For the same unbalanced force, a heavier object accelerates less than a lighter one. For the same mass, a bigger unbalanced force gives a bigger acceleration. Data from a force investigation show the pattern: acceleration goes up with force and down with mass.
A rugby front-rower and a winger sprint off the same line with the same push from the ground. Who gets going faster, and why?
You know that an unbalanced force changes motion. This lesson puts force, mass and acceleration together.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the trolley
ExploreTry it on the board and compare what happens with your prediction.
Explore: load the trolley
The push stays at 6 N. Change only the mass.
Try it yourselfYour edits are saved in this browser.
The push is 6 N. Set the mass to 2 kg and read the acceleration.
Use the board. This ticks when you get there.
Double the mass to 4 kg. What happens to the acceleration?
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: the pattern
In your book: draw two bar graphs from the data above: acceleration against force, and acceleration against mass. Under each, write the pattern in one sentence.
Same trolley, 2 kg. Pushing with 2 N gives 1 m/s², 4 N gives 2 m/s², 6 N gives 3 m/s². What acceleration would 10 N give (m/s²)?
Same push of 8 N. A 1 kg trolley accelerates at 8 m/s², 2 kg at 4 m/s², 4 kg at 2 m/s². What acceleration would 8 kg give (m/s²)?
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: hit a target
Try it yourselfYour edits are saved in this browser.
The trolley is 5 kg. Set the push so the acceleration is 2 m/s².
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: rockets and roads
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
For the same unbalanced force, a larger mass gives a smaller acceleration.
For the same mass, a larger unbalanced force gives a larger acceleration.
Data show the pattern: acceleration rises with force and falls as mass rises.
For teachers
Students state the qualitative relationship between force, mass and acceleration, predict which object accelerates more under the same force, and read the pattern from data (AC9S7U04, node 5, an integration node). About 17 minutes.
I can say what happens to acceleration when the force or the mass changes.
I can predict which of two objects accelerates more under the same force.
I can read a pattern from force data.
Before class
A short integration tutorial that follows the four Year 7 forces lessons. Set the width with ?width=minimum, typical (default) or wider. The Year 10 lessons put numbers on this (F = ma); here the aim is the qualitative pattern and the data.
Board notes: the trolley sits on a frictionless track; the board shows the acceleration before Play. The data tables in the questions are small, rounded numbers chosen for a clear pattern. Don't call the pattern F = ma yet.
Running the lesson
Engage: tally the predictions. Ask the 'full one' group what a heavier trolley resists.
Explore: ask what happened to the acceleration when the mass doubled.
Explain: the two statements and the data patterns are the success criteria.
In your own words: the two bar graphs and a sentence for each.
The hard part (on purpose)
Putting force and mass together. When a student says a heavier object accelerates more, ask which one they would rather push to get moving.
When students get stuck
A student says acceleration is the same speed. Ask: Is acceleration how fast, or how quickly speed changes? Fix: How quickly speed changes. Same force on a bigger mass changes speed more slowly.
A student says heavier means more acceleration. Ask: Which trolley is easier to get moving? Fix: The lighter one. Heavier means more resistance to changing motion.
Stretch and support
Stretch: A 4 kg trolley and a 1 kg trolley are pushed with the same force and the 4 kg one accelerates at 2 m/s². How fast does the 1 kg one accelerate? (8 m/s².)
Support: Give the data as two small tables with the acceleration column blank for the last row, and ask students to say 'doubles' or 'halves' for each step.
Where it leads
Next: Year 8 energy, then Year 10 Newton's laws, where the pattern becomes F = ma.
In class: a dynamics trolley on a ramp with a light string and different masses; a bowling ball and a tennis ball given the same push; data logging with light gates if available.
What each step is for
Engage: the trolley: Students say the heavier object accelerates more because it has more 'stuff', or that mass makes no difference. The pattern is that mass resists change in motion.
Explore: load the trolley: Changing only the mass, then only the force, and reading the acceleration each time is the controlled investigation the node's data analysis comes from.
Explain: the pattern: Stating the relationship in words, predicting which object accelerates more, and reading the pattern from two data tables meets the three success criteria.
Mix it up: Acceleration needs an unbalanced force, so the effect of a net force on motion comes back before force and mass are compared.
Elaborate: hit a target: Choosing the force to hit a target acceleration for a given mass uses the pattern in reverse.
Elaborate: rockets and roads: Engineers use the same pattern: to accelerate faster, raise the force or cut the mass.
Now you can
Say what happens to acceleration when the force or the mass changes.
Predict which of two objects accelerates more under the same force.