A multi-step problem becomes manageable with a routine: list what you know, work out the net force, find the acceleration with F = ma, use a = Δv ÷ t to link it to speed and time, then check the answer makes sense. Keep units (N, kg, m/s², m/s, s) with every number.
A car travelling at 25 m/s brakes. How long does it take to stop, and how big is the force? The answer takes three steps, and the hardest part is knowing which one comes first.
A short practice lesson with no simulation. You know F = ma, a = Δv ÷ t and the three laws. Now use them together.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explain: Use the particle model to explain what you saw.
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: which step first?
ExplainUse the particle model to explain what you saw.
Explain: practise the routine
In your book: write the routine in your own words as five steps, then solve this: a 400 kg motorbike (with its rider) has a driving force of 2400 N and 400 N of resistance. Find its acceleration and its speed after 5 s from rest.
A 1500 kg car has an engine force of 4000 N and a resistance of 1000 N. What is its acceleration (m/s²)?
It accelerates at 2 m/s² from rest. What is its speed after 6 s (m/s)?
A 1200 kg car travelling at 20 m/s brakes to a stop in 8 s. What is its acceleration (m/s²)? Use a negative sign for slowing down.
What braking force does the 1200 kg car need (N)? Give the size only.
Elaborate TypicalUse the same idea in a new context.
Elaborate: two-step problem
A 60 kg sprinter reaches 9 m/s from rest in 3 s. What net force acts on the sprinter (N)?
Elaborate WiderUse the same idea in a new context.
Elaborate: school zones
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
A routine: list what you know, find the net force, use F = ma for acceleration, use a = Δv ÷ t for speed and time, then check the answer is reasonable.
Keep units with every number: N, kg, m/s², m/s, s.
On a speed–time graph, the gradient is the acceleration.
A negative acceleration means slowing down.
For teachers
Students solve multi-step force and motion problems using F = ma and a = Δv ÷ t with correct units, use a graph to support a quantitative argument, and check answers for reasonableness (AC9S10U05, node 7, a practise node). About 20 minutes.
I can solve a problem that needs two or more steps.
I can keep the units straight.
I can tell whether an answer is reasonable.
Before class
A practice lesson with no simulation, following Free body diagrams. Set the width with ?width=minimum, typical (default) or wider. The Year 10 progression marks this node as a place for structured practice, so students need pencil and calculator more than a screen: ask them to show each line in their books before entering an answer.
Answers with a sign are entered as numbers (−2.5). Round figures are chosen for clean numbers; real braking and engine forces vary with speed.
Running the lesson
Engage: tally the predictions. Ask the 'acceleration' group what force the acceleration depends on.
Explain: the five-step routine is the success criterion; collect a different problem from each group.
In your own words: the routine and the motorbike problem.
The hard part (on purpose)
Choosing the step, not just the formula. When a student reaches for a number, ask what quantity it is and which step it belongs to.
When students get stuck
A student uses the engine force alone. Ask: What other force acts, and which way? Fix: Resistance, backwards. F in F = ma is the net force.
A student gets a huge or tiny answer. Ask: Does a mass in grams or a time in minutes appear anywhere? Fix: Convert to kg and s first.
Stretch and support
Stretch: A 1000 kg car has a braking force of 5000 N while travelling at 30 m/s. How long does it take to stop, and how far does it go? (6 s; 90 m, the area under the speed–time graph.)
Support: Give a five-line template (Known, Net force, Acceleration, Speed or time, Reasonable?) with the first line filled in.
Where it leads
Next: Newton's laws in the real world.
In class: collect braking data with a toy car and a ramp; use a speed–time graph from a data logger; work through a vehicle's stopping distance table.
What each step is for
Engage: which step first?: Students jump to a formula. The routine (net force first, then acceleration, then the motion) is the thing to build.
Explain: practise the routine: Each problem has two or three steps and asks for units, a check on reasonableness and a use of a graph: the node's three success criteria.
Elaborate: two-step problem: Connecting a graph to the formulas in a third context checks the transfer.
Elaborate: school zones: A quantitative argument from stopping distances connects the laws to a real decision.