# Question on Newton's law of motion

1. Apr 9, 2010

### gaobo9109

1. The problem statement, all variables and given/known data
A train consists of three coaches A,B and C linked together via tow bars. A and C are coaches with engines and have a mass of 10000kg each, while B is an un-powered coach having a mass of 8000kg. The driving force of the engines in A and C are 20000N and 18000N respectively. The drag force on A is 9000N while B and C experiences the same drag force of 6000N.

2. Relevant equations
Newton's three laws

3. The attempt at a solution
I tried to find the net force exerted on coach A by
F(net) = 20000 - 9000 = 11000N
a = 11000/10000 = 1.1m/s2
However, the answer given is 0.61m/s2
Can anyone tell me where did I go wrong? Thanks

2. Apr 9, 2010

### py1sl

Hi Gaobo,

The question gives you extra information which you haven't used which is key to getting answer.
I suggest you draw a picture and consider the entire train, currently you have missed the forces relating to coaches B and C.

Hope this helps