Physics question help please

Hi,
Please could I have some help on this question? I’m unsure about the markscheme, how could gravitational force provide centripetal force when I thought they are both separate forces acting towards the centre of the circle which are equal, and then the contact force is the sum of these magnitudes but in the opposite direction?
Question and markscheme: https://ibb.co/G4Qw9xsx
https://ibb.co/Nd77wfSD
Thanks!

Reply 1

Eimmanuel

Study Forum Helper

15

Original post by anonymous56754

Hi,
Please could I have some help on this question? I’m unsure about the markscheme, how could gravitational force provide centripetal force when I thought they are both separate forces acting towards the centre of the circle which are equal, and then the contact force is the sum of these magnitudes but in the opposite direction?
Question and markscheme: https://ibb.co/G4Qw9xsx
https://ibb.co/Nd77wfSD
Thanks!

Noted that centripetal force is not a real force but gravitational force is a real force.
Centripetal force is lnown as a net force or resultant force.
This should be in your text or note if you are taking CAIE A level physics.

Reply from mobile, so there may be typo.

Reply 2

anonymous56754

OP

12

Original post by Eimmanuel

Noted that centripetal force is not a real force but gravitational force is a real force.
Centripetal force is lnown as a net force or resultant force.
This should be in your text or note if you are taking CAIE A level physics.
Reply from mobile, so there may be typo.

so what does it mean by contact force is equal to the difference between centripetal and gravitational force when they are both the same? Surely it should say that contact force is equal to gravitational force so the resultant force which is the centripetal force=0? Thanks

Reply 3

Eimmanuel

Study Forum Helper

15

Original post by anonymous56754

so what does it mean by contact force is equal to the difference between centripetal and gravitational force when they are both the same? Surely it should say that contact force is equal to gravitational force so the resultant force which is the centripetal force=0? Thanks

It is highly recommended that you reread what you have written. The physics is wrong! Please restudy Newton’s laws of motion.

I would explain what “weightlessness” means by using another example.

Imagine you are in an elevator where you are standing on a weighing scale.
Writing Newton’s 2nd law:
R − mg = ma
where m is your mass and R is the normal (reaction) force (the reading of the weighing scale)

When the elevator is not moving or moving at a constant velocity, the weighing scale will give a reading of your “weight”:
R − mg = 0
R = mg

If the elevator is moving upward by accelerating up with an acceleration of a₁, the reading of the weighing scale will be
R − mg = ma₁
R = m(g + a₁)
This implies that the reading of the weighing scale increases, you would feel heavier.

Next, if the elevator at 300 storeys and moves down by accelerating down with an acceleration of a₂(less than the magnitude of g), the reading of the weighing scale will be
R − mg = −ma₂
R = m(g − a₂)
This implies that the reading of the weighing scale decreases, you would feel lighter. Yes, you will feel lighter indeed.
On the way of moving down, the elevator’s cable snaps, the elevator accelerates downwards at g, the reading of the weighing scale will be
R − mg = −mg
R = m(g − g)
R = 0
This is what we call the sense of weightlessness, when the contact force/normal force is zero.
Your true weight mg is the only force acting on you while you are accelerating at g, so it is your resultant force.

This is also what happens to an astronaut orbiting in a satellite where he experiences weightlessness when the contact force is zero NOT when the gravitational force is equal to normal force and this cannot happen. If you understand the physics, we should be able to explain.

Physics question help please

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