The force pulling on the trolley was increased so that the trolley accelerated. The force was then kept constant and different masses were put on the trolley. For each different mass the acceleration of the trolley was measured.
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Which of these is the independent variable?

In a scientific investigation: - The **independent variable** is the one you change. The description says "different masses were put on the trolley". - The **dependent variable** is the one you measure. The description says "the acceleration of the trolley was measured". - **Control variables** are the ones you keep the same. The description says "The force was then kept constant". Therefore, the total mass of the trolley is the independent variable.