Question:
A physics teacher explains to his students in class,the subject of motion under gravity.When a body is projected upwards from the ground with a certain velocity a,the velocity goes on decreasing at a constant rate due to gravitational pull of earth.At a certain height,this velocity becomes zero.The body cannot rise further.From this maximum height,the body begins to fall and its velocity goes on increasing at the same constant rate due to gravitational pull of earth.The body would strike the ground with exactly the same velocity (v = u),with which it was thrown initially. This is on the assumption that resistance due to air is zero, either way.
Read the above passage and answer the following questions:
(i) Why is velocity of body on striking the ground the same as initial velocity of projection of the body ?
(ii) Does the body take same time to fail to ground as the time it takes to reach the maximum height ?
(iii) What values of life do you learn from this brief study ?
Answer:
(i) The two velocities are equal in accordance with the principle of conservation of energy.During the upward journey,kinetic energy of the body is converted into its potential energy.And during the downward journey of the body,potential energy is converted back into the same kinetic energy.Of course, we assume that there are no stray losses of energy.
(ii) Yes, time taken to fall to ground from maximum height = time taken to go from ground to the maximum height,
(iii) From this brief study, we learn that output can at the most be equal to input,i.e., the degree of our success is always commensurate with our efforts.We reap what we harvest.The degree of sweetness is determined by the amount of sugar we put in.
Gravitation
Q 1.
State Kepler’s Laws of planetary motion. How did Newton guess inverse square law ?
Q 2.
A particle is dropped from a tower 180 m high.How long does it take to reach the ground ? What is the velocity when it touches the ground ? Take g =
Q 3.
Calculate the value of acceleration due to gravity on Moon. Given mass of Moon =
kg, radius of Moon = 1740 km
Q 4.
Why do sewing needles and all pins have a sharp pointed tip?
Q 5.
What is the value of ‘g’ at the centre of Earth ?
Q 6.
A boy on a clif£ 49 m high drops a stone.One second later,he throws a second stone after the first.They both hit the ground at the same time. With what speed did he throw the second stone ?
Q 7.
Two stones A and B are dropped from a multistoried building. A is dropped from 100 m and after some time B is dropped from 50 m height. Both of them reach the earth at the same time. Will they have equal velocity while reaching the ground ? Calculate and find out the answer (take g =10 m/s
2).
Q 8.
Which force is responsible for tides in the ocean at night ?
Q 9.
Fill in the blanks :
Every object in the universe ________ other bodies.
Q 10.
Can you calculate density of Earth from Newton’s law of gravitation ?
Q 11.
Suppose a planet exists whose mass and radius both,are half those of Earth.Calculate the acceleration due to gravity on the surface of this planet.
Q 12.
What do we call the gravitational force between the Earth and an object ?
Q 13.
The mass of Earth is
kg and that of moon is
kg. If the distance between the Earth and the Moon is
km, calculate the force exerted by Earth on the Moon. Given G =
.
Q 14.
What do you mean by acceleration due to gravity ?
Q 15.
At what height above the surface of the earth, the value of 'g' becomes 64% of its value at the surface of the earth. Take radius of the earth = 6400 km.
Q 16.
Fill in the blanks :
________ of an object is the force of gravity acting on it.
Q 17.
What is the importance of universal law of gravitation ?
Q 18.
What is value of ‘g’ on Earth ?
Q 19.
State and explain universal law of gravitation given by Newton.What is the importance of this law
Q 20.
Who measured the value of G first time, experimentally?
Q 21.
Calculate the force of gravitation between two objects of masses 80 kg and 1200 kg kept at a distance of 10 m from eachother.Given, G =
/
.
Q 22.
What happens to 'g' experienced by a body when it moves up the earth's surface?
Q 23.
Mass of an object is 10 kg. What is its weight on the Earth? (g = 9.8 m/s
2.)
Q 24.
Distinguish between mass and weight.
Q 26.
A stone drops from the edge of the roof.It passes a window 2 m high in 0.1 s.How far is the roof above the top of the window
Q 27.
An object is thrown vertically upwards and rises to a height of 10 m. Calculate (i) the velocity with which the object was thrown upwards and (ii) the time taken by the object to reach the highest point.
Q 28.
A body is projected upwards. What is its velocity at maximum height
Q 29.
Fill in the blanks :
Universal gravitational constant G = _____________.
Q 30.
The weight of a man is 690N which contains 5.3 x 10
-3 m
3 of blood. Find (a) blood's weight. Take density of blood is 1060 kg/m
3 (b) express it as a percentage of body weight.
Q 31.
Why will a sheet of paper fall slower than one that is crumpled into a ball?
Q 32.
Why does a block of plastic released under water come up to the surface of the water?
Q 33.
What is the SI unit of weight ?
Q 34.
Can a body have mass but no weight ?
Q 35.
Fill in the blanks :
When objects of different masses and sizes are dropped from same height would reach the ground at ______ (same time/different times), considering negligible air resistance.
Q 36.
Fill in the blanks :
The value of
g ________ (increases/decreases) as we go downwards from the earth's surface.
Q 37.
Why is G called the Universal Constant?
Q 38.
Does the force of attraction between two objects depend on the properties of intervening medium?
Q 40.
Fill in the blanks :
Assuming negligible air drag, if a steel ball and a table tennis ball are dropped from a height of 100 metres, both will reach the ground at ________ time(s). (same/different)
Q 41.
What are Plimsoll Lines?
Q 42.
Write the formula to find the magnitude of the gravitational force between the Earth and an object on the surface of the Earth.
Q 43.
Why is the weight of an object on the Moon (l/6)th its weight on the Earth ?
Q 44.
A stone is thrown vertically upward with an initial velocity of 40 m/s.Taking g =
,find the maximum height reached by the stone.What is the net displacement and the total distance covered by the stone ?
Q 45.
A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25 m/s. Calculate when and where the two stones will meet
Q 46.
Can a body have weight but no mass ?
Q 47.
Write down the equations of motion of objects under the influence of gravitational force of Earth.Explain the meaning of the symbols used.
Q 48.
Take a piece of thread.Tie a small piece of stone to one end of the thread. Hold the other end of the thread in hand and whirl it round.Note the motion of the stone.If the thread breaks suddenly,the stone flies off along tangent to the circle at that instant.
Read the above passage and answer the following questions:
(i) Which force Is whirling the stone ? Who is providing this force ?
(ii) Why does the stone fly off along the tangent to the circle at that instant ?
(iii) What values of life do you learn from this ?
Q 49.
Fill in the blanks :
The force which keeps the object moving in a circular path is called _____ force. It always acts towards the _____.
Q 50.
Fill in the blanks :
Kepler's Law of ______ states that a line that connects a planet to the Sun sweeps out equal ______ in the plane of the planet's orbit in _____time intervals.
