Conceptual Physics Fundamentals

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. Equilibrium and Linear Motion “When you’re over the hill, that’s when you pick up speed.” —Quincy Jones

4. Net Force • Net force is the combination of all forces that change an object’s state of motion. 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.

5. Net Force CHECK YOUR NEIGHBOR A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is A. 5 N to the left. • 5 N to the right. • 25 N to the left. • 25 N to the right.

6. Net Force CHECK YOUR ANSWER A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is A. 5 N to the left. • 5 N to the right. • 25 N to the left. • 25 N to the right.

7. Net Force Vector quantity • a quantity whose description requires both magnitude (how much) and direction (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

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

9. 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 equal in magnitude and opposite in direction when added, cancel to zero bag of flour remains at rest

10. The Equilibrium Rule CHECK YOUR NEIGHBOR The equilibrium rule, F = 0, applies to A. vector quantities. • scalar quantities. • both of the above. • neither of the above.

11. The Equilibrium Rule CHECK YOUR ANSWER The equilibrium rule, F = 0, applies to A. vector quantities. • scalar quantities. • both of the above. • neither of the above. Explanation: Vector addition takes into account + and - quantities that can cancel to zero. Two forces (vectors) can add to zero, but there is no way that two masses (scalars) can add to zero.

12. Support Force Support force (normal 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

13. The Support Force CHECK YOUR NEIGHBOR When you stand on two bathroom scales with one foot on each scale and with your weight evenly distributed, each scale will read A. your weight. • half your weight. • zero. • more than your weight.

14. The Support Force CHECK YOUR ANSWER When you stand on two bathroom scales, with one foot on each scale and with your weight evenly distributed, each scale will read A. your weight. • half your weight. • zero. • more than your weight. Explanation: You are at rest on the scales, so F = 0. The sum of the two upward support forces is equal to your weight.

15. 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

16. 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 steady speed across a floor, it is in dynamic equilibrium.

17. Equilibrium of Moving Things CHECK YOUR NEIGHBOR A bowling ball is in equilibrium when it A. is at rest. • moves steadily in a straight-line path. • both of the above • none of the above

18. Equilibrium of Moving Things CHECK YOUR ANSWER A bowling ball is in equilibrium when it A. is at rest. • moves steadily in a straight-line path. • both of the above • none of the above

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

20. 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

21. The Force of Friction CHECK YOUR NEIGHBOR The force of friction can occur A. with sliding objects. • in water. • in air. • all of the above

22. The Force of Friction CHECK YOUR ANSWER The force of friction can occur A. with sliding objects. • in water. • in air. • all of the above Comment: Friction can also occur for objects at rest. If you push horizontally on your book and it doesn’t move, then friction between the book and the table is equal and opposite to your push.

23. The Force of Friction • Friction is a Passive Force • It exists as a response to an Applied Force • 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 Fnet X will be unbalanced, and the object will slide. • The equation for maximum Force of Friction is μ●FN = coefficient of Friction x Normal Force