Question & answer-2 of ch-3 (force) of class-6

 11. What do you mean by the gravitational force? give an example to the illustrate it.

Ans. Gravitational force refers to the the force of of attraction between Any  two bodies  of the the universe. this was discovered bye Isaac Newton  in 1665, when he Saw an apple falling down the tree.eg. falling of a ball due to gravitational force.

12. Define  the term "weight of a body."

Ans. The weight of a body is the force by which are attracts that body .

W= m× g, 

w= weight

m= mass of a body

 g= acceleration due to gravity .g=9.8 ms-²  or nearly  taken as 10m/s².

Eg if a bodys mass is  10kg.

Weight of this body would be

W=m×g

    =10 kg×9.8 m/s²

    =98.0 kgm/s²

    =98.0 N

13. What do you understand by the term friction?

Ans. Friction is the force that opposes the relative motion between the two surfaces in contact with each other.

14. Give an an example to illustrate the existence of force of friction.

Ans. If we roll a ball on a plane ground , it moves up to a certain distance and then stops.Ball stops after covering some distance because of frictional force between the ball & the ground.

15. What is the cause of friction?

Ans. The  force of friction arises because of the interlocking of the irregular projections on the two surfaces in contact.

16. State two factor which directly affect the force of friction.

Ans. The two factors which affect the force of friction are:

  a)smoothness or roughness of the surface.

   b)The weight of the moving body on the surface.

17. In  which case will there be more friction between the truck and the road : when the truck is empty or when  it is loaded?

Ans we know that friction depends on the weight of a body. If a body is heavy then friction would be more.so loaded truck would be heavier,and hence friction would be more between the truck and the road.

18. Which offer more friction on a body : a glass surface or a wooden surface?

Ans. Since friction depends on the smoothness of a surface.more smoother a surface,lesser is the friction.wooden  surface is less smoother than the glass,so wooden surface offers more friction.

19. Name the three kinds of friction.

Ans.The three kinds of friction are:

a)Static friction- the the maximum opposing force between the object and the surface in contact with it ,so long as the object remains stationary even on applying the external force ,is called the static friction. static friction is self adjusting in nature.

b)Sliding friction- when the body begins to slide on a surface the force exerted by the surface on the object is called the sliding friction.

c)Rolling friction- when an object rolls over a surface the force which opposes the Rolling motion of the object is called the rolling friction.

20. List three disadvantages of friction.

Ans. Three Disadvantages of friction are as follows:

a) friction opposes the motion of a body so it decreases the efficiency.

b) friction causes wear and tear in the moving the moving parts of a body.

c) friction produces heat.