Need help with a force diagram question M1

for this question I can only solve what r is by resolving f which is asked in the second part of the question. How do you do a) without knowing what f is. The mark scheme suggests that rcos15 =4g but why isn't it 4g cos15 . Also how do you know the angle between r is 15? Alternate angle?

Thanks.

(edited 8 years ago)

Reply 1

Zacken

22

Original post by coconut64

for this question I can only solve what r is by resolving f which is asked in the second part of the question. How do you do a) without knowing what f is. The mark scheme suggests that rcos15 =4g but why isn't it 4g cos15 . Also how do you know the angle between r is 15? Alternate angle?

Thanks.

You can get rid of

F

by resolving vertically - since it is a purely horizontal force, it vanishes if you resolve vertically. If you draw a big diagram and label your angles, you should be able to find the angle between the reaction and the vertical - remember that the weight is vertical so there's no need to resolve really there.

Reply 2

TeeEm

19

Original post by coconut64

for this question I can only solve what r is by resolving f which is asked in the second part of the question. How do you do a) without knowing what f is. The mark scheme suggests that rcos15 =4g but why isn't it 4g cos15 . Also how do you know the angle between r is 15? Alternate angle?

Thanks.

you have some forces resolved horizontally and vertically and some parallel to the plane and perpendicular to the plane

Reply 3

WarwickPhysicist

1

It is not 4gcos(15) because there is a vertical force component, Fsin(15), to consider as well.

The mark scheme has resolved vertically and horizontally to the particle, this is a bit tidier because it means there is no horizontal force component to consider at this stage, you are resolving perpendicular and parallel to the plane, which is fine, but the mark scheme has gone for the former approach.

(edited 8 years ago)

Reply 4

coconut64

OP

16

Original post by Zacken

You can get rid of

F

by resolving vertically - since it is a purely horizontal force, it vanishes if you resolve vertically. If you draw a big diagram and label your angles, you should be able to find the angle between the reaction and the vertical - remember that the weight is vertical so there's no need to resolve really there.