Circular Motion, why is it R = MG - F and not R = MG + F? Watch
and I'm on part a)ii).
The mark scheme uses the fact that R = mg - F but I can't work out why. The reaction force is acting upwards, the weight is acting downwards, and the centripetal force, F, is acting downwards too.
The car isn't in the air or going through the floor, so in my mind, R must be equal to the sum of the downwards forces, aka R = MG + F.
Can someone explain this to me please? Thanks
F= mg - R (because F is downwards towards the centre of the circle)
Whenever you look at a circular motion question, look at the forces which are acting and make the resultant of these the centripetal force.
The reaction force is acting upwards, the weight is acting downwards, and the centripetal force, F, is acting downwards too.
Using Newton's 2nd Law, a force must act if there is an acceleration. If you do Mechanics 3, you will derive that this acceleration is towards the centre for circular motion (or you are just told this and never taught why).
Hence you know that the resultant of weight and normal reaction acts towards the centre, and ergo mg > R when in contact with a surface. The resultant (named centripetal for the sole purpose of tagging a name on things to help understand them) force is simply mg - R (resolving forces perpendicular to tangent of circular motion).