1. Forces in 1 Dimension
Interaction Forces

2. Warm-Up
You are driving down the highway and a bug splatters on your windshield. Which is greater: the force on the bug on the windshield, or the force of the windshield on the bug?

3. Lesson 3: Interaction Forces
New Vocabulary
Interaction pair
Newton’s third law
Demonstration
Two students sitting on rolling chairs
One student pushes on the other one
What happens?

4. Identifying Interaction Forces
Forces always come in pairs
When two systems interact, each one feels a force exerted by the other
Interaction pairs
Two forces in opposite directions and equal in magnitude
Result of contact between two systems
FA on B
FB on A

5. Newton’s Third Law All forces come in pairs The forces in a pair :
act on different objects
Are equal in strength and opposite in direction

6. Practice 1
A brick falls from a construction scaffold. Identify any forces acting on the brick. Also identify any forces that the brick exerts and the objects on which these forces are exerted.

7. Solution Forces acting on the brick: Forces that the brick exerts:
Gravity (attraction of the Earth’s mass)
Nothing else
Forces that the brick exerts:
An equal and opposite force on the Earth

8. Practice 2
A suitcase sits on a stationary airport luggage cart. Draw a free-body diagram for each object and specifically indicate any interaction pairs between the two.

9. Solution Suitcase Cart Fcart on suitcase FEarth’s surface on cart
FEarth’s mass on cart
FEarth’s mass on suitcase
Fsuitcase on cart