Newton’s Laws of Motion (AQA GCSE Physics): Revision Notes
Required practical - Force, mass and acceleration
What is this practical about?
This required practical helps you understand Newton's Second Law by investigating how changing the force on an object affects its acceleration. You'll use a glider on an air track and measure how fast it speeds up when different forces are applied to it.
This practical demonstrates one of the fundamental principles of physics - the relationship between force and acceleration. By keeping mass constant and varying force, you can observe this relationship directly.
Aim of the experiment
The main goal is to find out how force affects the acceleration of a moving object. You'll keep the mass the same but change the force, then measure what happens to the acceleration.
Equipment you need
For this practical, you'll use several pieces of equipment:
- Linear air track with gliders - provides a smooth, friction-free surface
- Light gates - electronic sensors that detect when the glider passes through
- Bench pulley - changes the direction of the string
- String - connects the glider to the weights
- Weight stack and weights - provides the force to pull the glider
- Card of known length - attached to the glider to trigger the light gates
- Clamp stands and clamps - hold equipment in position
- Vacuum cleaner on blow setting - creates the air cushion under the glider
The air cushion created by the vacuum cleaner is crucial for this experiment. It virtually eliminates friction between the glider and track, ensuring that the only significant horizontal force acting on the glider is the tension from the string. This allows you to study the pure relationship between applied force and acceleration.
Method - how to do the experiment
Step 1: Set up the equipment Set up the air track with the glider, making sure it's level. Attach light gates at two positions along the track with a known distance between them. Connect the string from the glider, over the pulley, to the weight stack.
Step 2: Prepare the timing system Set up the light gates so they can measure both velocity and time as the glider passes through them. Make sure the card attached to the glider will break both light beams clearly.
Step 3: Start the experiment Turn on the vacuum cleaner to create the air cushion. This reduces friction so the glider moves smoothly. Release the glider and let the weight pull it along the track.
Step 4: Collect data Record the velocity and time measurements from the light gates for different amounts of weight on the weight stack. This changes the force pulling the glider.
Step 5: Calculate acceleration Work out the acceleration by finding the difference between the two velocity readings and dividing by the time taken for the glider to travel between the light gates.
Worked Example: Calculating Acceleration
If the glider has velocity v₁ = 0.5 m/s at the first light gate and velocity v₂ = 1.2 m/s at the second light gate, with a time interval of Δt = 0.8 s:
Step 1: Find the change in velocity Δv = v₂ - v₁ = 1.2 - 0.5 = 0.7 m/s
Step 2: Calculate acceleration using a = 0.7 ÷ 0.8 = 0.875 m/s²
Safety considerations
Be careful when using this equipment. The glider can move quite fast and could be dangerous if it hits someone. Make sure the area around the air track is clear, and consider what might happen if the glider reaches the end of the track. Always ensure there are adequate stops or cushioning at the ends of the track.
Recording your results
Create a table to record your measurements. Include columns for the force applied, and then record acceleration readings. Take multiple readings for each force value to improve accuracy, then calculate a mean average.
Taking repeat measurements is essential for identifying and reducing random errors. Calculate the mean of your readings and consider the spread of values to assess the reliability of your data.
Key physics concepts
Understanding velocity and acceleration Velocity measures how fast something moves in a particular direction. It's calculated as distance moved divided by time taken:
Acceleration measures how quickly velocity changes. When something speeds up or slows down, it's accelerating. You calculate acceleration by dividing the change in velocity by the time taken for that change:
Why use light gates? Light gates are much more accurate than using a stopwatch and ruler. They automatically start and stop timing when the glider passes through, removing human reaction time errors. This gives you more precise measurements for your calculations.
Human reaction time is typically around 0.2-0.3 seconds, which would introduce significant error when timing fast-moving objects. Light gates eliminate this source of error and can measure time intervals to millisecond precision.
Expected results and conclusion
When you plot a graph of acceleration against force, you should get a straight line through the origin. This shows that acceleration is directly proportional to the applied force - when you double the force, you double the acceleration.
This relationship proves Newton's Second Law: the acceleration of an object is proportional to the force acting on it (when mass stays constant).
Mathematically, this relationship can be expressed as , where F is force, m is mass, and a is acceleration. Since mass remains constant in this experiment, (force is proportional to acceleration).
Summary
Key Points to Remember:
- The aim - investigate how force affects acceleration of a glider
- Key equipment - air track, light gates, weights, and pulley system
- Safety first - fast-moving glider can be dangerous
- Light gates are better - more accurate than stopwatches for timing
- Expected result - acceleration increases proportionally with force
- Physics principle - demonstrates Newton's Second Law ()