A student experiment

A group of students wishes to show that when the mass of an object is constant, the object's acceleration is proportional to the total force exerted on it. The y set up the following arrangement. A low-friction cart is allowed to move along a horizontal track on a table. A string at the front of the cart passes over a pulley at the end of the table. A hanger to which weights can be added is suspended from the dangling end of the string. The students reason that they can vary the force on the cart by varying the amount of weight hanging from the string. Why is this setup not satisfactory for demonstrating what the students intend to show?