Question:
A ball is thrown with some velocity 'u' m/s. Show that under free fall, it will fall the ground with same velocity.
Answer:
When the ball is thrown upwards, it will reach certain height (h) and starts falling. At height 'h', the final velocity will be (v) = 0.
Max. height reached by the ball, (acceleration = -g)
Using equation v2 - u2 =2aS
⇒ 02 - u2 = - 2gh
⇒ h = u2/2gh ... (i)
In second case when the ball starts to fall, the initial velocity = 0. It will accelerate due to gravity i.e. a = +g and reach ground with velocity (say v2)
Using equation v2 - u2 =2aS
⇒ v22 - 02 =2gh
⇒ v22 = 2g × u2/2g (from eqn. i)
⇒ v22 = u2
⇒ v2 = u
∴ the ball will reach ground with same velocity.
Gravitation
Q 1.
Calculate the force of gravitation between the Earth and the Sun,given that the mass of the Earth =
kg and mass of the Sun =
kg. The average distance between the two is
m.
Q 2.
State Kepler’s Laws of planetary motion. How did Newton guess inverse square law ?
Q 3.
What do we call the gravitational force between the Earth and an object ?
Q 4.
Can you calculate density of Earth from Newton’s law of gravitation ?
Q 5.
Fill in the blanks :
________ of an object is the force of gravity acting on it.
Q 6.
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 7.
What is the value of ‘g’ at the centre of Earth ?
Q 8.
Why do sewing needles and all pins have a sharp pointed tip?
Q 9.
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 10.
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 11.
What do you mean by acceleration due to gravity ?
Q 12.
Fill in the blanks :
Every object in the universe ________ other bodies.
Q 13.
Calculate the value of acceleration due to gravity on Moon. Given mass of Moon =
kg, radius of Moon = 1740 km
Q 14.
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 15.
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 16.
Calculate the force of gravitation between the earth and the Sun, given that the mass of the earth = 6 × 10
24 kg and of the Sun = 2 × 10
30 kg. The average distance between the two is 1.5 × 10
11 m.
Q 17.
Fill in the blanks :
Weight of an object is the product of ______ and ________.
Q 18.
Write the standard equations of motion when an object is thrown upwards with some initial velocity?
Q 19.
What is the gravitational acceleration of a spaceship at a distance equal to two Earth’s radius from the centre of the Earth ?
Q 20.
A ball is thrown up with a speed of 15 m/s.How high will it go before it begins to fall ? Take g=
Q 21.
Fill in the blanks :
The gravitational force between the earth and the object is generally called ____________.
Q 22.
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 23.
Which force revolves Moon around Earth ?
Q 24.
Which force is responsible for tides in the ocean at night ?
Q 25.
Does Newton’s third law of motion apply to gravitational force ?
Q 26.
Can a body have weight but no mass ?
Q 27.
Distinguish between gravitation and gravity.
Q 28.
Define g and G.Establish relation between them.
Q 29.
Whenever a body is thrown up with a certain velocity,the upward motion is opposed by gravitational pull of earth and also by the resistance of air.
Therefore, velocity of the body goes on decreasing. When this velocity becomes zero,the body cannot rise further.It has attained,what is called ‘maximum height’.From this height,the body begins to fall downwards under the action of gravity.
Read the above passage and answer the following questions:
(i) At the highest point,what are the velocity and acceleration of the body?
(ii) What is the maximum height attained by a body thrown upwards with a velocity of 19.6 m/s ? Take g =
.
(iii) What values in life do you learn from this concept ?
Q 30.
A ball is thrown upwards from the ground of a tower with a speed of 20 m/s. There is a window in the tower at the height of 15m from ground. How many times and when will the ball pass the window? (g = 10 m/s
2)
Q 31.
Fill in the blanks :
The average acceleration of free fall on the surface of earth is ______ .
Q 32.
What are Plimsoll Lines?
Q 33.
You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
Q 34.
The distance of planet Jupiter from the sun is 5-2 times that of Earth.Find the period of revolution of Jupiter around sun.
Q 35.
A force of 20 N acts upon a body whose weight is 9.8 N.What is the mass of the body and how much is its acceleration ?
Q 36.
What are the differences between the mass of an object and its weight ?
Q 37.
Explain what is meant by ‘free fall’and acceleration due to gravity.
Q 38.
Fill in the blanks :
The value of
g ________ (increases/decreases) as we go downwards from the earth's surface.
Q 39.
Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object?
Q 40.
Fill in the blanks :
Motion of body with constant speed in a circular path is called ______________.
Q 41.
Fill in the blanks :
All other variables remaining constant, the force of gravitation is ________ proportional to the square of distance between the two objects.
Q 42.
What do you mean by Thrust or Buoyant force?
Q 43.
The volume of a 500 g sealed packet is 350 cm
3. Will the packet float or sink in water if the density of water is 1 g cm
-3? What will be the mass of the water displaced by this packet?
Q 44.
A force of 200 N acts on a surface of area 10 cm
2. Calculate thrust and pressure. How much the pressure changes if the area of contact increases to 50 cm
2.
Q 45.
State the universal law of gravitation.
Q 46.
A stone is released from the top of a tower of height 19.6 m.Calculate its final velocity just before touching the ground.
Q 47.
What is the source of centripetal force that a planet requires to revolve around the Sun ? On what factors does that force depend ?
Q 48.
What is the value of G (i) on Earth (ii) on Moon ?
Q 49.
Does gravitational force between two objects depend on medium between them ?
Q 50.
A body is projected upwards. What is its velocity at maximum height
