Newton's Second Law

Unbalanced forces

curriculum-key-fact
A resultant force is the single force acting on the object when all the individual forces have been combined.

How to investigate experimentally Newton’s Second Law using light gates and a data logger

Resultant force and acceleration

Investigating experimentally Newton’s Second Law using light gates and a data logger

Method

  1. Friction compensated the runway so that the trolley moves at constant speed.
  2. Attach a length of card to the trolley. Measure the length of card with a 30 cm ruler.
  3. Set up light gates and the data logging software to measure acceleration from A to B. When prompted, type the length of card in cm.
  4. Put a 100g masses on to the mass hanger. Place 6 other 100g masses on the trolley.
  5. The 100g mass on the hanger acts as a resultant force of 1N accelerating the trolley along the runway. Record the force F in a suitable table.
  6. Release the trolley and allow the card to pass through light gates A and B. Record the acceleration.
  7. Repeat twice and average the acceleration.
  8. Move a 100g mass from the trolley to the hanger. This ensures that the mass being accelerated remains constant. Record F and repeat 6 and 7 above. Repeat for each mass in turn.

Results

It is important to record results in a suitable table, like the one below:

Force (N)Run 1 acceleration (m/s)2Run 2 acceleration (m/s)2Run 3 acceleration (m/s)2Mean acceleration (m/s)2

Graph

Graph showing a straight line through the origin. This tells us that resultant force F and acceleration a are directly proportional.F = force; A = acceleration

Plot a graph of resultant force F in N on the y-axis against acceleration a in m/s2 on the x-axis. Draw the line of best fit.

The graph is a straight line through the origin.

This tells us that resultant force, F, and acceleration, a, are directly proportional.

If you double the resultant force acting on an object, you double its acceleration.

HazardConsequenceControl measures
Electrical appliance (data logger and computer)Electrical fault - fire/shockDo not set up close to taps or sink. Check mains cable and plug are not broken, or wiring exposed before use.
Masses and hanger falling to floorObjects falling on feet - bruise/fractureUse relatively small masses and step back after releasing masses. Place bench stools as a cordon beneath the accelerating masses to prevent feet being placed beneath them.