You are Here: Home >< Maths

# doesn't part C of this question contradict what was asked for in part b? Watch

1. Question is attached. I accept part b has nothing wrong with it (solution attached) but part c clearly shows (part c solution is given as part of partb solution) that friction = 0.3 R = 0.3x(Mg+W) and so the mass does affect friction and so friction is not independent of the mass as stated in part b. I am totally split two ways by this question. Anyone who can help?
Attached Images

2. (Original post by thebrahmabull)
Question is attached. I accept part b has nothing wrong with it (solution attached) but part c clearly shows (part c solution is given as part of partb solution) that friction = 0.3 R = 0.3x(Mg+W) and so the mass does affect friction and so friction is not independent of the mass as stated in part b. I am totally split two ways by this question. Anyone who can help?
I can't see the solution very clearly (maybe I'm getting old), but it looks like it says something like rather than ?
3. (Original post by Implication)
I can't see the solution very clearly (maybe I'm getting old), but it looks like it says something like rather than ?
Yes. But mew x R is friction by mathematical definition right?
4. (Original post by thebrahmabull)
Yes. But mew x R is what friction is right?
is the maximum force due to friction I believe. The actual force, denoted by in this question, will be less than (or equal to) that.

I could be wrong though. It's been 4 years since I did A-level maths so I'm a bit fuzzy on this stuff
5. No. In B you were finding a formula for the friction force. In C you are applying a constraint on the friction.
(Original post by thebrahmabull)
Yes. But mew x R is friction by mathematical definition right?
No. Friction defined something like the force, parallel to the surface, that an object experiences due to interactions with the surface.

The case only applies when the object is moving or in limiting equilibrium.
6. (Original post by Implication)
is the maximum force due to friction I believe. The actual force, denoted by in this question, will be less than (or equal to) that.

I could be wrong though. It's been 4 years since I did A-level maths so I'm a bit fuzzy on this stuff
(Original post by morgan8002)
No. In B you were finding a formula for the friction force. In C you are applying a constraint on the friction.

No. Friction defined something like the force, parallel to the surface, that an object experiences due to interactions with the surface.

The case only applies when the object is moving or in limiting equilibrium.
So should I always visualise that mew x R is something different from friction, e.g.its a horizontal force, but not friction?
7. (Original post by thebrahmabull)
So should I always visualise that mew x R is something different from friction, e.g.its a horizontal force, but not friction?
No. Visualise it as the maximum possible value of friction.
8. (Original post by thebrahmabull)
So should I always visualise that mew x R is something different from friction, e.g.its a horizontal force, but not friction?
is the friction, but only in the limiting case i.e. when the object is moving or on the point of moving. Think of it as the maximum friction so . The more you try to move something, the stronger the frictional force resisting the motion gets - until it reaches its maximum value, when the object begins to move. If the object isn't moving and you aren't told it's in limiting equilibrium, you have to find the friction another way.
9. Thanks guys!
10. (Original post by morgan8002)
No. In B you were finding a formula for the friction force. In C you are applying a constraint on the friction.

No. Friction defined something like the force, parallel to the surface, that an object experiences due to interactions with the surface.

The case only applies when the object is moving or in limiting equilibrium.
Interesting question, I have read the thread but part B confuses me. How can friction exerted by ladder be independent of the mass of the brick? Since the ladder is not in equilibrium, it is now moving, so maximum friction must be acting on it... Mu x R value gets greater as mass of the brick increases till it becomes (3w/8) right ?
11. (Original post by chhhhelsie)
Interesting question, I have read the thread but part B confuses me. How can friction exerted by ladder be independent of the mass of the brick? Since the ladder is not in equilibrium, it is now moving, so maximum friction must be acting on it... Mu x R value gets greater as mass of the brick increases till it becomes (3w/8) right ?
We don't know that it's not in equilibrium though. Friction could be less than .
12. (Original post by morgan8002)
We don't know that it's not in equilibrium though. Friction could be less than .
Ohh right

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 12, 2016
Today on TSR

### Last-minute PS help

100s of personal statements examples here

### More pressure for kinky sex?

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.