quantity P ? If the value of P calculated using the above relation turns out to be 3.763, to what value should you round off the result ?

">A physical quantity P is related to four observables a, b, c and d as follows :

The percentage errors of measurement in a, b, c and d are 1%, 3%, 4% and 2%, respectively. What is the percentage error in the quantity P ? If the value of P calculated using the above relation turns out to be 3.763, to what value should you round off the result ?

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What amount of heat must be supplied to 2.0 x 10

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[Note: You will appreciate from this exercise why it is better to define average speed as total path length divided by time, and not as magnitude of average velocity. You would not like to tell the tired man on his return home that his average speed was zero!]

">A man walks on a straight road from his home to a market 2.5 km away with a speed of 5 km h

^{ –1}. Finding the market closed, he instantly turns and walks back home with a speed of 7.5 km h^{–1}. What is thea) magnitude of average velocity, and

b) average speed of the man over the interval of time (i) 0 to 30 min, (ii) 0 to 50 min, (iii) 0 to 40 min?

[Note: You will appreciate from this exercise why it is better to define average speed as total path length divided by time, and not as magnitude of average velocity. You would not like to tell the tired man on his return home that his average speed was zero!]

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">
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- Q:-
Fill in the blanks

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- Q:-
Given below are some functions of x and t to represent the displacement (transverse or longitudinal) of an elastic wave. State which of these represent (i) a traveling wave, (ii) a stationary wave or (iii) none at all:

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- Q:-
Fill in the blanks by suitable conversion of units:

(a) 1 kg m

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(c) 3.0 m s

^{–2}=.... km h^{–2}(d) G = 6.67 × 10

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A LASER is a source of very intense, monochromatic, and unidirectional beam of light. These properties of a laser light can be exploited to measure long distances. The distance of the Moon from the Earth has been already determined very precisely using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s to return after reflection at the Moon's surface. How much is the radius of the lunar orbit around the Earth?

Vakyanshsrivastav
2018-08-24 18:55:36

mast hai

Ambika
2018-07-30 11:59:57

It was of great help Thank you

Ansif
2018-02-26 23:31:19

Very helpful work

bhh
2018-02-24 19:15:24

bad

anami
2018-01-29 19:22:38

good work

anami
2018-01-29 19:22:17

great answers

anami
2018-01-29 19:19:11

good job

anami
2018-01-29 19:18:49

good work keep it up

Faizan
2016-09-22 17:08:39

Its was of great help .keep up the goodwork

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