Acceleration required practical

Aim: To investigate how the acceleration of a trolley depends on the resultant force acting on it and on its mass.

Variables

• Independent: Resultant force on the trolley (part 1); mass of the trolley (part 2)
• Dependent: Acceleration of the trolley
• Control: Total mass of the system when changing force (move masses from hanger to trolley); Force applied when changing mass; The runway surface and angle; Distance between the light gates

Equipment

• Trolley
• Long bench or runway
• Pulley clamped to the end of the bench
• String
• Slotted masses and hanger
• Light gates with data logger, or a metre rule and stopwatch
• Card to fix on the trolley for the light gates
• Extra masses to load the trolley
• Balance to find the trolley's mass

Method

1. Clamp a pulley to the end of the bench and attach a string from the trolley over the pulley to a mass hanger.
2. Set up two light gates along the runway (or mark start and finish lines a measured distance apart).
3. Put all the slotted masses on the trolley, then move one onto the hanger to give an accelerating force.
4. Release the trolley from the same starting point each time and record its acceleration from the light gates.
5. Move another mass from the trolley to the hanger and repeat, so the force increases but the total mass stays the same.
6. Repeat each force three times and calculate a mean acceleration.
7. For the effect of mass, keep the hanging mass the same and add extra masses to the trolley one at a time, recording the acceleration each time.
8. Convert each hanging mass to force using W = m g.

Safety

• Place a box of foam or a cushion on the floor below the hanging masses so they land safely.
• Stop the trolley at the end of the bench so it does not fall off, for example with a buffer.
• Keep feet out from under the falling masses.

Results and calculations

Record force (or mass) and mean acceleration in a table. Plot acceleration against force for part 1 (a straight line through the origin shows a ∝ F) and acceleration against mass, or 1 ÷ mass, for part 2 (a is inversely proportional to m). Use F = m a; if using a ruler and stopwatch, find a from a = Δv ÷ t or v² − u² = 2 a s.

Common mistakes and improvements

• Friction on the runway reduces the resultant force: tilt the runway slightly until the trolley just rolls at a steady speed when pushed, so friction is balanced.
• Timing with a hand stopwatch has large human reaction-time errors: use light gates and a data logger.
• Changing the hanging mass without keeping the total mass constant means two variables change: move masses between the trolley and hanger instead of adding new ones.
• The string rubs on the pulley or is not parallel to the runway: align the pulley so the string is horizontal and runs freely.

Exam tips

• Explain why masses are moved from the trolley to the hanger: the accelerating mass is the trolley plus hanger, so total mass must stay the same to make it a fair test.
• State Newton's second law in words: acceleration is directly proportional to resultant force and inversely proportional to mass.
• Say why light gates are better than a stopwatch: they remove human reaction time and give more precise readings.
• Practise calculating acceleration from light gate data: speed = card length ÷ time to pass through the gate.
• If a graph does not pass through the origin, suggest friction as the cause.