The force sensor measures the force on the sensor due to the bumper, but the cart's momentum change arises from the force on the
cart due to the bumper. Which of the following facts are needed to assert that the magnitude of these two forces are nearly equal at all times. 1. Their magnitudes differ by the magnitude of the net force on the bumper. 2. The force of friction is small. 3. The net force on the bumper is small. 4. The cart's weight is canceled by the the normal force exerted by the track.
Every bit of force applied to the bumper will be transmitted to the cart EXCEPT for the force needed to accelerate the bumper. This is the net force on the bumper.
If the bumper was heavy then a significant amount of force might be needed to accelerate the bumper so the amount transmitted to the cart would be substantially reduced.
If the net force on the bumper is small then the amount transmitted to the cart is almost the entire force applied.
