Thrust and Pressure 9th class(lec 3)

 video lecture for Thrust and Pressure

Question 1: It is difficult to lift a school bag with the help of a strap made of a thin and strong string, why?

Question 2: The atmosphere is held to the earth by

(a) gravity

(b) wind

(c) clouds

(d) earth’s magnetic field

Question 3: A girl stands on a box `60 cm` in length, `40 cm` in breadth, and `20 cm` in width in three ways. In which of the following cases, the pressure exerted by the brick will be

(a) maximum when length and breadth form the base

(b) maximum when breadth and width form the base

(c) maximum when width and length form the base

(d) the same in all the above three cases

Question 4: The weight of any person on the moon is about `1 /6` times that on the earth. He can lift a mass of `15 kg` on the earth. What will be the maximum mass, which can be lifted by the same force applied by the person on the moon?

Question 5: What is the effect of mass of the body on acceleration

due to gravity (g)?

Question 6:  If the ratio of the weights of a body of mass `‘m’` measured on two different planets `‘A’` and `‘B’` is `1 : 2` and the ratio of radii of two planets `‘A’` and `‘B’` is `2 : 4`,

respectively, then the ratio of the masses of two planets is, respectively _______.

a) `1/2`

b) `3/2`

c) `2/3`

d) `1/8`

Question 7: Explain how the apparent weight of a person, taking

a ride in a roller-coaster varies.

Question 8: If the mass of the Earth is decreased by `10%` keeping

its size constant, how is the weight of a body on the

Earth affected?

Question 9: Weights of two bodies are `20 N` and `30 N`. If they are separated by a distance of `2 m`, what is the force of gravitation acting between them? (`g=10ms^{-2}`)

a) `10.4\times10^{-11}N`

b) `9.4\times10^{-11}N`

c) `10.4\times10^{-10}N`

d) `10.4\times10^{11}N`

Question 10: A body weighs `98 N` on the Earth. How much 

does it weigh on the moon? (Take `g_{E} = 9.8 m s^{−2}, gm = 1.61 m s^{−2}`)

a) `10 N`

b) `12 N`

c) `14 N`

d) `16 N`

Question 11: The weight of a body of mass `3 kg` at a height of `12.8\times10^{6} m` from the surface of the Earth is _____.

(a) `9.75 N`  

(b) `1.46 N`

(c) `3.26 N`  

(d) `4.36 N`

Question 12: Spring balance measures ________ of a body in air.

(a) actual weight of a body

(b) apparent weight of a body

(c) mass of a body

(d) both mass and weight of a body

Question 13: The value of acceleration due to gravity on the Earth

at a distance of `29,000 km` from the surface is `0.3 m s^{-2}`. The value of acceleration due to gravity at the same height on a planet whose mass is `66.70\times10^{22}kg` and diameter is `8700 km` is_____ `ms^{-2}`.

 (Take `G = 6.67\times10^{-11} N m^{2}kg^{-2}`).

(a) `0.05`   

(b) `0.04`

(c) `0.06`   

(d) `0.09`

Answer Key

Q1:  Answer- If the school bag is lifted with the help of a strap made of a thin and robust string, then this task will be challenging because of the thrust, the school bag will impact the hand or shoulder vertically and on a relatively small area which will be painful.

Q2: (a) The atmosphere is held to the earth by gravity.

Q3:  Solution:

(b) As pressure=force/area=weight/area

Weight of girl is same in all the three cases. So, the pressure exerted by the brick will be maximum when area is least, i.e., when the breadth and width are from the base.

Q4:  Solution:

Since weight of a person on the moon is

`1/6` time that on earth

I.e. `W_{M}=1/6W_{E}`

`g_{M}=1/6g_{E}`                                             ……(1)

Force applied by the person to lift a mass of `15 kg` on earth is

`F=mg_{E}=15g_{E}N`

So maximum mass which can lift the same force applied by the person on moon is

`m'=\frac{F}{g_{M}}=\frac{15g_{E}}{g_{M}}=\frac{15g_{E}}{g_{E}/6}=15\times6=90 Kg`

Q5:   It will decreases.

Q6:   (d)

Q7:  if you were to sit on a scale during a roller coaster ride, you would see your "weight" change from point to point on the track.

Q8:   `10%` more.

Q9:   (a),   Q10:   (d),   Q11:   (C),   Q12:   (a),   Q13:   (b)