To link an observation to a model, say what you saw, choose the simplified picture that fits it, and use the model to predict or explain. The link is the model’s label, such as “pivot, force, distance”.
This lesson is part of the SPM subjects and learning resources section. The examples are original; follow your syllabus for the models you are expected to use.
What are the three steps?
- Observe: state what happens, with a comparison.
- Model: name the model and map the parts of the real object onto it.
- Predict or explain: use the model’s rule to explain the observation.
Worked example: the wheelbarrow
A student notices that a loaded wheelbarrow feels lighter to lift when the load sits close to the wheel, and heavier when it sits near the handles.
Observe. The same sand is easier to lift with the load near the wheel than near the handles.
Model. A wheelbarrow works as a lever. The wheel axle is the pivot, the load’s weight is one force (W) and the lifting force at the handles is the other (F). Call the distance from pivot to load d₁ and the distance from pivot to handles d₂.
Explain. To lift the barrow, the moment of the lifting force must equal the moment of the load about the pivot, so F × d₂ = W × d₁. The handle distance d₂ stays the same. If the load is moved closer to the wheel, d₁ gets smaller, so F gets smaller, which is why the barrow feels lighter.
Predict with numbers. With W = 400 N, d₂ = 1.2 m and d₁ = 0.6 m, F = (400 × 0.6) ÷ 1.2 = 240 ÷ 1.2 = 200 N. If the load moves to d₁ = 0.3 m, F = (400 × 0.3) ÷ 1.2 = 120 ÷ 1.2 = 100 N.
The mistake that costs marks
The common slip is to describe the barrow without a model: “It is easier because the load is near the wheel.” The model says why.
| Answer | Missing |
|---|---|
| “The load is near the wheel, so it is lighter.” | No model, no reason |
| “The wheelbarrow is a lever with pivot at the axle. A smaller load distance d₁ needs a smaller lifting force F.” | Nothing. The model explains the observation |
State your assumption
Every model leaves something out. Here the model ignores the weight of the barrow itself and assumes the lifting force is perpendicular to the handle. Say this in one sentence to show you understand the limits.
Check yourself
A nutcracker cracks a nut more easily when the nut is placed near the hinge than near the end of the handles. Name the model, label the parts and explain the observation.
Answer
Model: a lever, with the hinge as pivot. The nut pushes back with a force at a short distance from the pivot, and the hand applies a force at a long distance.
Explanation: the moment of the hand’s force equals the moment of the nut’s resistance about the hinge. When the nut is nearer the hinge, its distance is smaller, so the hand force needed for the same moment is smaller.
Assumption: the handles are rigid and the hand force is perpendicular to the handle.
What to study next
To test whether an explanation leaves something out, continue with review a written explanation for missing assumptions. Then try the Mechanical Engineering original practice set.
Check your own answers with the structured answer self-review. A teacher can work through your observations in online one-to-one Mechanical Engineering tuition.