Effect of friction on total energy?

[omar01]

hey guys,

I am trying to find the effect of friction on the total Energy of an object
my object is a small car with 2 motors with a known speed and a constant one (without an acceleration)
this object will be dragging a certain mass behind it

I already came out with an equation but not really sure if its working

The equation :
1/2 m*v₀²= 1/2 m*v²+ E_f
knowing that E_f = X + added mass

thanks,

 

[jbriggs444]

You speak of a cart with a single constant speed. Then you write an equation with two different variables (v₀ and v). This suggests that the speed is not, in fact, expected to be constant.

What exactly is it that you plan to measure? What is X? What is this mysterious "added mass"?

 

[DEvens]

Welcome to the forum.

It is not clear what you are trying to work out. If the car has constant speed, then the kinetic energy is constant. So why do you have ##v_0## and ##v##? This confuses me.

Also: Is the fact that the car has two motors important? What does ##X## refer to? Is the "added mass" the mass of the trailer? And why should ##E_f## be related to the added mass?

The friction might be causing something to get warmer. Or it might cause the car motor(s) to have to work harder so using up fuel faster. But at constant speed it does not change the energy of the car.

 

[omar01]

Yeah sorry this was a bit unclear
The experiement will be separated in two steps
The first is to add a known mass on my cart and measure it speed
The second part is to drag the same mass behind the cart and again measure it speed
By doing this I'll be able to see the effect of friction on the speed of the cart

Thanks for your help and hope this helps a bit

 

[jbriggs444]

Your expectation is that the cart towing the block will attain a constant speed lower than of the cart when it is not towing a block? You after an equation that predicts this? Well, let's figure out what parameters such an equation should have...

What do you think is responsible for the limited speed attained by the cart when it is not towing a block?

 

[omar01]

what I'm trying to prove is that with the friction between the floor and the weight will eventually slaw down the cart

 

[jbriggs444]

what I'm trying to prove is that with the friction between the floor and the weight will eventually slaw down the cart

Then run the experiment. See whether your prediction is correct.

 

[omar01]

Yeah this is what I am planing to do, but i need to compare my experimant with theoritacal values and also to calculate the friction effect on the speed
And i came up with these equations but not sure if they work

do you have any opignion about the equation (it will be helpful)

thanks,

 

[jbriggs444]

Yeah this is what I am planing to do, but i need to compare my experimant with theoritacal values and also to calculate the friction effect on the speed
And i came up with these equations but not sure if they work

The equations do not work.

Again, I ask, what do you think is responsible for the limited speed of the unladen cart? Where does that limitation show up in your equations?

 

[EM_Guy]

I think the experiment you are proposing is both complex and confusing.

You have wheels and motors involved here. Wheels roll. But wheels also slip and slide. Rolling friction is what makes rolling possible. That is, without friction, there would be no rolling. But when wheels slip and slide, then you have kinetic friction. You also have motors, which convert electrical energy into rotational mechanical energy.

http://www2.hcmuaf.edu.vn/data/phamducdung/thamkhao/truyendong/ly%20thuyet-truyendong/friction-rolling/Rolling%20Motion.htm

When you have pure rolling motion without any sliding, the instantaneous velocity of the point of the wheel in contact with the ground is zero.

 

[A.T.]

Rolling friction is what makes rolling possible

Static friction makes rolling possible. Rolling resistance opposes it.

 

[EM_Guy]

Thanks for the correction.

 

[omar01]

Yeah that is true
But in my experience I'm ignoring the friction of the wheels

And for the motor I am only using it to have a constant speed throw the experience

 

[EM_Guy]

What you are trying to do (I think) is get a thing moving at some speed. The thing has a force pulling or pushing it, and then it has kinetic friction opposing it. Then, you suddenly through more weight on the thing, while the pulling / pushing force remains the same? The kinetic friction (as I recall) is proportional to the normal force. When you increase the normal force, you increase the kinetic friction. If by increasing the normal force, the kinetic friction exceeds the applied pulling/pushing force, then the thing will decelerate.

But will the motors cause the pulling force to remain constant, or will the motors cause there to be an adjustment to the pulling force at some point?

 

[omar01]

Yeah you got it right
My motor should have the same power with or without the weight it won't readjust with the mass

 

[jbriggs444]

Yeah you got it right
My motor should have the same power with or without the weight it won't readjust with the mass

Will the motor generate the same power regardless of the speed at which the cart is moving? Will it produce the same forward force?

 

[omar01]

Yeah it should be doing it regardless of the mass
Or at least I programmed it to do so

 

[jbriggs444]

Yeah it should be doing it regardless of the mass
Or at least I programmed it to do so

Which is it? Same power or same force? And how do you know?

 

[omar01]

its the same power, the motor is programmed to do x number of cycles in a second

 

[EM_Guy]

its the same power, the motor is programmed to do x number of cycles in a second

x cylces per second is an angular velocity (rev/min or rad/s). Power = Torque x Angular Velocity. What kind of motor are you using? Do you know its torque versus speed characteristics?

 

[omar01]

the motor I am using if the nxt lego motor i think their is a way to know the troque versus on the internet