Year 7 Science · Unit 3 · Lesson 4
Challenge Worksheet
Learning Goals
Find the mistake
A student drew this free-body diagram and wrote this explanation
"I drew a free-body diagram of a book sitting on a table. I put the weight arrow pointing upward because weight is what holds the book up on the table. I also drew the normal force pointing downward because the table pushes down on the book to keep it in place. Since both forces are pointing in the same direction, the net force is zero and the book is balanced."
1. Identify ALL the mistakes in the student's answer. List each error clearly.
2. Rewrite the student's explanation correctly. Describe the correct direction of each force AND explain why the book stays still.
3. Explain why the mistake of drawing weight pointing upward is easy to make. What might a student be confusing it with?
1. A surfer on a wave is travelling at constant speed toward the shore. Their weight (500 N down) is balanced by buoyancy (500 N up), but the wave pushes them forward with 120 N and water drag acts backward with 80 N. Is the surfer in complete equilibrium? Calculate the net horizontal force and explain what happens next to the surfer's speed.
2. Design a rule: write two clear sentences a student could use to check whether any free-body diagram they draw is correct. Each rule should address a different common mistake.
Wrap Up
In one sentence, what was the main idea of this lesson?