# Homework Help: Direction of reaction force in a circular motion

1. Jul 1, 2015

### goldfish9776

1. The problem statement, all variables and given/known data
Thewheel is rotated in the anticlockwise direction...The correct reaction force is drawn using the blue pen . I don't understand why the reaction force isn't directed to the left ? which is drawn using the red colour ?
IMO, the red colour should be correct because the supporting surface is on the right , ( the person sticked to the right)

2. Relevant equations

3. The attempt at a solution

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2. Jul 1, 2015

### andrevdh

You are probably thinking of the resultant force which should be
to the left as indicated in red. The reaction force is from the floor which
is the friction and normal force combined which would point
upwards at an angle as indicated by the blue N3. The red force
is the blue N3 and W combined - the resultant of the forces.

3. Jul 1, 2015

### goldfish9776

So, which is correct ?
Centripetal force is the resultant force of R3 and W (mg ) ?
Or R3 is the resultant force of centripetal acceleration and W(mg ) ??

P/s : I know the red line should be centripetal acceleration force now .

4. Jul 1, 2015

### andrevdh

According to N2

a = Fresultant/m

5. Jul 1, 2015

### goldfish9776

what is the relationship of it and the question??

6. Jul 1, 2015

### goldfish9776

Can you explain this statement in more simple sentence??

7. Jul 1, 2015

### goldfish9776

which is the resultant force ???
the red line or the blue N3?

8. Jul 1, 2015

### andrevdh

The clue for me was "reaction force".
That is the person is pushing "down" on the floor due to being spun around.
The floor reacts (reaction force), your R3, by pushing back.
We can think of this reaction force as two forces combined, the normal force,
perpendicular to the surface, and friction, parallel to the surface.

9. Jul 1, 2015

### andrevdh

According to Newton's 2nd law a body accelerates in the direction of the resultant force
as I tried to show in one of my previous posts.
According to this what should the resultant force then be? Red or blue?

10. Jul 1, 2015

### goldfish9776

I can understand post # 8.
the resultant force should be the red line. but according to phytagoras's theorem , the R3 looks like resultant force , since W and Fc is perpendicular to each other ( as shown in the triangle drawn) which is making me confused now.

11. Jul 1, 2015

### andrevdh

It is just a question which forces the object is experiencing,
in this case W and blue N3. The resultant is the (vector)
sum of these. That means we can replace W and blue N3
with the red N3 in the diagram.

12. Jul 2, 2015

### goldfish9776

so , it has completely nothing to do with the hypotenuse of triangle (maths concept ) ?
The resultant force can be in any direction depending on the question although it does not comply with hypotenuse of triangle (maths concept )??

13. Jul 2, 2015

### CWatters

The resultant force is the vector sum of all the forces acting on the body. The vector sum of two forces only equals the hypotenuse of a triangle if the two forces are at right angles (orthogonal).

Your diagram does not appear to show _all_ the forces acting on the body. I understand W = Weight. Where is the centripetal force shown?

14. Jul 2, 2015

### andrevdh

As I understand it the person is standing in cage or on a platform that is swung around.
The blue N3 and W combined provides the centripetal force.