Announcements

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

Influencing Motion

Philosophy, definition, and reality

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.

What’s the point?

• What governs an object’s motion?

Newton’s First Law

• An object’s state of motion does not change unless an outside net force acts upon it.

Representation of Newton by William Blake, 1795

• If at rest, it remains at rest.

• If moving, it continues straight at constant speed.

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?

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.

What does Newton’s first law mean?

• Being at rest is nothing special; it is just another value (zero) of constant velocity.

constant velocities

rest

Newton’s Second Law

Fm

a =

equivalently,

F = ma

a = acceleration F = net force m = mass

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

Group Work

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

Group Work

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

Working with Commonly-Encountered Forces

Gravity

Support (Normal force)

Friction

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.

Group Work

A. What is the gravitational force, in N, acting on a 2-kg chicken at the surface of the earth?

B. 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.)

Poll Question

Gravity is constantly pulling us downward, but we are not accelerating downward. This means that

A. Newton’s first law does not apply here.

B. Gravity does not apply a physical force.

C. Some other force exactly opposes the force of gravity.

D. Gravity stops at the earth’s surface.

Support Force

• Exerted by a surface perpendicular (normal) outward

• Magnitude as needed to cancel inward forces support

weightweightweight

zero net force

Friction

• Commonplace but complicated• Surface friction acts parallel to a surface• Friction f opposes applied sliding force F

F f

N perpendicular (normal) force

Friction

F f

N perpendicular (normal) force

• f N (directions of f and N are different)• = coefficient of friction

• Coefficient depends only on surface materials, not mass, weight, area of contact, sliding speed (one exception).

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

Group Question

Rank the magnitude of frictional force on a car in these different situations.

I. Skidding on dry pavement.

II. Skidding on ice.

III. Braking just short of a skid on dry pavement.

Static vs. Kinetic Friction

• Start pushing an object

• Anti-lock brakes

• Learning to drive

• Stick-slip motion

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