1. Conceptual Physics Fundamentals Chapter 3: EQUILIBRIUM AND FRICTION Edited for TWHS Physics

2. This lecture will help you understand: Net Force The Equilibrium Rule Equilibrium of Moving Things The Force of Friction

3. Net Force Net force is – the combination of all forces that change an object’s state of motion. – it is a ________________ of the forces. example: If you pull on a box with 10 N and a friend pulls oppositely with 5 N, the net force is 5 N in the direction you are pulling.

4. Net Force Vector quantity a quantity whose description requires both __________ (how much) and _______ (which way) can be represented by arrows drawn to scale, called vectors – length of arrow represents magnitude and arrowhead shows direction examples: force, velocity, acceleration

5. The Equilibrium Rule The equilibrium rule the vector sum of forces acting on a non-accelerating object equals ________ in equation form:  F = 0

6. The Equilibrium Rule example: a string holding up a bag of flour two forces act on the bag of flour: –tension force acts upward –weight acts downward In Equilibrium, the Forces are ________ in magnitude and ____________ in direction when added, the sum is zero bag of flour remains at rest

7. Support Force Support force (______________) is an upward force on an object that is opposite to the force of gravity. example: a book on table a compresses atoms in the table, and the compressed atoms produce the support force

8. Equilibrium of Moving Things Equilibrium a state of no change with no net force acting – static equilibrium example: hockey puck at rest on slippery ice – dynamic equilibrium example: hockey puck sliding at constant speed on slippery ice

9. Equilibrium of Moving Things Equilibrium test whether something undergoes changes in motion example: A refrigerator at rest is in static equilibrium. If it is moved at a _____________ across a floor, it is in dynamic equilibrium.

10. The Force of Friction Friction occurs when objects rub against one another applies to solids, liquids, and gases acts in a direction to __________ motion example: When an object falls down through air, the force of friction (air resistance) acts upward.

11. The Force of Friction depends on the kinds of material and how much they are pressed together is due to tiny surface bumps and to “stickiness” of the atoms on a material’s surface example: friction between a crate on a smooth wooden floor is less than that on a rough floor

12. The Force of Friction Friction is a Passive Force – It exists as a response to an _____________ – As the Applied Force increases, the Friction Force also increases, until it reaches its maximum – When the Applied Force is greater than the maximum Friction Force, the F net X will be unbalanced, and the object will slide. – The equation for maximum Force of Friction is μ●F N = coefficient of Friction x Normal Force