# Explanation for Newton II giving negative mass in my Physics lab results please

• ChiralSuperfields
In summary, in your setup the mass of the hanging mass is not contributing to the force sensor on the cart, but the cart is in free fall and the tension is decreasing.

#### ChiralSuperfields

Homework Statement
Relevant Equations
Newton Second Law
I am trying to verify Newton II. The setup I am using is,

Where ##m_1 = 0.887 kg## is a cart and ##m_2 = 0.02016 kg## is a small hanging mass. There is a force sensor on ##m_1## to measure the force acting on it from the string and the acceleration of the cart.

To verify Newton's Second Law, we tried to plot we plot acceleration of ##m_2## vs force ##F## which is the same as the tension ##T##.

Apply Newton II to the ##m_2##: ##T = m_2a + m_2g## which is a linear equation (##y = mx + c##) of the form,
which is of the form ##y = T, m = m_2, x = a## and ##c = m_2g##

Therefore we graphed T vs a to get:

However, do you please know why I am getting ##m_2 = -0.2152 kg## from the graph when we measure it to be 0.02kg? I am not sure why the negative sign and why it is x10 larger than the measured ##m_2##. Also when I solve ##c = 0.5069 = m_2g##, for ##m_2## I also get a very different value for ##m_2##

Any help greatly appreciated!

Many thanks!

You mention a "cart" but I see no wheels at m1. Is it a cart with frictionless wheel bearings, or is it on a frictionless surface in your experimental setup, or should you have included a friction term in your equations?

ChiralSuperfields
berkeman said:
You mention a "cart" but I see no wheels at m1. Is it a cart with frictionless wheel bearings, or is it on a frictionless surface in your experimental setup, or should you have included a friction term in your equations?
Thank you for your reply @berkeman ! Yes we were told to ignore friction (in the wheel bearings).

Many thanks!

ChiralSuperfields said:
Relevant Equations: Newton Second Law

I am trying to verify Newton II. The setup I am using is,
View attachment 326971Apply Newton II to the ##m_2##: ##T = m_2a + m_2g## which is a linear equation (##y = mx + c##) of the form,
If ##a## = ##g## what does ##T## equal?

scottdave and ChiralSuperfields
erobz said:
If ##a## = ##g## what does ##T## equal?

##T = 2m_2g##

ChiralSuperfields said:

##T = 2m_2g##
Does that make sense to you?

It shouldn't.

hutchphd, ChiralSuperfields and robphy
@ChiralSuperfields ,

Is the mass of the force sensor included as contributing to ##m_1## ?

ChiralSuperfields
As @erobz hinted at, if T is zero, what situation is that similar to?

ChiralSuperfields and erobz
erobz said:
Does that make sense to you?

It shouldn't.

Sorry how not?

Many thanks!

SammyS said:
@ChiralSuperfields ,

Is the mass of the force sensor included as contributing to ##m_1## ?

Yes

Many thanks!

scottdave said:
As @erobz hinted at, if T is zero, what situation is that similar to?

Free fall.

Many thanks!

ChiralSuperfields said:

Sorry how not?

Many thanks!
##a=g## implies ##m_2## is in free fall. Is there anything tugging on ##m_2## (holding it back) in that case?

scottdave and ChiralSuperfields
ChiralSuperfields said:

Yes

Many thanks!
I'm surprised at that.

The data clearly show that ##m_1## must be approximately 2 kg.

Afterall, you have the net force, ##T##, exerted on ##m_1## and you have its acceleration ,

If this were an ideal situation, no friction and no stretching of the string, the acceleration and the tension would not vary throughout your table.

Last edited:
ChiralSuperfields
erobz said:
##a=g## implies ##m_2## is in free fall. Is there anything tugging on ##m_2## (holding it back) in that case?

Yes the inertia of ##m_1## is decreaseing the acceleration of ##m_2##

Many thanks!

SammyS said:
I'm surprised at that.

The data clearly show that ##m_1## must be approximately 2 kg.

Afterall, you have the net force, ##T##, exerted on ##m_1## and its acceleration ,

If this were an ideal situation, no friction and no stretching of the string, the acceleration and the tension would not vary throughout your table.

Do you please know why the data would give a wrong mass for ##m_1##?

Many thanks !

ChiralSuperfields said:

Yes the inertia of ##m_1## is decreaseing the acceleration of ##m_2##

Many thanks!
No. If ##a=g##, there is only a single force acting on ##m_2##. What is that force? You need to be thinking about what happens in the limit as ##m_1 \to 0##

ChiralSuperfields
erobz said:
No. If ##a=g##, there is only a single force acting on ##m_2##. What is that force? You need to be thinking about what happens in the limit as ##m_1 \to 0##

If ##a = g## then the only force acting on the system is force of gravity on ##m_2##

Many thanks!

ChiralSuperfields said:

If ##a = g## then the only force acting on the system is force of gravity on ##m_2##

Many thanks!
Right, so what is the tension ##T## in that case?

ChiralSuperfields
erobz said:
Right, so what is the tension ##T## in that case?
Thank you for you reply @erobz!

##T = 0## since ##m_1 = 0## basically

Many thanks!

ChiralSuperfields said:
Thank you for you reply @erobz!

##T = 0## since ##m_1 = 0## basically

Many thanks!
But you told me above subbing in ##a=g##, that ##T=2m_2g##. Does ##T=0## in your equation when ##a=g##?

ChiralSuperfields
erobz said:
But you told me above subbing in ##a=g##, that ##T=2m_2g##. Does ##T=0## in your equation when ##a=g##?

True! No, I don't think T = 0 when a = g

Many thanks!

ChiralSuperfields said:
To verify Newton's Second Law, we tried to plot we plot acceleration of m2 vs force F which is the same as the tension T.
Well, no, the F in Newton II is the net force. There are two forces acting on m2.
ChiralSuperfields said:
Apply Newton II to the ##m_2##: ##T = m_2a + m_2g##
The first thing to be clear about is your sign conventions. Which way are you taking as positive for a? Does T act on m2 in that direction? What about m1?
ChiralSuperfields said:
we graphed T vs a to get
I don’t understand what you are plotting here. The leftmost column is timestamps in increments of 1/20s, Implying these are measurements in a single drop. These should hardly vary. The only reason they do is some inaccuracy in the readings near the start. The slope of the graph you plotted is of no interest here.

You note that ##T = m_2a + m_2g## is a linear relationship, but if you want the slope to represent the mass you must get it in the form ##y=m_2x+c##.

scottdave and ChiralSuperfields
haruspex said:
Well, no, the F in Newton II is the net force. There are two forces acting on m2.

The first thing to be clear about is your sign conventions. Which way are you taking as positive for a? Does T act on m2 in that direction? What about m1?

I don’t understand what you are plotting here. The leftmost column is timestamps in increments of 1/20s, Implying these are measurements in a single drop. These should hardly vary. The only reason they do is some inaccuracy in the readings near the start. The slope of the graph you plotted is of no interest here.

You note that ##T = m_2a + m_2g## is a linear relationship, but if you want the slope to represent the mass you must get it in the form ##y=m_2x+c##.

For ##m_2##, I am taking upwards as positive and downwards as negative so if ##T < m_2g## then ##a < 0##.

Yeah the measurements are in a single drop. Sorry, how is the slope of the graph of no interest?

Getting that linear relationship for ##T## in ##y = mx + c## form we get ##T = (m_2)(a + g)##.

I decided to plot the graph again and set the y-intercept equal to zero since ##T = (m_2)(a + g)## is in ##y = mx## form to get the graph:

This is closer to the original value for ##m_2##, however, why is ##m_2## so large from the graph?

Many thanks!

Do you really that linear fit describes your data?

scottdave and ChiralSuperfields
ChiralSuperfields said:
Relevant Equations: Newton Second Law

I am trying to verify Newton II. The setup I am using is,
View attachment 326971
Where ##m_1 = 0.887 kg## is a cart and ##m_2 = 0.02016 kg## is a small hanging mass. There is a force sensor on ##m_1## to measure the force acting on it from the string and the acceleration of the cart.

To verify Newton's Second Law, we tried to plot we plot acceleration of ##m_2## vs force ##F## which is the same as the tension ##T##.

Apply Newton II to the ##m_2##: ##T = m_2a + m_2g## which is a linear equation (##y = mx + c##) of the form,
which is of the form ##y = T, m = m_2, x = a## and ##c = m_2g##
These equations are wrong.

The only force acting on the cart & sensor combination (mass, m_1) is the tension, T, measured by the sensor.

There are two forces acting on the hanging mass, m_2 . Do you have any idea as to what these two forces are?

ChiralSuperfields
malawi_glenn said:
Do you really that linear fit describes your data?

Yes I do. I don't have any reason to not think that the linear fit is for the data.

Many thanks!

What is the R-squared value?

ChiralSuperfields
malawi_glenn said:
What is the R-squared value?
Thank you for your reply @malawi_glenn ! The value is 0.95 (2 sig figs).

Many thanks!

SammyS said:
These equations are wrong.

The only force acting on the cart & sensor combination (mass, m_1) is the tension, T, measured by the sensor.

There are two forces acting on the hanging mass, m_2 . Do you have any idea as to what these two forces are?

Two forces acting on hanging mass is tension and gravity.

Sorry how are the equations wrong?

Many thanks!

ChiralSuperfields said:
I am taking upwards as positive and downwards as negative
But in your table you have written positive values for acceleration, so did m2 accelerate upwards? If so, I guess the mass must be negative.
ChiralSuperfields said:
since T=(m2)(a+g) is in y=mx form
Yes, but you plotted against a. What is x according to what you wrote above?
ChiralSuperfields said:
how is the slope of the graph of no interest?
As I wrote in post #22, in a single drop all the values should be the same: all the forces should be the same and all the accelerations should be the same. Your data have the acceleration increasing at first. To explain that, there must be some systematic error in the early readings. I would ignore all the data before it settles down to a fairly constant acceleration. The slope you found merely tracks how the error evolves during the drop.

2. Take the average T and average a of what remains,
3. Correct the sign error in T=(m2)(a+g), and
4. … use that to find the mass, not T=(m2)a.

ChiralSuperfields
ChiralSuperfields said:

True! No, I don't think T = 0 when a = g

Many thanks!
Yeah, I can see that! What I’ve been trying to get you to realize with this escapade is that you better think again…

Last edited:
ChiralSuperfields
ChiralSuperfields said:
For ##m_2##, I am taking upwards as positive and downwards as negative so if ##T < m_2g## then ##a < 0##.

So which way is ##m_2## accelerating in this experiment w.r.t your convention? is ##a## positive or negative? I'm asking because you are plotting it as a positive value...which is fine so long as your equation describing ##T## makes sense for ##a## being a positive value. Does your equation describing ##T## make sense for ##a## being a positive value?

Last edited:
ChiralSuperfields
Hi @ChiralSuperfields. It might help if you take a few steps back. Can you work-through the questions below?

Take the positive direction for each mass to be the direction in which it accelerates.

Since ##m_1## and ##m_2## have the same magnitude acceleration, use the same symbol, ##a##, for the acceleration of each,

##T## is the tension in the string and ##g## is acceleration due to gravity.

Q1. What is the net force on ##m_1## alone?

Q2. Using your answer to Q1, apply Newton’s 2nd law to ##m_1## to write an equation for ##m_1##.

Q3. Give an expression for the net force on ##m_2## alone.

Q4. Using your answer to Q3, apply Newton’s 2nd law to ##m_2## to write an equation for ##m_2##.

Q5. Combine your two equations (from Q2 and Q4) by eliminating ##T##. Reaarange the resulting equation to make ##a## the subject.

To verify Newton’s second law you will need to check that (within the limits of experimental error) your values of ##a, m_1## and ##m_2## [edit: and ##g##] agree with the (correct) equation from Q5.

Last edited:
ChiralSuperfields, malawi_glenn and erobz
ChiralSuperfields said:

Two forces acting on hanging mass is tension and gravity.

Sorry how are the equations wrong?

Many thanks!
If ##m_1## is the mass of the cart along with everything on the cart, then as I said, the tension is related to ##m_1## by ##T=m_1\, a##.

For the hanging mass, we have ##m_2\, g -T = m_2\, a##.

By the way:
You can eliminate ##T## from these two equations, to get ##\displaystyle \quad a=\dfrac{m_2}{m_1+m_2}\,g ##.

For the values that you have, this gives an acceleration of approximately ##0.22\,\rm{m/s^2 }## . This agrees fairly well with your data.

However, this gives a value for the tension of approximately ##0.19\,\rm N ##.
This is problematic.

Last edited:
ChiralSuperfields and erobz
The tension in the string cannot be more than the weight of m2, which is round about 0.2 N. Something is wrong with your measurements.

ChiralSuperfields, haruspex and SammyS