How do you investigate the effect of the object’s mass on the acceleration produced by a constant resultant force?

To investigate the effects of changing the object’s mass on the acceleration produced by a constant resultant force, attach a 100g mass to the end of the string. Then attach a 200g mass to the toy car. Record the car as it acerates along the bench. Now repeat the experiment, increasing the mass attached to the toy car. Newton’s third law states the acceleration of an object is inversely proportional to the mass of an object. Therefore as the mass of the toy car increases the acceleration should decrease.