Required practical activity 7: investigate the effect of varying the force…

The idea

This practical investigates Newton's Second Law, F = ma. You will use a trolley on a friction-compensated ramp, connected by a string over a pulley to suspended masses. To investigate varying force with constant mass, transfer masses one by one from the trolley to the weight hanger. This increases the accelerating force while keeping the total system mass constant. To investigate varying mass with constant force, keep the hanging mass constant and add masses directly to the trolley. Acceleration is measured using light gates and a data logger, or by timing the motion over a set distance. Plotting graphs of acceleration against force, and acceleration against inverse mass, should yield straight lines through the origin, confirming the relationships.

What earns the marks

• Describe a valid method to measure the acceleration of the trolley, such as using two light gates to find initial and final velocities over a known distance.
• Explain how to vary the accelerating force while keeping the total mass of the system constant by transferring masses between the trolley and the weight hanger.
• Explain how to vary the mass of the system while keeping the accelerating force constant by adding masses to the trolley but leaving the hanging mass unchanged.
• Identify the independent, dependent, and control variables for both parts of the investigation.
• Describe how to use the collected data to plot appropriate graphs to verify that acceleration is directly proportional to force and inversely proportional to mass.

Common mistakes and how to put them right

• Changing the accelerating force by adding new masses to the hanger from an external supply.

  Put it right: Transfer masses from the trolley to the hanger so the total mass of the system remains constant.

• Failing to compensate for friction.

  Put it right: Slightly tilt the ramp until the trolley moves at a constant velocity when given a small push, ensuring the hanging weight is the only resultant force.

• Confusing the mass of the trolley with the total mass of the system.

  Put it right: The total mass being accelerated includes the trolley, the string, and the hanging masses.

Exam tips

• Clearly state the equipment used to measure each variable, for example using a balance to measure mass and light gates to measure velocity and time.
• When describing the method, explicitly state which variables are kept constant in each part of the experiment to ensure a fair test.
• Remember that a graph of acceleration against mass produces a curve, but a graph of acceleration against the inverse of mass produces a straight line through the origin.