chapter 2-sec 3 & chapter 3. forces force —a push or pull that one body exerts on another

Chapter 2-Sec 3

& Chapter 3

ForcesForce—a push or pull that one body exerts on another

Balanced Forces Net Force—when 2 or

more forces act on an object at the same time

Balanced Forces—forces that are equal in size and opposite in direction.–An object will remain

still

Unbalanced Forces Unbalanced Forces—The size and

direction of forces acting on an object are unequal.–cause objects to move.

Balanced & Unbalanced Forces

Inertia Inertia—the tendency of an object

to resist a change in its motion–Velocity of an object only changes if a

force changes it.

The inertia of an object is related to mass– The greater the mass, the greater its

inertia.

Newton’s 1st Law of Motion Newton’s 1st Law: The Law of

Inertia–An object at rest stays at rest and an

object in motion stays in motion with the same speed and direction unless acted on by an unbalanced force.

Newton’s 1st Law of Motion

Punkin Chunkin: Inertia II

Punkin Chunkin: Inertia

Shooting Objects at RestThe object’s inertia causes it to remain still

even when a bullet passes through it.

Friction Friction: A force that opposes motion

between two surfaces that are touching each other

The amount of friction is dependant upon– Surfaces– Force pressing the surfaces together

Types of Friction Static Friction

– Friction between two objects that are NOT moving.

Sliding Friction– Friction between two surfaces moving past

each other. Rolling Friction

– Friction between a rolling object & the surface it is rolling on.

Fluid Friction – Friction between an object and a gas or liquid

Air Resistance Air Resistance: Collisions of the object's leading

surface with air molecules. Dependent upon a variety of factors, most

importantly– the speed of the object

Increased speeds = increased air resistance. – the cross-sectional area of the object

Increased cross-sectional area=increased air resistance

Newton’s Second Law Newton’s 2nd Law: The acceleration of an object is

dependent upon the force acting upon the object and the mass of the object.

Newton’s Second Law as an equation: F = m × a    F = force (N)  Newton is the SI Unit of force m = mass (kg)    N = kg m / s2

a = acceleration (m/s/s)

Gravity Universal law of Gravity:

– Any 2 objects will exert an attractive force on each other

– The size of the force is dependant on 2 things Mass of both objects Distance between the objects

– The range of gravity never disappears Therefore it is not possible for an

object with mass to be weightless.

Weight vs. Mass Weight: Force of gravity pulling you toward the earth

– F = ma becomes W = mg – Weight = mass × gravitational acceleration

g on Earth is 9.8m/s2

– SI Unit = Newton– Weight can change with a change in location.

Mass: A measure of how much matter an object has– You know an object has mass because

it has inertia– SI Unit = Kilograms

Centripetal Force Centripetal Force: The force exerted toward

the center of a curved path. Earth’s gravity exerts a centripetal force on

the Moon that keeps it in a nearly circular orbit.

Real World/space station & centripetal force Roller coaster Loops

Falling Objects on Earth Near Earth’s surface

acceleration due to gravity is 9.8 m/s2

If 2 objects with different masses are dropped from the same height, which would hit the ground first?

http://www.youtube.com/watch?v=aRhkQTQxm4w&feature=plcp

dropping objects on the moon

Projectile Motion Anything dropped, thrown, or shot through the air

is a projectile Because of gravity and inertia, you can get a

curved path.

http://video.pbs.org/video/1602463762/

http://science360.gov/obj/tkn-video/fc729ef0-22ee-4f61-bb2a-b6c07685fb02/science-nfl-football-projectile-motion-parabolas

Monkey and the Zookeeper The zookeeper must shoot the

banana from the banana cannon to the monkey who hangs from the limb of a tree.

This particular monkey has a habit of dropping from the tree the moment that the banana leaves the muzzle of the cannon.

The zookeeper is faced with the dilemma of where to aim the banana cannon in order to hit the monkey.

If the monkey lets go of the tree the moment that the banana is fired, then where should she aim the banana cannon?

Monkey and the ZookeeperWhere should the zookeeper aim?Taking Gravity into effect The zookeeper aims above the monkeyDoes the speed of the banana change the effect?The zookeeper aims at the monkey and shoots the banana very fast The zookeeper aims at the monkey, yet shoots the banana very slow. ZOO in SPACEIn a low gravity environment, where should the zookeeper shoot the banana?

The Truck and The Ball A pickup truck is moving with a constant speed. In the course of its motion, a ball is projected straight

upwards by a launcher located in the bed of the truck. Assume the ball does not encounter a significant amount

of air resistance. What will be the path of the ball and where will it be

located with respect to the pickup truck?

Many would insist that there is a horizontal force acting upon the ball since it has a horizontal motion.

Yet this is simply not the case. The horizontal motion of the ball is the result of its own inertia.

When projected from the truck, the ball already possessed a horizontal motion, and thus will maintain this state of horizontal motion unless acted upon by a horizontal force.

An object in motion will continue in motion with the same speed and in the same direction ... (Newton's first law).

Newton's Law Review physclips newton's laws http://www.phys.unsw.edu.au/einsteinlight/

NEWTON’S 3rd LAW

For every action, there is an equal and opposite reaction

“Action” and “Reaction” are names of forces Forces ALWAYS occur in pairs Single forces NEVER happen action reaction pairs

“Equal & Opposite” In Newton’s Third Law,

“equal” means:

Equal in size.– The action and reaction

forces are EXACTLY the same size.

Equal in time.– The action and reaction

forces occur at EXACTLY the same time.

In Newton’s Third Law, “opposite” means:

Opposite in direction– The action and

reaction forces are EXACTLY 180o apart in direction.

Why don’t the forces cancel each other out?

Only forces pushing or pulling on an object affect the object’s motion.

Only forces that act on the same object can cancel.

Newton’s Third Law action and reaction forces act on different objects, so they don’t cancel.

Writing Action Reaction Forces

Action force: “A action verb B”

Reaction force: “B action verb A”

– ACTION

Bowling ball hits the pin to the left.

– REACTION

Pin hits the bowling ball to the right

The action and reaction forces don’t cancel since they push on different objects.