1. Interaction Forces

2. Identifying Interaction Forces
Forces always come in pairs.
An interaction pair is two forces that are in opposite directions and have equal magnitude.
Sometimes, this is called an action-reaction pair of forces.

3. Newton’s 3rd Law
For every action there is an equal and opposite reaction.
F A on B = -F B on A
The force of A on B is equal in magnitude and opposite in direction of the force of B on A.

4. Newton’s Third Law
Mini-Lab – Partner Up!

5. Practice
When a softball with a mass of 0.18 kg is dropped, its acceleration toward Earth is equal to g, the acceleration due to gravity. What is the force on Earth due to the ball, and what is Earth’s resulting acceleration? Earth’s mass is 6.0 x 1024 kg.

6. Practice
You lift a relatively light bowling ball with your hand, accelerating it upward. What are the forces on the ball? What forces does the ball exert? What objects are these forces exerted on?

7. Practice
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. (ignore air resistance).

8. Practice
You toss a ball up in the air. Draw a free-body diagram for the ball while it is still moving upward. Identify any forces acting on the ball. Also identify any forces that the ball exerts and the objects on which these forces are exerted.

9. Forces of Ropes and Strings
Tension is force exerted by a string or rope.
We will assume all strings and ropes are massless.

10. Practice
A 50.0 kg bucket is being lifted by a rope. The rope will not break if the tension is 525 N or less. The bucket started at rest, and after being lifted 3.0 m, it is moving at 3.0 m/s. if the acceleration is constant, is the rope in danger of breaking?

11. The Normal Force
Any time two objects are in contact, they each exert a force on each other.
The normal force is the perpendicular contact force exerted by a surface on another object.

12. Practice
Ashley hands a 13-kg box to 61-kg Stephanie, who stands on a platform. What is the normal force exerted by the platform on Stephanie?

13. Practice
Pg #s 1-3