iv MASS AND ITS MEASUREMENT 53 



A Gram is the mass of pure water which will exactly fill a cubic 

 centimetre at a temperature of 4 C. 



The same prefixes are used to designate the multiples and sub- 

 multiples of the gram as are used in the case of the metre. 



Law of Gravitation. Every body in nature attracts every other 

 body with a force directly proportional to the product of their 

 masses and inversely proportional to the square of the distance 

 between them ; and the direction of the force is in the line joining 

 the centres of the bodies. 



The Weight of a body is the force with which it tends to move 

 towards the earth. 



The Weight of a given mass varies from place to place. Its weight 

 at the sea-level is more than on the top of a mountain. Its weight 

 is also greater when it is in the neighbourhood of the poles than 

 when at the equator. 



Masses are determined by means of the Balance. It is found that 

 when the pans of the balance are at equal distances from the sup- 

 ports, there is equilibrium when the masses are equal. 



The Moment of a Force about any point is the product obtained by 

 multiplying this force by the vertical distance between the point 

 and the line of action of the force. 



When a Balance is in equilibrium the moments, about the point of 

 suspension, of the forces equal to the weights of the masses are equal. 

 When the balance is true the arms are of equal lengths and con- 

 sequently the weights are equal also. 



QUESTIONS ON CHAPTER IV. 



1. Define the mass and weight of a material body, carefully dis- 

 tinguishing between the terms. 



2. Give the British and metric measures of mass. 



3. State Newton's law of gravitation, explaining clearly the rule 

 of inverse squares. 



4. What is a spring balance and what can be measured with it ? 



5. Explain fully what different readings, if any, are given by a 

 spring balance under the following circumstances and account for 

 them as far as you can. 



(a) When a spring balance carrying a given mass is taken 

 from the equator to the north pole. 



(b) When the same balance is carried down a deep mine. 



(f) When the balance is taken to the summit of a high 

 mountain. 



6. Describe an instrument for determining the mass of a material 

 body and explain how it is used. 



7. What do you understand by the moment of a force ? Explain 

 your definition by an example. . 



8. What is the condition of things, from the point of view of 

 moments, when a balance with both pans loaded is in equilibrium. 



9. What particulars about a given mass could you determine (a) 

 by using a spring balance, (b) by means of a pair of scales ? 



10. Multiply 10 "4 square centimetres by 15 '5 decimetres, and 

 state the result both in cubic centimetres and in litres. 



