1. Interaction Forces
4.3
Page 102

2. Identifying Interaction Forces
Forces result from the interaction of two objects. (roller skating).
Forces always come in pairs.
Interaction pair – two forces that are in opposite directions and have equal magnitudes (a.k.a. action-reaction pair).
One does not cause the other.

3. Interaction Forces
Notice the symmetry in the subscripts: A on B and B on A.
These are an interaction pair.

4. When identifying interaction pairs…
The always occur in two different free-body diagrams.
They always have the symmetry of subscripts noted on previous slide.
Each object has a weight.
Weight force is due to an interaction between the object and Earth’s mass (gravity) so each object will also exert a force on Earth.
The acceleration caused by the force of an object interacting with Earth is usually a very small number. Therefore, for problems involving falling or stationary objects, Earth can be treated as part of the external world rather than a second system.

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

6. Page 104
28. You lift a relatively light bowling ball with you 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. Page 104
29. 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. (Air resistance can be ignored).

8. Forces of Strings
Tension is the force exerted by a string or rope. We will consider all string and rope massless.
Before the rope breaks, there must be forces holding the rope together.
The force that the top part of the rope exerts on the bottom part is Ftop on bottom .
Newton’s 3rd law states that this force must be part of an interaction pair.
The other member of the pair is FBottom on top.
The forces are equal in magnitude, but opposite in direction.

9. Forces of Strings Before rope breaks, the bucket is in equilibrium.
Force of the bucket’s weight downward must equal in magnitude but opposite in direction to the tension in rope upward.

10. Forces of Ropes
Read last paragraph on page. 105

11. The Normal Force…
The perpendicular contact force exerted by a surface on another object.
Normal force is always perpendicular to the plane of contact between two objects.
It is NOT always equal to the weight of the object.
Lifting up on the object, but not enough to cause movement.
Pushing down on the object.
FN + F string on box – Fg = ma = 0
FN = Fg – F string on box

12. The Normal Force