A cart with an unknown mass is at rest on one side of a track. A student must find the mass of the cart by using newton’s second law. The student attaches a force probe to the cart and pulls it while keeping the force constant. A motion detector rests on the opposite end of the track to record the acceleration of the cart as it is pulled. The student uses the measured force and acceleration values and determines that the cart’s mass is 0. 4kg. When placed on a balance, the cart’s mass is found to be 0. 5kg. Which of the following could explain the difference in mass?.