You are Here: Home >< Maths

# Mechanics, Friction help Watch

1. A sack of weight 600N is being loaded onto a truck up a ramp inclined at 18 degrees to the horizontal. A rope attached to the sack is held at an angle of 25 degrees to the ramp. The coeffeicient of friction between the sack and the ramp in 0.3.
What tension in the rope is needed

a) To prevent the sack from sliding down the ramp
b) to pull the sack up at a steady speed

I was able to get
Tcos(23) = F + 600sin(18)

and

600cos(18) = Tsin(23) + R

The equation im trying to use is flim=uR
but if i cant work out R, then i cant work out flim either

So i was hoping someone could help me get past just this 1 step
2. (Original post by Notsocleverstudent)
A sack of weight 600N is being loaded onto a truck up a ramp inclined at 18 degrees to the horizontal. A rope attached to the sack is held at an angle of 25 degrees to the ramp. The coeffeicient of friction between the sack and the ramp in 0.3.
What tension in the rope is needed

a) To prevent the sack from sliding down the ramp
b) to pull the sack up at a steady speed

I was able to get
Tcos(23) = F + 600sin(18)
Looks like you've resolved parallel to the ramp. Which way is friction acting, given it's on the point of slipping down?

and

600cos(18) = Tsin(23) + R

The equation im trying to use is flim=uR
but if i cant work out R, then i cant work out flim either

So i was hoping someone could help me get past just this 1 step
As you're asked for the tension needed, it's in limiting equilibrium and flim is F

So just use F=uR

And you then have 3 equations and 3 unknowns.

PS is the angle 23 or 25?
3. (Original post by ghostwalker)
Looks like you've resolved parallel to the ramp. Which way is friction acting, given it's on the point of slipping down?
Friction is acting against the driving force which if you draw on a diagram, the friction would be acting down the slope(ramp)
And the problem is, I get this far but without knowing either T or F, I'm kind of stuck

(Original post by ghostwalker)
As you're asked for the tension needed, it's in limiting equilibrium and flim is F

So just use F=uR

And you then have 3 equations and 3 unknowns.

PS is the angle 23 or 25?
Sorry, it's 25, my head kind of dazed off

And, the problem i get with using Flim = uR is that I can't work out R because the Tension T is acting both vertically so without T I can't work out R and vice versa

And was there a 3rd equation I'm not aware of :s

Thank you so far
4. (Original post by Notsocleverstudent)
Friction is acting against the driving force which if you draw on a diagram, the friction would be acting down the slope(ramp)
And the problem is, I get this far but without knowing either T or F, I'm kind of stuck
No, friction acts against the direction in which the body is tending to move. If the mass is moving up, friction acts down, and if the mass is about to slip down, friction acts upwards, so to speak.

And, the problem i get with using Flim = uR is that I can't work out R because the Tension T is acting both vertically so without T I can't work out R and vice versa

And was there a 3rd equation I'm not aware of :s

Thank you so far
That is the third equation F=uR.
5. (Original post by ghostwalker)
No, friction acts against the direction in which the body is tending to move. If the mass is moving up, friction acts down, and if the mass is about to slip down, friction acts upwards, so to speak.
Thank you. I actually didn't see it from that perespective. I'll draw a new diagram, and see if I can get further with that

That is the third equation F=uR.
So just to be sure, the 3 equations are:

Tcos(23) = F + 600sin(18)
600cos(18) = Tsin(23) + R
Flim = uR

And, if I rearrange the 1st 2, into the 3rd one, I would get

Tcos(23)-600sin(18) = 0.3 x 600cos(18)-Tsin(23)

Right so far? :/
6. (Original post by Notsocleverstudent)
So just to be sure, the 3 equations are:

Tcos(23) = F + 600sin(18)
600cos(18) = Tsin(23) + R
Flim = uR

And, if I rearrange the 1st 2, into the 3rd one, I would get

Tcos(23)-600sin(18) = 0.3 x 600cos(18)-Tsin(23)

Right so far? :/
As I said in my first post, which direction is friction acting? Your first equation is incorrect.

Regarding your 3rd equation, don't forget Flim = F here.

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: December 23, 2010
Today on TSR

### Degrees to get rich!

... and the ones that won't

### Women equal with Men?

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.