# Newtons third law of motion

## 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.

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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.

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]

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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.