EXAMKRACKERS: Mistake is EK Physics #180?

[Lunasly]

Q. An object with mass m sits on a plane inclined from the horizontal at an angle theta. Which of the following represents the force on the object due to gravity?

A) mg(sin) theta
B) mg(cos) theta
C) mg
D) g(sin)theta

Answer: C (highlight)

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I thought that the normal force on the block was mg(cos)theta and thus the force of gravity on the block (due to its incline) would be an equal and opposite for to the normal force. So wouldn't the answer actually be B?

Thanks,
Lunasly.

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[Arctangent]

Gravity is always downwards, or mg for any given object. The way I think of the relationship between gravity and normal force on an inclined plane is that:
1. Gravity must be mg
2. mg*cos represents the component perpendicular to the plane
3. Because of point #2, the normal force is necessarily also mg*cos
It seems the difference between the thought processes then is whether normal force is dependent on gravity, or whether gravity is dependent on normal force. In all cases, force due to gravity is mg, and normal force corresponds keeping in consideration gravitational force and the plane itself.

 

[Lunasly]

So we shouldn't always think that the normal force cancels out the force of gravity. The normal force is just the force perpendicular to the surface.

Thanks.

 

[FutureDoc2]

So we shouldn't always think that the normal force cancels out the force of gravity. The normal force is just the force perpendicular to the surface.

Thanks.

I usually think about normal force canceling out the force of gravity on a flat surface. Since this problem is dealing with inclined planes, you have to take into account the angle between them. Try drawing the question out to get a better idea. The normal force will be perpendicular to the box, but when you include the W (mg), you should be able to see why the Weight and Normal force do not cancel out. I hope I didnt make you more confused!

 

[FutureDoc2]

So we shouldn't always think that the normal force cancels out the force of gravity. The normal force is just the force perpendicular to the surface.

Thanks.

Also, an easy way to attack incline plans (on a frictionless surface) as mentioned in EK physics lec 2 is Force always equals mgsintheta while Normal force is always mgcostheta. The total force will only change from mgsintheta when there is other forces acting, i.e. friction or tension..

 

[nindra]

But wouldn't the force perpendicular to the inclined plane be mgcos(theta)? Or are we looking at the overall force due to gravity instead of the component forces??

 

[Lunasly]

Yes, we are looking at the overall force due to gravity rather then the component forces. In any case, every object with a mass has an 'mg' if they are on earth.