Announcements

1. Announcements • Exam 1/2 today • On-line on Moodle • Available 11 AM–midnight • Contact me if you need accommodations

2. Influencing Motion Philosophy, definition, and reality

3. Objectives • Relate uniform motion and zero net force. • Relate force, acceleration, and mass. • Compute the direction and magnitude of the forces of gravity, support, and friction.

4. What’s the point? • What governs an object’s motion?

5. Newton’s First Law • An object’s state of motion does not change unless an outside net force acts upon it. • If at rest, it remains at rest. • If moving, it continues straight at constant speed. Representation of Newton by William Blake, 1795

6. Does it Make Sense? • Do you feel a force while cruising in a plane? A train? An automobile? • If you were moving in a perfectly straight line at a constant velocity inside a closed box, could you tell?

7. How Can We Use It? • Equilibrium Rule: An object moving at a constant velocity experiences zero net force. • That means that the forces acting on it add to zero. • A body with zero net force is in mechanical equilibrium.

8. What does Newton’s first law mean? Being at rest is nothing special; it is just another value (zero) of constant velocity. rest constant velocities

9. F a= m a=acceleration F=netforce m=mass Newton’s Second Law equivalently, F=ma

10. Quantify Force • Unit of force: 1 newton (N) = force needed to accelerate 1 kg at 1 m/s2 • Unit named for a person • Unit name lowercase • Abbreviation capitalized

11. Group Work F = ma.If m = 10 kg and a = 1 m/s2, what is F?

12. Group Work A net force of 100 N acts on a 10-kg steel block. What is its acceleration?

13. Working with Commonly-Encountered Forces Gravity Support (Normal force) Friction

14. Gravity • Magnitude of gravitational force is proportional to mass: F = mg. • (g is gravitational field strength.) • Direction of gravitational force is toward the center of the earth. • At earth’s surface, g  9.8 N/kg.

15. Group Work • What is the gravitational force, in N, acting on a 2-kg chicken at the surface of the earth? • If gravity is the only force acting on it, what will the chicken’s acceleration be? (Acceleration is a vector, so specify both magnitude—including units— and direction.)

16. Poll Question Gravity is constantly pulling us downward, but we are not accelerating downward. This means that • Newton’s first law does not apply here. • Gravity does not apply a physical force. • Some other force exactly opposes the force of gravity. • Gravity stops at the earth’s surface.

17. support weight weight weight Support Force • Exerted by a surface perpendicular (normal) outward • Magnitude as needed to cancel inward forces zero net force

18. perpendicular (normal) force N F f Friction • Commonplace but complicated • Surface friction acts parallel to a surface • Friction fopposes applied sliding force F

19. perpendicular (normal) force N F f Friction • f mN(directions of f and N are different) • m = coefficient of friction • Coefficient m depends only on surface materials, not mass, weight, area of contact, sliding speed (one exception).

20. Friction • Static friction: when the two surfaces are not moving past each other • Kinetic friction: the two surfaces slide along each other • Static friction >kinetic friction

21. Group Question Rank the magnitude of frictional force on a car in these different situations. • Skidding on dry pavement. • Skidding on ice. • Braking just short of a skid on dry pavement.

22. Static vs. Kinetic Friction • Start pushing an object • Anti-lock brakes • Learning to drive • Stick-slip motion

23. Reading for Next Time • Projectiles: falling in gravity • Big ideas: • Vertical acceleration = g downward • Horizontal acceleration = 0 • Horizontal and vertical motion (x, vx, ax; y, vy, ay) are independent