Finding friction without coefficient?

[kelseybrahe]

Homework Statement

We did a Prac where we attached ticker tape to a cart (905g), and the cart to a mass (249g) through a pulley. We then had to assess the tape and answer questions. We are asked to calculate the force acting upon the whole system and then calculate the force acting only upon the cart. We are not given the coefficient of friction.
I do not know the applied force (can I work this out?)

Average acceleration = 1.05m/s (initial velocity was zero/at rest?)

There is no inclination. The cart was on a flat, smooth desk.

Homework Equations

F=ma a(av)= v-u/t
Maybe Ek=1/2mv2

The Attempt at a Solution

Read somewhere that Ek=1/2mv2 can give you the coefficient of friction, but that didn't seem right at all. Other than that, completely lost.

 

[haruspex]

the force acting upon the whole system

Define the system.

the force acting only upon the cart

Net force, or just the force from the string tension?

 

[kelseybrahe]

Define the system.

Net force, or just the force from the string tension?

I'm not entirely sure about either of these. The questions are very vague. I'm assuming by 'whole system' it means the combined mass of the cart and the mass pulling it. Which is 1154g.

For the second, the question simply states "it could be possible to use the same ticker tape to determine the friction acting on the cart only. How?"

 

[haruspex]

I'm not entirely sure about either of these. The questions are very vague. I'm assuming by 'whole system' it means the combined mass of the cart and the mass pulling it. Which is 1154g.

Ok. This is awkward, and I rather suspect the question is phrased wrongly. But let's take it at face value for now. What are the forces acting on the system (cart+string+weight).

For the second, the question simply states "it could be possible to use the same ticker tape to determine the friction acting on the cart only. How?"

Can you calculate what the acceleration would have been without friction?

 

[kelseybrahe]

Ok. This is awkward, and I rather suspect the question is phrased wrongly. But let's take it at face value for now. What are the forces acting on the system (cart+string+weight).Can you calculate what the acceleration would have been without friction?

Okay, thank you.
If it helps this is the first question:
"Calculate the difference in theoretical and measured acceleration, and then calculate The force of friction acting on the system."

Cart, string, pulley, weight and also the ticker time itself?

Yes! If I've done it correctly it's 0.47m/s^2.

 

[kelseybrahe]

Yes! If I've done it correctly it's 0.47m/s.

Sorry, part of my response has gone to the wrong place in there.

 

[haruspex]

If it helps this is the first question:
"Calculate the difference in theoretical and measured acceleration, and then calculate The force of friction acting on the system."

do I have this right:
The first part asks for the frictional force acting on the system (as you have now defined it).
The second part does not ask you to calculate the frictional force acting on the cart only, but does ask how you could do that using the equipment.
?

You were advised to calculate the theoretical acceleration without friction. Do that.

 

[kelseybrahe]

do I have this right:
The first part asks for the frictional force acting on the system (as you have now defined it).
The second part does not ask you to calculate the frictional force acting on the cart only, but does ask how you could do that using the equipment.
?

You were advised to calculate the theoretical acceleration without friction. Do that.

Yes, I think so.
You've definitely got the first part right.
For the second (I have a cold and am a bit foggy - I'm sorry if I seem slow!) I think that it's asking for a formula or something. Do you know of one? If you don't, just go with equipment and see where that leads?

The theoretical acceleration is 0.47m/s^2

 

[haruspex]

The theoretical acceleration is 0.47m/s^2

That cannot be right. Please post your working.

 

[kelseybrahe]

That cannot be right. Please post your working.

m+M (mass total) divided by weight

(m= 249 (mass on string) M=905 (mass of cart) )

= 1.154/2.44
= 0.47

This was being told to assume the weight force was the only force acting on the system for that question.
Perhaps that can extend to the other questions as well.

Have I used the wrong formula?
I converted it to Kg, is that incorrect?

 

[haruspex]

m+M (mass total) divided by weight

Small but important error. What is the relationship between force, mass and acceleration?

 

[kelseybrahe]

Small but important error. What is the relationship between force, mass and acceleration?

They are proportional to each other?
Increase force and mass, acceleration increases?

 

[haruspex]

They are proportional to each other?

Yes, but what is the equation?

 

[kelseybrahe]

Yes, but what is the equation?

Oh! F = ma

 

[haruspex]

Oh! F = ma

Right. So is that what you wrote here?

m+M (mass total) divided by weight

 

[kelseybrahe]

Right. So is that what you wrote here?

No... Oh, okay.
So I would re-arrange it to be a=f/m?

 

[haruspex]

No... Oh, okay.
So I would re-arrange it to be a=f/m?

Yes.

 

[kelseybrahe]

Yes.

Right. But how do I find f?
I have an answer for it but that is using the calculated acceleration of 1.05m/s^2. And that wouldn't be accurate, would it?

(Thank you for taking the time to go through this with me - I really appreciate it!)

 

[haruspex]

Right. But how do I find f?

Let's get this step done first:

"Calculate the difference in theoretical and measured acceleration..."

 

[kelseybrahe]

Let's get this step done first:

Oh sorry, I mean the f in the equation a=f/m
Because that's force, right? But the only way I can find force is by doing f=ma. I think?

 

[haruspex]

Oh sorry, I mean the f in the equation a=f/m
Because that's force, right? But the only way I can find force is by doing f=ma. I think?

That's the calculation you did in post #10, just doing the divide the right way around this time.

 

[kelseybrahe]

That's the calculation you did in post #10, just doing the divide the right way around this time.

Oh sorry! I'm such a ditz.
Okay.
2.44/1.154

= 2.15m/s^2

 

[haruspex]

Oh sorry! I'm such a ditz.
Okay.
2.44/1.154

= 2.15m/s^2

Right. So how much less is the observed acceleration?

 

[kelseybrahe]

Right. So how much less is the observed acceleration?

1.10m/s^2

 

[haruspex]

1.10m/s^2

Right. Can you figure out the total frictional force from that?

 

[kelseybrahe]

Right. Can you figure out the total frictional force from that?

I'm not sure how to do that... Do you use the difference in acceleration and multiply that by the mass?

 

[haruspex]

I'm not sure how to do that... Do you use the difference in acceleration and multiply that by the mass?

That will work, but it would be better to understand why.
If you do not, I urge you to use the more standard approach of solving ΣF=ma. You know one force, you know the total mass, and you observd the accelration. There is one force, friction, that you do not know.

 

[kelseybrahe]

That will work, but it would be better to understand why.
If you do not, I urge you to use the more standard approach of solving ΣF=ma. You know one force, you know the total mass, and you observd the accelration. There is one force, friction, that you do not know.

I'm really sorry, I don't understand what you mean.
So I would substitute the total mass and observed acceleration into that equation in order to find (sum)F. So, friction is only included in that total, it is not THE total, right?
Is there something I have to do to isolate friction, or does that answer cover it?

(Sorry for my ignorance here)

 

[haruspex]

So I would substitute the total mass and observed acceleration into that equation in order to find (sum)F. So, friction is only included in that total, it is not THE total, right?

Yes.

Is there something I have to do to isolate friction,

Yes. What other force(s) are there affecting the motion?

 

[kelseybrahe]

Yes.

Yes. What other force(s) are there affecting the motion?

String, pulley, ticker timer, Gravity?