Law of Inertia & Balanced vs. Unbalanced Forces

Question: What allows for a snowboarder to

be able to ride on a halfpipe?

Galileo and the Concept of Inertia

Using a pair of inclined planes facing each other, Galileo observed that a ball would roll down one plane and up the opposite plane to approximately the same height.

Any difference between initial and final heights was due to the presence of friction. If friction could be entirely

eliminated, then the ball would reach exactly the same height.

If the slope of the opposite incline were reduced, then the ball would roll a further distance in order to reach that original height.

What if there was no second incline?

If there was no friction, the ball would never stop because it is trying to reach the original height.

DEMONSTRATION 1

Tennis ball drop Run in a straight line toward

the target. Try to drop the ball on the target as you run past.

Newton’s First Law of Motion

The state of motion of an object does not change as long as the net force acting on the object is zero

An object at rest remains at rest

An object in motion remains in motion with the same speed and direction (i.e. no acceleration)

What is a force?

A push or pull that acts on an object

Forces can cause a resting object to move (or accelerate) Acceleration = a

change in an object’s speed or direction

MISCONCEPTION ALERT:Forces Don't Keep Objects Moving A force is not needed to keep

an object in motion

Example: Slide a book across a table and watch it slide to a stop. Why does the book come to a stop?

Units of Force Measured in Newtons (N)

This unit is named after Sir Isaac Newton (1642-1727) Scientist who explained how force, mass

and acceleration are related

Definition: The SI unit of force. It is equal to the force that would give a mass of one kilogram an acceleration of one meter per second

A Ball at Rest…

With your group, list at least five ways you could affect the motion of the kick ball.

How will the motion of the ball be different if we increase the magnitude of the force we use to move the ball?

TUG-O-WAR TIME!!!

Combining Forces

More than one force can be acting on an object at a given time

Forces in the same direction add together

Forces in the opposite direction subtract from one another

Net Force The overall force acting on an

object after all the forces are combined

Combining Forces

= Adding Forces

= 0 Equal and opposite

forces

=Subtracting Forces

Sometimes the net force acting on an object is zero

Balanced Forces Forces that combine to produce a net force of

zero Results in NO CHANGE in an object’s motion

ARM WRESTLING DEMONSTRATION TUG-O-WAR DEMONSTRATION

Common Examples

Two people locked in an arm wrestling match

Tug-o-War match with two evenly matched teams!

Two football players pushing against one another at the line of scrimmage

Human Table Demonstration

Unbalanced Forces Results when the net force acting on an object

is NOT equal to zero

When an unbalanced force acts on an object, the object accelerates

TUG-O-WAR

DEMONSTRATION

Force Diagrams

Use arrows to represent the direction and strength of a force (like a vector!)

Door

10 N10 N

Does the door move?

Door

10 N

Does the door move? In which direction?

3 N

Inertia

The tendency of an object to resist changes in motion

An object at rest stays at rest unless a force acts on it

An object in motion stays in motion unless a force acts on it

What would happen if the Earth stopped spinning?

http://www.youtube.com/watch?v=K0-GxoJ_Pcg

Mass & Inertia The tendency of an object to resist changes in its

state of motion varies with mass

The mass of an object depends on the inertia of the object. The more inertia that an object has, the more mass that it has.

A more massive object has a greater tendency to resist changes in its state of motion Think of a brick made of clay vs. a brick made of

Styrofoam.

Inertia Mini Labs

Teacher Demo

Marker and Bottle Demonstration

Penny Elbow Trick

You will be practicing your skills with an old parlor trick: the penny catch. First, bend your elbow completely, so your wrist is as close to your shoulder as possible. Now, raise your elbow directly out from your body so that your forearm is completely horizontal (parallel with the ground). By balancing a stack of pennies right on your elbow (or as close to it as possible) and then quickly swinging your elbow down, you can amaze your friends and demonstrate Newton’s 1st Law (with some practice). Explain how this is an example of inertia.

Rolling Chair

Use a chair that has wheels.  Let the person with the least mass be the "pilot" of the scooter. Have another member of the group push them enough to get them moving. Do the same with another member of the group, and compare the relative inertia of both persons. How do you know that one has more mass (and inertia) than the other?

Now we will try to explain one of the aspects of projectile motion we have used in the past. With one person pushing the scooter, the "pilot" will hold a ball out at arm’s length and drop it while they are moving past a target. How is Newton’s 1st Law applicable to this situation?

Swirling Thoughts

Watch your teacher spin the brain stress ball on a string.

What will be the trajectory of the brain when the teacher lets go? How does Newton’s First Law relate to this demonstration?

Brain Teaser

An astronaut is outside of her spaceship with no jetpack or tie line, repairing the engine. Suddenly, she finds herself away from the ship, and only has a wrench in her hands. As she stares at her ship, she tries to think of a way to get back. Since there is nothing to push off from, she thinks that she may be lost forever, but suddenly remembers the wrench. Using the wrench, how can she get back to the ship? How does this demonstrate Newton’s 1st Law?

EXTRA CREDIT!

Crash Test Dummy Video Clip

EXIT CARD QUESTION

“Ohio’s mandatory safety belt usage law requires drivers and front-seat occupants of most passenger vehicles to wear safety belts whenever they drive or ride in a motor vehicle on Ohio’s roadways.” (www.bmv.ohio.gov) Evaluate this law in terms of how it relates to Newton’s first law of motion. Take a position for or against this law and provide at least three arguments to support your position. Use your knowledge of forces of motion to support your position with evidence. (2 points for each argument and explanation; 6 points total)

Two minutes to think, two minutes to share, three minutes to respond.