# Center of mass of a vehicle

#### Altairs

Ah … now I see …

No, when you said 1/4 th of the car, I thought you meant that only one tyre was on the platform
Oh...right..

#### tiny-tim

Homework Helper
The idea is that the whole car will be over the platform. Then the car will be lifted from it back by men or something so that they will be standing on the ground. My point was that there is no difference in this and simply putting front two tyres over and rear two tyres off the platform but my friends wouldn't agree.
If the men are lifting it by holding onto the rear axle, then your friends are wrong, and you are right … the distribution of the forces (or reactions) depends only on the positions of the points of support on the car.

(But if the men are holding it by the tow-bar under the rear bumper, then the reading will change, because the c.o.m. is further from the tow-bar.)
Second point is that will this procedure (the tipping one) when performed for both the front and the rear tyres equal the weight ?
Sorry … I'm not following this at all … why would you get men to tip the car when you can get the same reading by leaving the rear tyres on the ground?

#### Whatevr

when we tilt the car and find the reaction thn v'l hav a frictional force acting on the tyre,in contact with the ground apart from the surface reaction so will we consider it as limiting friction or how do we calculate it?

#### tiny-tim

Homework Helper
Welcome to PF!

when we tilt the car and find the reaction thn v'l hav a frictional force acting on the tyre,in contact with the ground apart from the surface reaction so will we consider it as limiting friction or how do we calculate it?
Hi Whatevr! Welcome to PF!

The weighing platform will only measure the vertical reaction, and the friction force is entirely horizontal, so the friction force will make no difference.

#### Whatevr

but when u find the z distance u take moments abt a point nd therefore u hav 2 kno all the forces how else wud u claculate the distances

#### tiny-tim

Homework Helper
but when u find the z distance u take moments abt a point nd therefore u hav 2 kno all the forces how else wud u claculate the distances
Hi w'v'r!

ah, you've made a good point … if the higher pair of tyres is supported on a sloping surface, then the friction will spoil the equations.

So it is important that the higher pair of tyres (or its axle) be supported on a raised horizontal surface.

#### Whatevr

no i dont get it:( that wont b possible i thnk or the frictional component wud stil exist ??
friction will exist if we raise the car frm one end

#### tiny-tim

Homework Helper
no i dont get it:( that wont b possible i thnk or the frictional component wud stil exist ??
friction will exist if we raise the car frm one end
If all the wheels are on horizontal surfaces, why would the car want to move anywhere (even with the brakes off)?

There'd be no gravitational advantage in moving.

#### Whatevr

no but thn how'd we calculate the z distance i.e. the height part?

#### Altairs

no but thn how'd we calculate the z distance i.e. the height part?
If we can do away with the friction part than it is quite simple.

#### Whatevr

ummmmmm thats the prob that friction exists and we hav 2 b as exact as possible so v hav 2 consider friction

#### Altairs

I still don't see any point in taking friction. Secondly, I don't have the wildest idea that how will we calculate friction in our case( I am even confused that while the car is in horizontal position with no external horizontal force acting over the platform the friction is acting or not !!!). Its definitely not limiting. Thirdly, we have to give a 'standard procedure'. With friction involved it won't remain standard as we would need to know the different values of $$\mu$$ for different contacting surfaces (tyres and platforms).

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#### Altairs

if the higher pair of tyres is supported on a sloping surface, then the friction will spoil the equations.

So it is important that the higher pair of tyres (or its axle) be supported on a raised horizontal surface.
This situation can also be simulated by hooking the front end of the car (from bumper or something) and then lifting.

#### tiny-tim

Homework Helper
This situation can also be simulated by hooking the front end of the car (from bumper or something) and then lifting.
Hi Altairs!

Yes … provided you ensure that the rope is vertical once the car is raised!

Though there is a practical problem … I don't think there's anything at the front of a car that's strong enough not to be damaged.

#### Altairs

Though there is a practical problem … I don't think there's anything at the front of a car that's strong enough not to be damaged.
Something can be attached as an extension to the front side and then the it can be hooked.

#### Whatevr

i dont get how ur trying 2 remove friction........

#### Altairs

How are we supposed to calculate the friction present ?

#### tiny-tim

Homework Helper
How are we supposed to calculate the friction present ?
By fixing it so that the friction is zero, which means supporting the wheels on horizontal surfaces, or the towbar on a vertical rope.

#### Altairs

By fixing it so that the friction is zero, which means supporting the wheels on horizontal surfaces, or the towbar on a vertical rope.
That is exactly what I thought (post 62).

$$4F_{s} = 0$$ (fixed and no external forces present horizontally)

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#### tiny-tim

Homework Helper
That is exactly what I thought (post 62).

$$4F_{s} = 0$$ (fixed and no external forces present horizontally)
Hi Altairs!

(btw, don't send a Visitor Message if it's important, since there's no notification of Visitor Messages, and I might not see it for days. Send PM if you think I've missed a post.)

Yes … ignore friction …

… your post #62, replying to Whatevr, was completely right (which is why I didn't comment on it).

#### Whatevr

ok so can u tell me how do we calculate the z-distance i.e. the height co-ordinate. we can get both the horizontal distances but wat abt the vertical one??

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