Newtons third law of motion

[Ballox]

Homework Statement

A train consisting of two cars pulled by a locomotive experiences an acceleration of 0.33m/s^2 [fwd]. Friction is negligible. Each car has a mass of 3.1 * 10^4 KG.

a. ) Determine the force exerted by the first car on the second car.
b. ) Determine the force exerted by the locomotive on the first car.

Homework Equations

Applying Newtons third law of motion which states:

for every action force, there is a simultaneous force equal in magnitude, but opposite in direction

The Attempt at a Solution

I drew a FBD of the situation and solved for part a) by simply applying Newtons second law equation [ F= mA ]

I solved for F and got 1.0 * 10^4 N.

However I have trouble solving for part b.)
The answer is supposed to be 2.0 * 10^4 N, but I can't get it.


Would appreciate any help/advice!
Many thanks in advance.

 

[benhou]

Since your answer for part a) is the force of the first car pulling on the second, now you can apply Newton's third law.

 

[Ballox]

Never mind. I've figured out the solution. The force of the locomotive on the first car involves the masses of the 1st and 2nd cars. Therefore I add the two masses together to get the total mass however I keep acceleration the same...

Using Newton's second law equation I determine the force.

F = mA
F = (62000)(0.33)
Therefore F = 2.0 * 10^4 N [fwd]

 

[benhou]

Ok. Since car 1 pulls car 2 with such a force you got in part a), car 2 must pull car 1 with an equal and oppose force according to Newton's third law. From there you can calculate the force exerted on car one by the locomotive. Maybe drawing a free body diagram would help.