At the local grocery store, you push a 15.2 kg shopping cart. You stop for a moment to add a bag of dog food to your cart. With a force of 12.0 N, you accelerate the cart from rest through a distance of 2.24 m in 3.00 s. What was the mass of the dog food? F=ma m=f/a I rearranged x(f)=x(i)+v(i)+1/2at^2 to be a=(1/2t^2)/x so, 1/2(3)^2/2.24= 2.01 m/s^2. m=f/a m=12.0/2.01=5.97kg. This defenately looks wrong. How can the cart's mass of decreased? I'm confused what did I do wrong? I tried 15.2-5.97 and that was also wrong.