Physics & Engineering 12 · Week 14 · Wednesday

Read a Force–Time Graph

Learning objective: Calculate signed impulse as area and distinguish average force from peak force.

45–60 minutes · Digital-only investigations · Student work stays locally

Complete science reading

The area is impulse

The signed area under a force–time curve equals the impulse. A rectangular constant-force pulse at −30 N lasting 0.40 s gives −12 N·s. For a symmetric triangular pulse that rises linearly from zero to a peak and then returns to zero during that same interval, the area is half the peak times duration.

Same impulse, different peaks

To deliver a prescribed −12 N·s over 0.4 s, a rectangular pulse must have constant −30 N. A triangular pulse of the same signed area and duration needs peak −60 N. Both have mean force −30 N, but the triangle's peak magnitude is twice the rectangle's constant magnitude. The time-integrated force—not the graphical peak alone—determines Δp.

Compare duration and shape independently

With the impulse held at −12 N·s and duration increased from 0.20 s to 0.60 s, the mean magnitude falls from 60 N to 20 N. Pulse shape changes the peak: for our simple symmetric triangle, peak = 2×mean; for the rectangle, peak = mean. The rectangle and triangle are specified ideal shapes, not measured airbags, helmets or padding.

What a model does not predict

Graph area fixes impulse, but knowing impulse and duration alone does not reveal the real peak force. Real-world force curves may be irregular, include repeated impacts, distributed loads and multiple directions. The Force–Time Pulse Studio uses explicit rectangular and triangular assumptions so peaks are mathematical outputs, not safety ratings.

Vocabulary and definitions

force–time graph
Graph of net force against elapsed time.
signed area
Integral above or below the time axis.
peak force
Largest force magnitude within a specified pulse model.
rectangular pulse
Ideal constant-force interval.
triangular pulse
Ideal linear rise to peak followed by linear fall to zero.

Worked example

For J=−12 N·s and Δt=0.4 s, mean F=−30 N. Rectangular peak is −30 N; symmetric triangular peak is −60 N; both areas are −12 N·s.

Show the chosen body, coordinate sign convention, equation, units and interval.

Student evidence portfolio

Use Pulse Studio to compare equal impulses with two durations and two pulse shapes. Record average force, signed graph area and peak separately.

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Daily Retrieval · 4 Questions

Public open-book formative practice. Correct missed work toward the 80% learning target; formal exams must remain separately supervised.