Physics IA · A · Space, time and motion · topic 2 of 30
Friction: is the coefficient really a constant? · IB Physics IA idea: guide, sample graph and step-by-step plan
plain kit, careful physics. good.Friction is meant to be F = μN, with μ a fixed number for two surfaces. You're going to test that model and see how far it holds.
- ✓ you like testing simple models
- ✓ you can use a force sensor
- ✓ you don't mind patient repeat readings
The big idea
← test the MODEL, not just μThe simple model says friction is proportional to the normal force, so a graph of friction against N is a straight line through the origin with gradient μ. Static friction and kinetic friction are different quantities, so decide which one you measure and how. Change the load and the surface, and ask whether μ stays constant, and whether the line really goes through the origin. A good IA tests the model, not just a number for μ.
Make it yours: pick one twist
Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.
Swipe for more twists →
Use strips of paper, felt, rubber and sandpaper under the same block.
→ μ becomes a property of the pair of surfaces; your analysis compares gradients and their uncertainties.Turn the block onto a face with a different contact area, same mass.
→ The model predicts no change; a clean test, with the evaluation asking why any difference appears.Record the peak force just before sliding and the force during steady sliding.
→ Two gradients on one graph; needs a force sensor to see the peak, and needs care to separate them.Pull at two or three controlled speeds, for instance with a motor.
→ Tests whether kinetic friction depends on speed; the graph becomes μ against speed.Increase the angle until the block slides, or time a slide down.
→ A second route to μ (tan θ at the point of slipping) to compare with the pull method.Shoe soles on a gym floor, tyre rubber on tarmac or a brake pad.
→ Gives your introduction a purpose; the physics and graph stay the same.Your Physics IA research question: run the check
name static OR kinetic hereWrite your own question first. Then make sure it ticks every box:
- Names one independent variable and its range (the surface, or the normal force from added masses).
- Names one dependent variable and how it is found (static or kinetic, and from which part of the sensor trace).
- Says what stays fixed (pulling speed, string angle, surface condition, block).
- Says what you compare with (the model F = μN, and what you will conclude about μ).
- Is narrow enough for about 10 hours, with at least 6 loads or surfaces and repeats.
The set-up
One way to arrange it. Draw yours with the real block, surface and sensor, and show the direction of the pull.
How your Physics IA could be laid out
The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.
Your question; where friction matters (shoes, brakes, tyres); what you expect and why.
Normal force and friction; the model F = μN; static against kinetic; why the gradient of F against N is μ.
Independent, dependent and controlled variables, each with how you set, measure or hold it, and why it matters.
How the block is pulled level and at steady speed; how the sensor is zeroed; how you read static and kinetic values; surface preparation; repeats.
Short: falling masses, dust from rough surfaces.
One table: added mass, normal force, each force reading, units and uncertainties in the headers.
Mass to normal force, averaging repeats, uncertainty in F and N, and how you find μ from your data.
F against N with error bars and fit; gradient and intercept and what they mean; one line for each surface if relevant.
Answer with numbers: μ for each case, whether the line passes through the origin, and what that says about the model.
Weaknesses ranked by their effect on your result, each linked to a realistic improvement.
Every source for theory and method.
What your Physics IA graph could look like
Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only
- The axes make the model a straight line through the origin: easy to test.
- Error bars on both axes: the sensor uncertainty on F and the weighing uncertainty on N.
- Say which friction (static or kinetic) each line shows, and how you read it from the trace.
- A line that misses the origin is a result, not a nuisance: what could cause an offset?
- Another graph to consider: μ against N for each point, to see directly whether μ drifts with load.
Which uncertainty matters most?
Rank by the effect on your result, here μ from one point. Scatter in repeats and a slightly tilted string often beat the instrument's resolution.
Your game plan
the trial run shows if it sticks & slipsTick steps off as you go. We'll remember on this device.
0 of 11 done
- Read and predictFind the friction model and the difference between static and kinetic friction. Predict how F should change with N, and what the intercept should be.
- Trial run (1 lesson)Pull the block along each surface. Do you get a steady trace? Does the block stick and slip? Is the string level? Fix the problems now.
- Decide what you measureChoose static or kinetic, or both, and decide how you read it from the trace. Write down why: it becomes method justification.
- Decide range, repeats and surfacesChoose at least 6 loads or surfaces, how many pulls each, and the pulling speed. Control the surface condition (clean, dry, flat).
- Collect the dataZero the sensor before each run. Record raw readings with units and resolution, and note stick-slip or other oddities.
- ProcessConvert masses to normal force, average the repeats and carry the uncertainty through.
- Graph and fitPlot F against N with error bars. Fit best and worst lines and read off the gradient and the intercept.
- Compare with the modelIs the line straight, through the origin and the same across loads, within the uncertainties? What does that say about F = μN?
- EvaluateRank the weaknesses by effect on μ. Make each improvement specific to a weakness you found.
- Write, then cutWrite in the order of the layout and stay within 3,000 words. Tables, graphs and references do not count.
- Check before your teacher's draftRun through the checklist below, then get your RQ or draft checked. Your teacher reads one draft only.
Physics you'll need
Traps to avoid in this Physics IA
- Treating static and kinetic friction as the same thing, and quoting one μ for a trace that shows both.
- A string that is not horizontal, which changes the normal force.
- Uneven or dusty surfaces that change between runs.
- Pulling by hand at an uneven speed, then picking a reading without saying how.
- Forcing the line through the origin without checking whether the data does.
- Weighing the masses once and reusing the weight for a block that has gained dust or tape.
- Calling stick-slip "error" instead of describing it as an effect.
More Physics IA ideas like this
- Paper cones: what sets the terminal speed?●●○
- Trolley on a ramp: how much energy does friction take?●●○
- Projectile: is the best launch angle really 45°?●●○
This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.