Year 9 Science · Unit 4 · Lesson 17
Challenge Worksheet
Learning Goals
Read the free-body diagrams
Three objects are shown below with force arrows. Arrow length represents magnitude. For each object: (i) calculate the net force (show direction); (ii) state whether the object accelerates, moves at constant velocity, or is stationary; (iii) give a real-world example of that situation.
| Object | Net force (magnitude & direction) | Motion state | Real-world example |
|---|---|---|---|
| A | |||
| B | |||
| C |
A physics teacher claims…
"If the net force on an object is zero, it must be stationary. Zero force means nothing is happening — the object has to be sitting still."
(a) Construct a rebuttal using Newton's First Law. Quote or paraphrase the law precisely and explain how it directly contradicts the teacher's claim. What does "zero net force" actually tell us about motion?
(b) Give two real-world examples of objects with zero net force that are NOT stationary. For each example: name the object, state its velocity, and name the balanced forces acting on it.
(c) Explain the crucial distinction between "at rest" and "constant velocity" that the teacher's claim ignores. Why do both states have zero net force, and what is the key difference between them?
Wrap Up
In one sentence, what was the main idea of this lesson?