Transcript AP Physics 9/16/09 A whimsical Wednesday

Physics
10/05/09
A magical Monday
1) Attendance – If your life had a theme song, what
would it be?
2) Inertia
3) Reminder Lab Thursday
Newton’s Laws
• Newton’s First Law
– An object at rest remains at rest and an object in
motion stays in motion unless the object
experiences a net force.
• Newton’s Second Law
• Newton’s Third Law
Inertia
• Object’s resistance to changes of motion
– think of it as the “laziness”
• The greater the mass, the more inertia the object has.
– Examples:
» acceleration of a car and you are pulled back into the seat
» driver hits the breaks hard and you fly forward
» seat belt locking mechanism (below)
When the brakes are hit,
the red block slides
forward due to inertia
causing the ratchet to
lock.
Acceleration is determined by net external force
moving with constant velocity means all forces are balanced
F ground
F car
F friction
F gravity
If the person takes his foot off the gas, the force from the car decrease and the net
force is greater on the side of friction, thus slowing the car
F water
F person
F fish
F gravity
bobber only stays on surface if all force are balanced
• A person is pulling a crate with a force of 70.0 N directed at an
angle of 30 degrees above the horizon. There is a frictional
force from the crate of 50.0 N. Is the crate moving?
Since Fnet y = 0, we do not have to do any work there.
F gr
50.0 N
30°
Fy
Fx
Fg
Since this does not equal zero, the book would accelerate to the right.
• Melissa leaves her Physics book on a drafting table that is
inclined at an angle of 35 degrees. There is a gravitational
force of 22N, a force from the table to the book of 18N,
and a frictional force of 11N. Does the book move?
18 N
We could use our trig function twice or we could
rotate the x-y axes 35° to get the following:
18 N
11 N
35°
Fy
35°
22 N
22 N
Fx
But we knew that because it had to
equal the force from the table
Yes, accelerates down the slope
• A gust of wind blows an apple from a tree. As the apple falls,
it has a gravitational force of 9.25 N and the wind exerts a
force of 1.05 N to the right. Find the magnitude and direction
of the net external force.
1.05 N
1.05 N
θ
Fnet
9.25 N
9.25 N