Force and Acceleration RP

This practical is about testing Newton's Law. You could be investigating either:

How changing the force affects acceleration when mass stays constant

How changing the mass affects acceleration when force stays constant.

Formula:

Equipment:

In either experiment, you start with a trolley on a track. To apply a force to the trolley, you need to attach a string to the trolley and hang it over a pulley. You can attach masses on a mass hanger to a loop on the end of the string - this will provide the accelerating force. The accelerating force can be calculated using the formula:

Remember gravitational field strength on Earth is .

In both experiments, you need to measure acceleration. You can do this using a stopwatch or using light gates.

Part 1: Investigating How Force Affects Acceleration

Stopwatch Version:

• Set up your bench as above. Measure and mark a distance for the trolley to travel, around works well.

• Attach the pulley to the end of the bench and make sure it's secure

• Tie string to your toy car, pass it over the pulley, and attach the full weight stack (e.g. ) to the other end

• Make sure the string is horizontal and in line with the car

• Hold the car at the start, then release it while starting your stopwatch

• Stop the stopwatch when the trolley reaches the end of the marked distance

• Remove mass from the weight stack (e.g. reduce to , then , etc.)

• IMPORTANT: Add the removed masses onto the top of the car each time - this keeps the total mass constant

• Repeat for each force value

To calculate the acceleration of the trolley, you need to use the acceleration equation:

Formulae:

The initial velocity of the trolley should be , and the final velocity of the trolley should be twice the average speed of the trolley. To calculate the average speed of the trolley:

We can double this average velocity to give the final velocity, then use it in the above acceleration equation.

Light Gate Version:

• Set up the trolley track that has an interrupt card attached (usually long)

• Position two light gates along the track at a measured distance apart

• Attach string to the trolley, pass over pulley, attach masses to create the pulling force

• When you release the trolley, each light gate measures the time for the interrupt card to pass through

• The computer calculates velocity at each light gate using: velocity = card length / time

• The computer then calculates acceleration using the change in velocity and the time between gates

• Repeat with different forces - but ensure the total mass stays the same

Why do we add the masses to the trolley?

This is a control variable situation. If you just reduce the hanging mass, you're changing two things at once - the force pulling the car and the total mass of the system (car + hanging weights). By transferring the masses from the string to the car, you keep the total mass constant, so you're only investigating the effect of changing force.

Part 2: Investigating how mass affects acceleration

Method:

• Use the same setup as Part 1

• This time, choose ONE force value and keep it constant

• Start with just the trolley, then add masses to the top of the trolley

• Calculate acceleration for each different mass using either a stopwatch or light gates.

You'll need to present your data in a table, then calculate acceleration for each trial. You should then draw a graph. If you are plotting force against acceleration you should find that they are directly proportional. This can be determined by the line of best fit - it should go straight and through the origin. If you are plotting mass against acceleration you should find that they are inversely proportional, as one increases the other decreases. This can be seen as a curving line downwards.

Improving Your Results

• Using an air track reduces friction to almost zero, making results more accurate.

• Light gates measure time much more precisely than human reaction time with a stopwatch.

• Raising one end of the ramp slightly means a component of the trolley's weight will be parallel to the trolly increasing the resultant force. This can be used to oppose any friction on the trolley.

Safety Considerations:

• Make sure the pulley is securely attached - you don't want it falling off

• Keep the area around the bench clear so the car doesn't hit anything

• Don't use masses that are too heavy - the car might crash into the pulley

• Catch falling masses before they hit the floor to avoid damage

• Be careful with the air track (if using) - keep fingers away from the blower