Edexcel IGCSE Physics · Spec 1.7
Newton's Second Law and F = ma Practical
Newton's second law and the F = ma required practical.
Physics revision video
Newton's Second Law and F = ma Practical
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Explained
Force, mass and acceleration, and the practical that tests the link
Newton's second law says that the resultant force on an object equals its mass multiplied by its acceleration. In symbols, F equals m a. The word resultant is doing real work in that sentence, and most of the mistakes in this topic come from ignoring it.
Resultant force
An object usually has several forces acting on it at once. The resultant is the single force that would have the same effect as all of them together. Forces along the same line add if they act in the same direction and subtract if they oppose.
If the resultant is zero, the object either stays still or keeps moving at constant velocity. It does not accelerate. If the resultant is not zero, the object accelerates in the direction of that resultant.
So a car travelling at a steady speed has a driving force, but it also has friction and air resistance that exactly balance it. The resultant is zero even though the individual forces are large.
What the equation is telling you
For a fixed mass, doubling the resultant force doubles the acceleration. Force and acceleration are directly proportional.
For a fixed force, doubling the mass halves the acceleration. Mass and acceleration are inversely proportional. This is why a loaded lorry pulls away more slowly than an empty one with the same engine.
The practical
The experiment tests both relationships, one at a time, which is what makes it a fair test.
A trolley runs along a track, pulled by a string over a pulley with masses hanging from it. Light gates, or a ticker tape, measure the acceleration. The pulling force is the weight of the hanging masses.
To investigate force, keep the total mass constant and change the pulling force. The trick is to move masses from the trolley to the hanger rather than adding new ones, so the total mass being accelerated does not change. Plot acceleration against force and you get a straight line through the origin.
To investigate mass, keep the hanging masses the same and load extra mass onto the trolley. Acceleration falls as mass rises. Plotting acceleration against one over the mass gives the straight line.
Friction is the main source of error. Compensating for it by tilting the track slightly is a standard improvement to suggest.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Physics gives advice that applies to every calculation in this topic. Candidates who find calculations challenging should substitute the data before attempting a rearrangement, because once a mistake has been made in rearranging, no marks can be given, and substitution into an incorrectly rearranged formula is not permitted.
The same report notes that derived units such as newtons require inputs in SI units, so converting grams to kilograms at the start is a good strategy for gaining marks even if you cannot complete the solution.
Both points are procedural rather than scientific, and both are entirely within your control. Write the equation, substitute, convert to kilograms and metres, then rearrange.
Weight is a force, mass is not
Mass is the amount of matter, measured in kilograms, and it does not change when you move the object. Weight is the force of gravity on that mass, measured in newtons, and it does change with gravitational field strength.
In the practical, the hanging masses provide the force through their weight, so you multiply by gravitational field strength before using F equals m a. Skipping that step is a common way to lose the calculation.
Spec 1.7
What you need to know
- State Newton's second law
- Explain how force and mass affect acceleration
- Describe the experiment that tests it
Active recall
Quick check
Answer each question before opening the answer.
State Newton's second law as an equation.
Resultant force = mass × acceleration (F = m × a).
If the resultant force doubles and mass stays the same, what happens to acceleration?
The acceleration also doubles (they are directly proportional).
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