Required practical activity 19: investigate the effect of varying the…

The idea

This required practical tests Newton's Second Law, a = F ÷ m, in two ways. In the first, keep the mass constant and change the force, then measure the acceleration; a graph of acceleration against force should be a straight line through the origin, showing that acceleration is directly proportional to force. In the second, keep the force constant and change the mass, then measure the acceleration; a graph of acceleration against mass curves downwards, and a graph of acceleration against 1 ÷ mass is a straight line through the origin, showing that acceleration is inversely proportional to mass. A typical method uses a trolley on a runway, a string over a pulley, and hanging masses to provide the force, with light gates or a motion sensor to time the trolley. Control variables include the same runway, the same trolley and the same starting position.

What earns the marks

• Identifies the independent variable, dependent variable and control variables for each part of the investigation, for example force changed and acceleration measured while mass is fixed in part one.
• Describes a workable method to apply a known force to a trolley of constant mass, such as hanging masses on a string over a pulley, and to measure the acceleration, such as using light gates or a motion sensor.
• Explains how to reduce the effect of friction, for example by tilting the runway slightly until the trolley moves at constant velocity before the force is applied.
• Processes results by plotting a graph of acceleration against force and recognising a straight line through the origin as evidence that acceleration is directly proportional to force.
• Evaluates the investigation by identifying sources of uncertainty, such as timing errors or friction, and suggesting improvements such as repeating readings and calculating a mean.

Common mistakes and how to put them right

• Changing both force and mass in the same set of readings.

  Put it right: Change only one variable at a time, keeping the other quantity constant so the effect on acceleration can be attributed to the chosen variable.

• Ignoring friction and treating the hanging weight as the only force on the trolley.

  Put it right: Compensate for friction by tilting the runway, or discuss friction as a source of error that makes the measured acceleration smaller than expected.

• Plotting acceleration against mass and expecting a straight line through the origin.

  Put it right: Acceleration is inversely proportional to mass, so the straight-line graph is acceleration against 1 ÷ mass.

Exam tips

• State clearly which quantity you change, which you measure and which you keep the same; examiners look for this control of variables in practical questions.
• When describing the graph, name both axes and the shape you expect, and link the shape to the relationship between the variables rather than just saying 'it goes up'.
• Include one realistic improvement, such as repeating each reading and taking a mean, and explain how it reduces the effect of random error.