Newton’s laws and resultant force

Repair the common break

Common misconception: Newton’s third-law forces cancel on one body.

Correct model: Third-law forces act on different bodies; use the resultant force on the chosen body and F = ma only for constant mass.

Worked transfer

For an accelerating lift, the forces on the passenger are normal contact N upward and weight mg downward, so N − mg = ma. The reaction to N acts on the lift, not on the passenger.

Guided practice

Explain why “Newton’s third-law forces cancel on one body.” fails. Then construct one example that correctly applies newton’s laws and resultant force.

Independent practice

A crate is pulled right while accelerating left. Draw its free-body diagram and explain how this is possible without treating a third-law pair as cancelling forces.

Return to the recorded repair plan when the explanation and application are secure enough to re-test.