Newton's second law experiment

QUESTION:

My son has recently been working on his AS physics course work. The investigation is into acceleration and he has been using an air track and data logger to time acceleration of a moving truck carrying a given mass. A wire was connected to the front of the truck and passed over a wheel at the end of the bench where an accelerating mass was attached. He has been told to remove 0.1 g of mass from the truck at a time and add it onto the accelerating mass in order to change the accelerating force. Surely this gives two variables.

My question is why has he been told to remove mass from the object on the air track and add it to the accelerating mass on the end of the wire. Surely he should only add mass to the accelerating mass and not remove any from the truck. Can you confirm this please or tell me why I am wrong.

Answer:

The point is this.

The gravitational attraction on the masses hanging over the pulley accelerate BOTH trolley and the suspended masses! So if the mass of the trolley is M and the mass of the masses is m the TOTAL mass being accelerated is (M+m).

Now this is being accelerated by a force (mg) giving an acceleration of mg/(M+m).

The two variables in the experiment are the force and the TOTAL mass.

If we want to vary the TOTAL mass it is simple – just remove mass from the truck and keep the accelerating force mg the same.

If we want to vary the accelerating force (mg) and keep the total mass the same we remove a mass from the suspended masses to give a new accelerating force (say m'g) but in order to keep the TOTAL mass of the system the same we must put the mass that we have removed from the suspended masses onto the truck.