Year 9 Science · Unit 4 · Lesson 17

Balanced and Unbalanced Forces

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Learning Goals

Trace the cause-and-effect chain from free fall to terminal velocity.
Calculate weight and air resistance force for a skydiver at terminal velocity.
Predict changes in net force and acceleration when a parachute opens.

Warm Up

Because… chain

Fill in the missing effects. Each step leads to the next in the skydiver's journey to terminal velocity. Use the words provided if needed: drag / weight / net force / speed / zero / constant / increases / decreases / greater / equal

Skydiver jumps — weight (gravity) pulls downward

→

Unbalanced net force acts downward

→

Skydiver's speed increases

→

Air resistance increases as speed increases

→

Air resistance equals weight

→

Overall outcome (terminal velocity reached):

Your Turn

Skydiving scenario

A 75 kg skydiver falls from an aircraft over Moruya, NSW. After some time she reaches terminal velocity of 57 m/s in a face-down spread-eagle position. Use g = 9.8 N/kg for all calculations.

(a) Calculate the skydiver's weight. Use W = mg. Show your working.

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(b) At terminal velocity, state the size of the air resistance force acting on the skydiver. Explain how you know this without measuring the air resistance directly.

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(c) The skydiver opens her parachute. Air resistance suddenly increases to 4000 N while her weight stays the same. Explain: (i) what is the new net force and in which direction? (ii) how does this change her acceleration? (iii) what eventually happens to her speed and why?

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Wrap Up

In one sentence, what was the main idea of this lesson?