Lesson 9 — Force and Work: A Quantitative Language for Motion

Summary

Learners meet force and work quantitatively. Force is measured in newtons, and work is the force applied along a distance: W = F × d, measured in joules. They solve worked examples and practice problems, and notice that direction matters — carrying something sideways does no work against gravity.

Objectives

• Explain motion, force, and energy quantitatively in everyday and engineered systems (such as work, power, and momentum). (D06.S3.10.01)

Connection

Lift a full bucket of water: you have done work against gravity. Push a stalled cart along the road: more work. Work is not just “effort” — it is a precise, measurable thing: how much force you apply, over how much distance, in the direction you push or lift. Knowing that number tells an engineer how big a motor to build, a farmer how much a pump must lift, and you how much a task will actually cost your muscles.

Materials

• Work worksheet
• Science journal

Preparation

• Copy or draw the work worksheet.
• Retrieval: from Grade 7, force, mass, and acceleration (D06.S3.07.01); from Grade 6, balanced and unbalanced forces (D06.S3.06.02). Today we add the quantitative measure of work.
• Prepare one worked example and practice problems.

Facilitator note

This lesson is written to the learner (“you”). The idea to land: force is measured in newtons; work = force × displacement in the direction of the force (W = F·d), in joules; and direction matters — perpendicular force does no work. Teach the formula with a worked example and unit discipline (S-011), then let learners practice and check. Watch the common error: students multiply force by total path length even when the force and motion are not aligned — model the “force in the direction of motion” step explicitly.

The intellectual lens: work is defined so that energy is conserved and transferable — the definition is a tool, not an arbitrary rule. The technology lens: every machine, from a hand pump to a crane, is a work calculation in physical form; the units (newtons, joules) are the shared language engineers use worldwide. The egalitarian lens: measuring work precisely is what lets us design tools that reduce human toil — the same force that once took many hands can be done by a motor, freeing people’s labor (though whether that freedom is shared fairly is a separate question). The ethics lens: who does the hardest work, and who gets the benefit of machines that do it instead? A work calculation is neutral; its use is not. Preview: Lesson 10 adds power (how fast the work happens) and momentum (motion in collisions).

Procedure

1. Recall (5 min). From Grade 7, what is force, and what does a net force do to motion? Name one task you did this week that felt like “work.”
2. Meet work (15 min). Force is measured in newtons (N). Work = force × distance moved in the direction of the force:
   • Worked example: you lift a 100 N bucket straight up 2 m. W = 100 N × 2 m = 200 J (joules).
   • Direction matters: if you carry the bucket 10 m horizontally at constant height, the upward force does no work (the force and the motion are perpendicular).
   • A push example: you push a cart with 50 N of force for 4 m. W = 50 N × 4 m = 200 J.
3. Guided practice (15 min). With a partner, solve: (a) lift 40 N up 3 m; (b) push 80 N for 2.5 m; (c) carry 60 N horizontally 10 m — how much work against gravity? Show units on every step. Compare and agree before moving on.
4. Independent practice (15 min). In your journal, write one real task you did today, estimate the force (in newtons, comparing to a 10 N ≈ 1 kg feel) and the distance, and compute the work. Then explain in one sentence why direction matters in the definition of work.
5. Close (5 min). In one sentence: what is work, what are its units, and why does direction matter?

Differentiation

• Support: Use a bag or bucket to feel force and distance first; give a pre-set formula with blanks to fill.
• Extension: Solve a multi-step problem: find the work to push a 200 N crate up a 5 m ramp, then discuss why ramps (not straight lifts) reduce the needed force.

Assessment

• Formative (peer + self): Can the learner compute W = F × d with correct units, and explain why perpendicular force does no work?
• Portfolio artifact (unit): The completed work worksheet, added to the physical-systems section.

Home connection

Ask someone at home about a lifting or pushing task they do often. Estimate the force and distance together and compute the work.

Resources

• On force and motion as background: Newton, Principia Mathematica (1687) (S-302); and Khan Academy, forces and Newton’s laws, https://www.khanacademy.org/science/physics/forces-newtons-laws (S-241).
• On worked examples and guided practice: Kirschner, Sweller & Clark (2006), https://doi.org/10.1207/s15326985ep4102_1 (S-011).