INDIAN SCHOOL AL WADI AL KABIR

DEPARTMENT OF SCIENCE 2017 – 2018

CLASS: IX PHYSICS DATE: 21/8/2017

WS. NO.03 TOPIC : GRAVITATION NAME:

1) What is the magnitude of the gravitational force acting on a body of mass ‘x’ ? 2) (a) Prove that if the earth attracts two bodies placed at the same distance from the centre of earth, with equal force; then their masses will be the same. (b) Mathematically express the acceleration due to gravity that is expressed by a free falling (c) Why is ‘G’ called a universal constant ? 3) State two factors on which value of ‘g’ depends. 4) A stone is dropped from a height of 20 m. (a) How long will it take to reach the ground ? (b) What will be its speed when it hits the ground ? (g=10m/s2). (Ans: (a)2s (b)20m/s) 5) Write three differences between gravitational acceleration (g) and gravitational constant (G). 6) How does the force of gravitation between two objects change when the distance between them is reduced to half ? 7) Two electrons each of mass 9.1× 10-31 kg are at a distance of 10×10-10m. Calculate the gravitational force of attraction between them (G = 6.67 ×10-11 Nm2kg-2) 8) State the universal law of gravitation. Derive its expression. 9) When is an object said to be in free fall ? What is meant by acceleration due to gravity ? Derive an expression for acceleration for an object falling freely. 10) List three differences between mass and weight. 11) State any four natural phenomena explained by universal law of Gravitation 12) A) What keeps the moon in uniform circular motion around the earth? B) Why do astronauts in space feel weightless? 13) Name the positions on earth where the value of ‘g’ is (i)maximum (ii) minimum? Justify your answer Page 1 of 2

14) Account for the following i) On moon, man feels lighter than earth ii) Mass is scalar while weight is a vector quantity 15) Give reason – stone falls towards the earth but earth do not rise towards the stone. 16) Explain what happens to the force between two objects if (i)the mass of one object is doubled? (ii) the distance between the object is tripled? 17) A ball is dropped from the roof of a building . It takes 10 seconds to reach the ground. Find the height of the building. (Ans:490m) 18) If the distance between two objects is increased four times , then by how many times will the mass of one of the objects be changed to maintain the same gravitational force. 19) An object is dropped from a tower 180 m high. How long does it take to reach the ground? (Take g =10 m/s2 ) (Ans: 6s) 20) Ravi weighs few grams of sugar first at the poles and then at the equator. Will he observe the same weight in both cases ? Justify your answer. 21) Two solid objects of masses 1kg and 2kg are dropped from a helicopter at the same time. Which one will reach the ground earlier? Justify your answer with suitable reason. PREPARED BY : MS. VIPINA GANGADHARAN Page 2 of 2