## AravindG 4 years ago doubts on newton laws of motion

1. AravindG

k thx for cming

2. AravindG

how many types of inertia are there?

3. AravindG

easmore?

4. anonymous

Typically, we consider two. That associated with linear motion and that associated with rotational motion.

5. AravindG

oh i was asking like inertia of rest ,inertia of motion,inertia of direction etc

6. anonymous

They are all the same. Inertia is simply a measure of how an object resists a change in motion.

7. AravindG

hmm. can u xplain inertia of direction and based on tht reason this: wheels of vehicles are provided with mudguards

8. anonymous

The mud has a tendency to always maintain the same direction of travel. Eventually, this tendency exceeds the forces acting on the mud by the tire. When this happens, the mud will leave the tire in a tangential manner.

9. AravindG

doubt 2:a body of mass 2kg moves with an acceleration of 3 m/s^2 find change in momentum in in one second

10. AravindG

y is it the answer force ma?

11. anonymous

That is the definition of Newton's Second Law. $F = {d \vec p \over dt}$

12. AravindG

doubt 3:what is average force?

13. anonymous

$F_{avg} = {\Delta \vec p \over \Delta t}$

14. AravindG

doubt 4:A shell explodes in mid air into 2 equal fragments.what is direction of motion of 2 particles .explain this

15. anonymous

This is a result of internal forces only. Since Newton's Second Law considers only external forces, there are couple inferences we can draw. Let's assume the shell is at rest when it explodes. First, the center of mass of the two particles will remain at the same point in space. Second, the net momentum of the two particles will equal zero (because the object is initially at rest).

16. AravindG

can u xplain more i am cnfused

17. anonymous

The exact direction and nature of the motion of the two particles after the explosion is dependent on the nature of the explosion force.

18. AravindG

hw can we make such an assumption

19. AravindG

y dont we consider gravity as external force?

20. AravindG

?

21. anonymous

Gravity can be considered. The center of mass of the two particles will follow the path prescribed by the external forces.

22. anonymous

This is definitely classical mechanics.

23. AravindG

eashmore?

24. anonymous

Yes?

25. AravindG

xplain

26. anonymous

27. anonymous

Let's keep things simple for now. Air resistance will change things.

28. AravindG

wel wat is direction of motion

29. anonymous

The motion of the individual particles is dependent on the nature of the explosion. The center of mass however, will have motion as described by external forces.

30. AravindG

my text says they move in opp directions

31. AravindG

for cinservation of momentum

32. anonymous

Their net momentum after the explosion must equal the momentum of the shell before explosion. Because momentum is a vector quantity, they will travel away from each other (i.e. their velocities will have opposite signs).

33. AravindG

we assume it was at rest??

34. anonymous

Let's take the simplest example. Take the shell to be on the ground, which is smooth. After the explosion the net momentum will be zero.

35. AravindG

mth3v4 pls dont mess around

36. anonymous

If the shell is in motion. The net momentum of the two particles will equal the momentum of the shell before the explosion.

37. AravindG

k

38. AravindG

hw do we recognize an internal force ?? u see i am vonfused with that

39. anonymous

I already established this as being classical mechanics. Please don't comment if you don't have anything valuable to add.

40. anonymous

I don't understand the question Aravind

41. AravindG

wel u see an example like i hav a fan kept in a boat

42. AravindG

then i turn the fan on facing the sail

43. anonymous

Nvm. I understand. An internal force is one that does not mechanical energy of a system.

44. AravindG

the boat doesnt move

45. anonymous

An internal force is one that does not change the mechanical energy of a system.

46. AravindG

hw do we recognize that . u seei thought he wind from fan can move the boat

47. anonymous

Newton's Third Law. The fan pushes the wind against the sail, but the wind pushes back on the fan. Since the fan and sail are coupled by the boat, the boat doesn't move.

48. AravindG

ya u r spam

49. AravindG

can u show a fig

50. AravindG

eashmore?

51. anonymous

|dw:1328338036916:dw|

52. AravindG

53. anonymous

Those are forces. The square below represents the boat system. There are not external forces. |dw:1328338113551:dw|

54. AravindG

srry easmore bt i am not getting u

55. AravindG

:(

56. AravindG

i dont understand hww a return force acts on fan

57. anonymous

Do you doubt the existence of the return force or what causes the return force?

58. anonymous

Instead of a fan. Let's say you push directly on the sail mast. In this case, would the boat move?

59. anonymous

I have to leave soon.

60. AravindG

hey gogind can we cntinu with this discussion?

61. AravindG

k pls xplain

62. anonymous

hmm..lets see...You are having trouble with imagining the force that is acting on the fan, correct?

63. AravindG

ys

64. anonymous

Try imagine in like this. Fan pushes a bunch of molecules to the sail of the boat (we call that wind) , but since all these little molecules have some mass, newtons 3rd laws says that: As the blade of the fan exerts force on the molecules, all those molecules must exert the same force on the blade but it in opposite direction. OK! So now there is a bunch of molecules traveling towards the sail and eventually hitting it. When they hit the sail the same thing happens, they exert force on the sail, and since the sail is connected to the boat, they actually exert in on the boat. So in conclusion: The boat wants to go backwards, because of the force molecules exert on the blade of the fan which is connected to the boat, but the same molecules hit the sail which is also connected to the boat, causing the boat wanting to go forward. So the net result of this two forces is 0, because they have the same magnitude but opposite direction. What do you think would happen if there was no sail?

65. AravindG

hmmm..it would have remained in rest

66. AravindG

isnt it?

67. anonymous

if there IS a sail it would remain at rest, because the forces cancel each other. If there were NO sail at all, the boat would travel backwards. In the boat with no sail, molecules that acted on the blade of the fan would just travel in the opposite direction of the boat with nothing to stop them (no sail), so the only force that is acting on the boat is the one acting on the fan, since there is not force to cancel in out the boat would move backwards.

68. AravindG

k i understood

69. AravindG

but is it sure that alll molecules from fan would hit the sail??

70. anonymous

of course not. What we are talking about here is the ideal situation. In real life a good part of molecules would not hit sail. Imagine you had a really small sail and a huge fan, some molecules would hit the sail but a bigger part of them would not. So the net effect would be that you are moving backwards. In this case the force acting on the blades is much garter then the one acting on the sail...

71. AravindG

k i understood it cmpletely thx

72. AravindG

i hav some othr doubts too can u help?

73. anonymous

I don't have time, since I'm studying right now. Just post the question in the group and someone will answer, if they don't I'll answer later if I can

74. AravindG

wen will u be free?

75. anonymous

I don't know, when I get tired i guess :D. Just post on the group I'm sure someone will take a look at it

76. anonymous

what doubts??