Mechanics question (1 Viewer)

[richz]

hello there,

im a bit confused here, can someone please explain to me an issue about circular motion in a horizontal plane (7.4 of cambridge). For reaction forces, how do u know when N(normal force) is perpendicular to the surface and when its at an angle. I have always known that the normal force is always perpendicular to the surface . For example figure 7.15 shows that the normal force is equal and perpendicular to the surface while in example 8, the normal force is at an angle to the surface

thnx

 

[shafqat]

It is always perpendicular. In example 8 the slant of the surface at that point is the tangent, so the normal force is perpendicular to it.

 

[richz]

o ok, so how would u explain qs 5 and 6 from ex 7.4 because in 5 the normal force is at an angle to the horizontal plane, but in 6 the normal is perpendicular to the surface. In each question how would u determine the normal force?

 

[shafqat]

In 5 its perpendicular to the tangent to the point of contact on the hemisphere.
In 6 its perpendicular to the table, that is straight up.

 

[richz]

yep thats true, but wat words in the question give it away?

 

[shafqat]

In 5 it says it sits on a bowl. Wherever it sits, the normal acts perpendicular to the surface, ie perp to the tangent. Similarly in 6 the force is perpendicular to the surface, which is the table.

 

[richz]

so ur saying most of the time when it sits on the bowl the normal is perp to the tangent of the bowl. so it is at an angle to a horizontal surface

 

[shafqat]

Forget horizontal and vertical. The normal force is always perpendicular to the surface on which it sits, whatever the situation. On the hemisphere, the surface is not horizontal, it is at an angle. For the table the surface just happens to be horizontal.

 

[richz]

o ok thnx.