Final answer:
In an experiment investigating the velocity of a trolley using a ticker timer, the independent variable would be the force applied to the trolley, the dependent variable would be the trolley's velocity, and the controlled variables could include the weight of the trolley, the slope angle, air resistance and friction.
Step-by-step explanation:
The independent variable in this experiment would be the force applied to the trolley, as this is the variable you would manipulate to observe changes in the trolley's velocity. The dependent variable would be the velocity of the trolley, since this is being measured and can change based on the independent variable. The controlled variables could include factors such as the weight of the trolley, the angle of the slope it's traveling on (if any), air resistance and friction, which should all be kept constant during the experiment to ensure that any changes in the velocity can be attributed solely to changes in the applied force.
Learn more about Independent, Dependent and Controlled Variables