Mechanics..Friction questions...

Hello I have 2 frition questions which i need help on please..

(1) A coin is projected with speed 6m/s along the horizontal surface of a bench. The coefficient of friction between the coin and the surface is 0.7. Modelling the coin as a particle and assuming no air resistance, find the speed of the coin after 0.3s.

(2)A heavy ring of mass 5kg is threaded on a fixed rough horizontal rod. The coefficient of friction between the rod and the ring is 0.5. A light string is attached to the ring and pulled downwards with a force acting at a constant angle of 30° to the horizontal. The magnitude of the force T is newtons, and is gradually increased from zero. Find the value of T that is just sufficient to make the equilibrium limiting.

Thanks a lot
From John

Reply 1

Sollytear

1) Friction = 0.7mg
u = 6
v = v
a: f=ma, -0.7mg/m=a = -0.7g
s = s
t = 0.3

v = u + at, v = 6 - 0.7*0.3*9.8 = 3.942m/s

I dont think you use conservation of energy, but hey i might be wrong. If u do, you end up using newtonian mechanics for the distance.

Reply 2

Neo1

Yeah number 1 is correct, though for number 2 I cant seem to figure it out...

Any help please?
Thanks

Reply 3

steve2005

Neo1

Yeah number 1 is correct, though for number 2 I cant seem to figure it out...

Any help please?
Thanks

Picture 49.png20.4KB

Reply 4

vv2006

i did this in my gcse physics.

Reply 5

Neo1

Q2 was wrong the answer is meant to be 39N.

"vv2006" if you did this in GCSE perhaps you can work out the answer to question 2..

From John

Reply 6

steve2005

Neo1

Q2 was wrong the answer is meant to be 39N.

"vv2006" if you did this in GCSE perhaps you can work out the answer to question 2..

From John

I made an error when I put cos 30 = 1/ rt2 BUT when I did the calculation I used the correct value and my answer remains 28.86N

IF the angle had been 60 degrees the answer would be 49N Perhaps you mean 49 N and not 39N. Perhaps the question is not posted correctly. Perhaps I am completely wrong. If I am wrong could someone please tell me.

Edit. I failed to take account of downward tension of string. See below for corrected version.

Picture 79.png86.3KB

Reply 7

steve2005

Neo1

Q2 was wrong the answer is meant to be 39N.

"vv2006" if you did this in GCSE perhaps you can work out the answer to question 2..

From John

Yes, I agree the answer is 39.8N
I forgot to take into account the downward tension on the string would increase R and consequently affect the frictional force.

Picture 91.png34.6KB

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