Transcript Chap. 12 P.P - Moline High School

Chapter 12
Forces
http://www.youtube.com/watch?v=8zsE3mpZ6Hw
http://www.youtube.com/watch?v=4NQbeZ0EXZQ
http://www.youtube.com/watch?v=bkivr6bB-og&feature=related
Newton’s First Law of Motion
1st Law of Motion(Law of Inertia)
-Objects in motion will stay in motion in a straight line
and objects at rest will stay at rest, unless acted on by
an outside force
-Ex. Stack of books on a
car seat. What happens
when you suddenly stop?
Inertia - objects resistance to a change in
motion
Mass affects an objects inertia
Increase mass = Increase inertia
Lower mass = Lower inertia
Seat belts and inertia
Seat belts work because they keep your
body from flying forward.
If you get hit from behind will seatbelts or
air bags help?
No, you will fly back into the
seat.
Newton’s Second Law of Motion
Force is equal to the mass of an object and its
acceleration.
Force = Mass x Acceleration
F=mxa
Force is measured in Newtons
1 N = 1 kg x 1 m/s2 or 1kgm/s2
( 1 lb is = to 4.45 N or 1 N is = to .225 lb)
Newton’s Second Law of Motion
Newton’s second law explains how
acceleration is dependant on force and mass.
Newton’s Second Law of Motion
Newton’s Second Law of Motion
Newton’s second law could also be written
as: - When a force is applied to an object,
the object accelerates in the direction of
the greater force.
OR
The acceleration of an object is directly
proportional to the net force on the object
and inversely proportional to the object’s
mass.
Newton’s Second Law of Motion
Acceleration = net force/mass
or a = F/m
Gravity
Gravity - Force of attraction between two
objects.
- force of gravity depends on two
variables:
a. depends on mass
-larger = more gravity (attraction)
- smaller = less gravity (attraction)
Gravity
- force of gravity depends on two
variables:(continued)
b. depends on distance
-closer = more gravity (attraction)
- further = less gravity (attraction)
Weight
A measurement that can be taken because of
gravity is weight.
Weight – force of gravity pulling on the
mass of an object.
- weight is a force that can change.
Mass never changes!
Free Fall
when only gravity is acting on a falling object.
constant rate of acceleration. This is called
gravitational acceleration (g)
* g = 9.8 m/s²
- which means everything accelerates
as it falls at the rate of 9.8 m/s² each
second!
Free Fall and Weight
We can use this knowledge to mathematically
find weight!
Equation to find weight is same as force!
Force (weight) = m a
or
weight = m X g (acceleration of gravity)
Weight
Ex. Find the weight of a 58 kg man on
Earth.
Weight = mg
Weight = 58kg x 10 m/s2
= 580 kg m/s2 or 580 N
Now find his weight on:
Venus g=8.8 m/s2
58kg x 8.8 m/s2 = 510N
Mars g= 3.7 m/s2
58kg x 3.7 m/s2 = 220N
Neptune g=11.8 m/s2
58kgx11.8 m/s2 = 680N
Newton’s 2nd Law
Newton’s 2nd law explains why heavy objects
and light objects fall at the same rate.
-because
if the mass is
large, it is harder to
accelerate (has more
inertia).
So it takes more time to
fall!
Terminal Velocity
On Earth we have air resistance
- causes friction, causing the falling objects to
slow down
Terminal Velocity
- maximum velocity reached by a falling
object.
(320 km/h or 120 mph)
- occurs when air resistance is equal to the
gravitational acceleration.
Terminal Velocity
Projectile Motion
curved path an object follows when projected
near the Earth’s surface.
Projectile Motion
- 2 components make up the path of projectile
motion.
Both are independent of each other.
a. horizontal motion
-force stays constant
b. vertical motion
-pulling it down at a rate of 9.8 m/s2
Projectile Motion
What if your object you are shooting at is
falling too.
Where should you aim?
http://www.physicsclassroom.com/mmedia/vectors
/mzng.cfm
Projectile Motion
-Orbiting objects are in projectile motion.
- so they are NOT weightless or floating,
they are just constantly falling!
Newton’s 3rd Law of Motion
For every action there is an equal and
opposite reaction.
Forces act in pairs
- don’t necessarily act on the same objects.
Ex. Escaped balloon
Action – air escaping from
opening in balloon
Reaction - balloon flies forward.
Momentum
The product of the mass and velocity of a
moving object.
Momentum(p) = mass (kg) x velocity (m/s)
p = kgm/s
Ex. Calculate the momentum of a 6.00kg bowling ball
Moving at 10.0 m/s down the lane.
6.00kg x 10 m/s = 60.0 kgm/s down the lane
The Law of Conservation of
Momentum
Total amount of momentum in a system is
conserved.
Ex.