Required Practical 14: Force, Mass, and Acceleration

Section 1 of 7Idea behind the practical

Newton's second law

• Newton's second law Resultant force = mass × acceleration, F = m × a, with force in newtons (N), mass in kilograms (kg) and acceleration in m/s².
• So acceleration is directly proportional to force and inversely proportional to mass.

Two investigations

• Two investigations Investigation 1 changes the force while the total mass stays the same.
• Investigation 2 changes the mass while the force stays the same.

Section 2 of 7Method: changing the force

1

Set up a trolley on a level, friction-compensated runway, attached by a string over a pulley to a hanging mass holder.

2

Fix a card of known length to the trolley so it can pass through a light gate connected to a data logger.

3

Put five 100 g masses on the trolley, so the total mass of the system stays constant.

4

Release the trolley and record its acceleration from the light-gate readings.

5

Move one 100 g mass at a time from the trolley to the holder, which raises the pulling force without changing total mass.

6

Repeat each setting three times and calculate a mean acceleration.

Section 3 of 7Method: changing the mass

1

Keep a constant pulling mass, for example 100 g (weight 1 N), on the holder.

2

Start with the trolley carrying no extra load, and record its acceleration.

3

Add 200 g to the trolley each time and repeat the run, again taking three readings for each mass.

4

Use the total mass of trolley plus load plus hanging mass when working out the relationship.

Section 4 of 7Variables and safety

Type	Force investigation	Mass investigation
Independent	Pulling force (weight of hanging masses)	Mass of the trolley and its load
Dependent	Acceleration of the trolley	Acceleration of the trolley
Control	Total mass, runway slope, string length	Pulling force, runway slope, string length
Safety	Catch the trolley with a buffer or sandbox; keep feet clear of falling masses	Same precautions; do not let masses drop onto the floor

Section 5 of 7Processing the results

1Plot a graph

• Plot a graph Plot force against acceleration for the first investigation.

2Straight line through the origin

• Straight line through the origin This shows acceleration is directly proportional to force.

3Plot acceleration against 1/m

• Plot acceleration against 1/m Plot acceleration against 1 ÷ mass for the second investigation.

4Conclude

• Conclude A straight line through the origin shows acceleration is inversely proportional to mass.

Section 6 of 7Key terms

Term	Meaning
Resultant force	The single force that has the same effect as all the forces acting on an object.
Light gate	A sensor that records the time an object takes to cut a light beam.
Friction compensation	Tilting the runway slightly so that the component of weight cancels friction.
Directly proportional	Doubling one quantity doubles the other; the graph is a straight line through the origin.
Inversely proportional	Doubling one quantity halves the other.

Section 7 of 7Exam technique

1

Adding masses to the hanger from outside the system; always move them from the trolley so total mass is constant.

2

Using grams in F = m × a; convert to kilograms before calculating.

3

Forgetting to compensate for friction, which makes the graph miss the origin.

4

Drawing a line to the plotted points that joins dot to dot rather than a best-fit line.