AP Physics 1- 2.3 Newton’s Third Law- Study Notes- New Syllabus
AP Physics 1-2.3 Newton’s Third Law – Study Notes
AP Physics 1-2.3 Newton’s Third Law – Study Notes -AP Physics 1 – per latest Syllabus.
Key Concepts:
- Interaction of Two Objects – Newton’s Third Law
Interaction of Two Objects – Newton’s Third Law
Newton’s Third Law states: For every action, there is an equal and opposite reaction. When two objects interact, they exert forces on each other that are:
- Equal in magnitude
- Opposite in direction
- Act on different objects
Representation of Paired Forces:
- If Object A exerts a force \( F_{AB} \) on Object B, then Object B exerts a force \( F_{BA} \) on Object A.
- These forces form an action–reaction pair:
\( F_{AB} = -F_{BA} \)
Important Note:
The action and reaction forces do not cancel each other because they act on different objects.
Example 1: Pushing a Wall
A person pushes a wall with force 100 N. The wall pushes back on the person with 100 N in the opposite direction. Even though the wall doesn’t move, the reaction force exists and balances the push on the person.
Example 2: Book on a Table
The book exerts a downward force (weight) on the table: \( F_{bt} = mg \).
The table exerts an upward normal force on the book: \( F_{tb} = -mg \).
These two forces are an action–reaction pair.
Example 3: Rocket Propulsion
The rocket expels gas backward with a force \( F_{rg} \). The gas pushes the rocket forward with equal and opposite force \( F_{gr} \).
This reaction is what propels rockets in space.
Example :
A 2 kg block rests on a horizontal table. The block exerts a force on the table due to its weight. Find:
- The force exerted by the block on the table.
- The force exerted by the table on the block.
▶️ Answer/Explanation
1. Weight of block (force on table): \( F_{bt} = m g = 2 \times 9.8 = 19.6 \, \text{N} \) (downward).
2. By Newton’s Third Law, the table exerts an equal and opposite normal force on the block: \( F_{tb} = 19.6 \, \text{N} \) (upward).
Answer: – Block on table: 19.6 N downward – Table on block: 19.6 N upward These two forces are an action–reaction pair.