Law of Inertia & Balanced vs. Unbalanced Forces. Question: What allows for a snowboarder to be able...
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Transcript of Law of Inertia & Balanced vs. Unbalanced Forces. Question: What allows for a snowboarder to be able...
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.