Newton‘s 3rd Law

Newton’s Third Law For every action there exists an equal and opposite reaction. If body A exerts a force F on body B, then body B exerts a force of -F on body A.

Examples of Newton’s Third Law

Sample Problem You rest an empty glass on a table. a) How many forces act upon the glass? b) Identify these forces with a free body diagram. c) Are these forces equal and opposite? d) Are these forces an action-reaction pair?

Mass and Weight Many people think mass and weight are the same thing. They are not. Mass is inertia, or resistance to acceleration. Weight can be defined as the force due to gravitational attraction. W = mg

Normal Force The normal force is a force that keeps one object from penetrating into another object. The normal force is always perpendicular to a surface. The normal exactly cancels out the components of all applied forces that are perpendicular to a surface.

Normal Force on a Flat Surface The normal force is equal to the weight of an object for objects resting on horizontal surfaces. N = W = mg N mg

The normal force most often equals the weight of an object…

…but this is by no means always the case! Normal Force …but this is by no means always the case!

Sample Problem A 5.0 kg bag of potatoes sits on the bottom of a stationary shopping cart. Sketch a free-body diagram for the bag of potatoes. Now suppose the cart moves with a constant velocity. How does this affect the free-body diagram?

Apparent Weight

Apparent Weight If an object subject to gravity is not in free fall, then there must be a reaction force to act in opposition to gravity. We sometimes refer to this reaction force as apparent weight. In 1-D force problems, the apparent weight is the same thing as the normal force (it is the force that a scale would read).

Elevator Rides When you are in an elevator, your actual weight (mg) never changes. You feel lighter or heavier during the ride because your apparent weight increases when you are accelerating up, decreases when you are accelerating down, and is equal to your weight when you are not accelerating at all.

Going Up? v = 0 a = 0 v > 0 a > 0 v > 0 a = 0 v > 0 W Wapp Ground floor Normal feeling v = 0 a = 0 W Wapp Just starting up Heavy feeling v > 0 a > 0 W Wapp Between floors Normal feeling v > 0 a = 0 W Wapp Arriving at top floor Light feeling v > 0 a < 0

Going Down? v = 0 a = 0 v < 0 a < 0 v < 0 a = 0 v < 0 Wapp Top floor Normal feeling v = 0 a = 0 W Wapp Beginning descent Light feeling v < 0 a < 0 W Wapp Between floors Normal feeling v < 0 a = 0 W Wapp Arriving at Ground floor Heavy feeling v < 0 a > 0

Sample Problem An 85 kg person is standing on a bathroom scale in an elevator. What is the person’s apparent weight a) when the elevator accelerates upward at 2.0 m/s2? b) when the elevator is moving at constant velocity between floors? c) when the elevator begins to slow at the top floor at 2.0 m/s2?

Sample Problem A 5 kg salmon is hanging from a fish scale in an elevator. What is the salmon’s apparent weight when the elevator is a) at rest? b) moving upward and slowing at 3.2 m/s2? c) moving downward and speeding up at 3.2 m/s2? d) moving upward and speeding up at 3.2 m/s2?