Hi, this problem’s been bugging me for ages. I’m sure it’s just a simple misunderstanding somewhere on my behalf but I just can’t seem to get my head round it. 1. The problem statement, all variables and given/known data A monorail car with front wheel drive and mass of 5 tonnes is suspended from two wheels 10m apart as shown. Its centre of mass G is midway between the wheels and 4m below the rail. Starting from rest, the train is designed to achieve a maximum speed of 20km/h in 40s with uniform acceleration. Calculate the minimum coefficient of friction between the front wheel and the rail to achieve this. Part 2 Determine the acceleration that could be achieved with the same coefficient of friction if rear wheel drive was used, and hence the time to reach 200km/hr. The answers given are 0.254 and 49.1 I've attached a photo of the diagram. 2. Relevant equations F=ma F=uR V=at 3. The attempt at a solution 1. Firstly calculate the acceleration in m/s^2 To do this I converted the speed 200 km/hr is equivalent to 200x10^3 m/hr Divide into seconds 200x10^3/3600 = 55.5 m/s Then divide by 40s to get the acceleration needed (v/t=a)so a = 1.38ms^-2 or 25/18 2. Calculate the force needed to accelerate the mass F=ma, m=5000kg, a=1.38ms^-2 5000 x 1.38 = 6944.4N or 62500/9 3. Work out the coefficient of friction. a) Work out the reaction force of the front wheel on the beam, using moments and resolving forces, giving you 24525N (2500kg x 9.81) b) Use F=uR Calculated F needed as 6944.4, and R as 24525N Rearrange formula F/R=u 6944.4/24525 = 0.283 which is wrong!? Surely the centre of mass G must affect it somehow but I don’t understand how to incorporate that. I can’t do the second part of the question because of the reason above. If the centre of mass was off centre then I could understand how the coefficient of friction and acceleration between front and back would differ. Any help would be greatly appreciated.