How Does Friction Impact Bobsled Acceleration?

[Balsam]

Homework Statement

In the men's four-man bobsled event in the winter olympics, the maximum mas of a bobsled with pilot, two riders, and a brakeman is 630 kg. During a practice run, the rear rider and the brakeman exert average forces of 1220N and 1200N [forward] respectively to accelerate a bobsled of mass 225kg, a pilot of mass 98 kg, and a forward rider of mass 97 kg(who have already gotten into the sled). Then they jump in for the challenging ride down the 1300m course. During the pushing, the magnitude of the force of friction acting on the bobsled is 430N. Calculate the average acceleration of the bobsled, pilot, and the forward rider.

Homework Equations

Fnet=ma

The Attempt at a Solution

I added all the masses and got m=420kg. I added all the forces and got Fnet= 1990N[forward]. But, when I divided the force by the mass, I got a=4.7m/s^2[forward]. The answer in the book is 4.4 m/s^2[forward]. I don't know where I went wrong.

 

[Dr. Courtney]

It's always a mistake not to draw a picture. In this case, make it a good free body diagram.

Formula roulette is, well, risky.