Register to reply

Three same-mass freight cars, why force on each is not same ?

by ato
Tags: cars, force, freight, samemass
Share this thread:
Dec25-12, 03:16 AM
P: 30
three cars of mass m are pulled with force F by a locomotive. Find forces on each cars ?
assuming the force F is exerted on car C and nearest to C is car B and remaining one is A.
[tex]\vec{F}_{A}[/tex] is total sum of all (interbody-) forces on car A . similarly [tex]\vec{F}_{B}[/tex] and [tex]\vec{F}_{C}[/tex] are defined.

assuming the forces that the question asks is [tex]\vec{F}_{A}[/tex],[tex]\vec{F}_{B}[/tex] and [tex]\vec{F}_{C}[/tex] and the given information is

according to newton's 2nd law
$$\vec{F}_{B}=\frac{d^{2}}{dt^{2}}m\vec{r}_{B}$$ and

but since
$$\frac{d^{2}}{dt^{2}}\vec{r}_{A}=\frac{d^{2}}{dt^{2}}\vec{r}_{B}=\frac {d^{2}}{dt^{2}}\vec{r}_{C}$$, $$\frac{d}{dt}m =0$$

so above five equations would give

according to superposition principle,
because there is only one force i.e tension force due to string, exerted on A.
because two tension forces (from both A and C) is acting on B.
because one external force of magnitude F and one tension force from B.

according to 3rd law we also have,
$$\vec{F}_{AB}=-\vec{F}_{BA}$$ and

so from above five equations,
but i dont undertand why its wrong because the solution says
$$\vec{F}_{B}=\frac{2\vec{F}}{3}$$ and

the only problem think i can think of is may be the question is asking for different forces , because there are forces that have same value for example ,

thank you
Phys.Org News Partner Physics news on
UCI team is first to capture motion of single molecule in real time
And so they beat on, flagella against the cantilever
Tandem microwave destroys hazmat, disinfects
Doc Al
Dec25-12, 03:41 AM
Doc Al's Avatar
P: 41,566
The question is not asking for the net force on each car, which of course must be equal. It is asking for the force that C exerts on B and B exerts on A in terms of the force F that the locomotive exerts on C.
Dec25-12, 04:01 AM
P: 30
so the question was indeed asking for $$\vec{F}_{CO}$$,$$\vec{F}_{BC}$$ and $$\vec{F}_A$$ .

i dont understand why not say so in the question, instead of being so short and confusing. i though the book was teaching physics not reading mind.

thanks for the help Doc Al

Register to reply

Related Discussions
Conservation of momentum continous mass flow freight cart. Introductory Physics Homework 4
Freight car-caboose collision finding mass without velocities Introductory Physics Homework 12
Cars and Center of Mass Introductory Physics Homework 1
A freight train has a mass of 4.5*10^7 kg Introductory Physics Homework 2
2 Cars Colliding with same mass Introductory Physics Homework 1