Physics — Newtonian Forces · Force-diagram demonstration and discussion

Free Body Diagram Practice

Distinguish forces acting on a chosen object from equal-and-opposite interaction forces acting on different objects.

Landscape activity · use fullscreen on a phone for larger controls

↗ New Window

📝 Description

Free Body Diagram Practice helps students identify and represent forces acting on an object. The activity supports force analysis, diagram reading, and problem-solving in mechanics. It is useful for building a strong foundation before solving numerical physics problems.

📋 Instructions

Study the object and identify the forces acting on it. Use the diagram to connect force arrows with the physical situation.

🎓 Learning Guide: Free Body Diagram Practice

Learning focus: Distinguish forces acting on a chosen object from equal-and-opposite interaction forces acting on different objects.

Physics — Newtonian Forces Force-diagram demonstration and discussion

Learning objectives

Choose a system before drawing a free-body diagram.
Identify weight and contact-normal force on a resting block.
Distinguish Newton’s third-law pairs from the forces included in one object’s FBD.

Concepts and principles

A free-body diagram includes only external forces acting on the object being analysed.
For a block resting on a horizontal table, Earth’s gravity pulls it downward and the table’s normal force acts upward.
The block pushes downward on the table; that force is the third-law partner of the table’s upward force on the block, not a second force on the block.

Examples and worked solutions

A 2 kg block is at rest on a horizontal table (g≈9.8 m/s²). Find its weight and normal force.: Worked solution: Weight=mg=19.6 N downward. If no other vertical forces act, the table’s normal force on the block is 19.6 N upward.

Try this activity

Use the displayed block/table demonstration and inspect force labels.
List forces on the block and compare your list to the labelled diagram.
Reframe the question for the table, identifying which object exerts each force.

Expected observations and explanations

Opposite forces on the same object can balance; they are not automatically Newton’s third-law pairs.
Every interaction pair acts on two different objects.

Assessment questions and answers

What is the reaction to the table’s normal force on the block?: Answer: The block exerts an equal-magnitude downward normal force on the table.
Should the table’s force on the block and the block’s force on the table appear on one FBD?: Answer: No. They act on different objects and belong on separate free-body diagrams.