Happy Friday Eve

•Today: Newton’s 1st and 3rd Laws •HW: WebAssign Due Friday at Midnight

Warm-Up

•In your notes: •Draw a small circle with the term “Newton’s 1st Law” written inside.

•Draw a larger circle around the small circle.•Inside the large circle write everything you have learned in the past or already know about Newton’s 1st Law.

DYNAMICSNewton’s Laws

Newton’s Laws of Motion•Sir Isaac Newton (1642 – 1726)

•3 Laws of Motion•Law of Gravitational Motion•Optics•Reflecting Telescope•Calculus

•Dynamics•The study of why things are moving. Newton’s 3 Laws of Motion explain the motion of an object resulting from net forces acting on the object.

Nature and nature’s laws lay hid in night;God said “Let Newton be” and all was light.~Alexander Pope (poet)

Newton’s 1st Law of Motion

•If a body is at rest, it will remain at rest. If a body is moving with constant velocity, it will continue to do so unless acted upon by a net external force.

What is a force?

•An interaction between two objects. •Any push or pull on an object.•Always think of forces acting ON an object.

Forces

•Forces are vectors•They have a magnitude and direction.

•Along an axis, note their direction as simply + or – , then add.

Net Forces

•Net means a “total” or “sum of”•1st Law does not mean that no force at all is acting on the body. It means that the forces must add to zero along each axis.

NOTE: the net force must be external: it must come from somewhere else, you cannot cause yourself to change your state of motion.

Inertia•Newton’s 1st Law is also called the Law of Inertia because is defines the “modern” definition of inertia.•Inertia: the resistance of a body to any change in its state of motion

•Considered a measure of mass in kilograms (kg)•MORE MASS = MORE INERTIA = MORE resistance to changes in motion

•If you’re a quarterback, who do you want on your offensive line?

Newton’s 3rd Law

•For every force that acts in nature, there must be an equal and opposite force.

•Commonly stated: •For every action, there is an equal and opposite reaction.

•Together: For every action force, there is an equal and opposite reaction force.

Newton’s 3rd Law

•When you push on the way, the wall pushes back on your with a force equal in strength.

•As you plummet toward Earth, the force of the Earth on you and the force of you on the Earth are equal.•Why doesn’t the earth move?

•Bug on a windshield?

Don’t the cancel?

•Action/Reaction forces do not cancel each other out because they act on two different objects.

•When you jump, you push down on the ground (you might leave an imprint). The ground pushes back up on you which is what propels you in the air.

Examples

•Birds flying•The bird’s wings push air down and back.

•The air then pushes up and forward on the bird lifting it and causing it to move forward.

•Swimming/walking

Types of Forces•Weight (Force of Gravity) – measure of gravitational attraction

•Normal Force – applied by a surface perpendicular to the surface•

•Applied Force – any push or pull on the object

Types of Forces

•Friction – resistive force due to motion between two surfaces, parallel to surface

•Tension – within ropes, string, cables. Applied into the line and away from the object.•

•Drag Forces (Fdrag), Spring Forces (Fspring), Bouyant Forces (Fbuoy), others…

Warm-Up:

•Discuss the following questions with your shoulder partner: •If an elephant were chasing you, its enormous mass would be very threatening. But if you zigzagged, the elephant’s mass would be to your advantage. Why?

•If you were in a spaceship and fired a cannonball into frictionless space, how much force would have to be exerted on the ball to keep it going?

It’s Friday!!!

•WebAssign Due tonight at MIDNIGHT!

•Today: Freebody Diagrams and Equilibrium

Free Body Diagrams

•Diagrams used to show the relative magnitude and direction of all forces acting on an object.

•Draw a box to represent object

•Draw arrows to represent each force •Arrows come from the box•Arrows point in same direction as force•Never draw components

Sample

Draw a free body diagram for each of the following:

a) A book is at rest on a tabletop.

b) An egg is free-falling from a nest in a tree.

c) A wagon is pulled at a 30 degree angle from the horizontal along a flat surface (consider friction).