Transcript forces ch2,3

Newton’s First Law of Motion
-An object moving at a constant
velocity(constant speed and direction) keeps
moving at that velocity unless an unbalanced
net force acts on it.
-AKA: The law of Inertia
Newton’s Second Law
-States: The acceleration of an object is
in the same direction as the Net Force on
the object and can be calculated from
this equation:
acceleration = Net force (N)
Mass (kg)
-It basically tells us how forces, mass,
and acceleration are all related!
- The greater FORCE you apply to an object =
the greater ACCELERATION of the object
Tapping a soccer ball with your foot produces
a small acceleration but kicking it produces a
large acceleration!
-The greater the mass of an object = the less
acceleration of the object with the same force
applied
Using the same force (strength), you can hit a
baseball farther than a softball
You can also use Newton’s Second
Law formula to calculate Net Force:
just reorder it!
Net Force = mass X acceleration
You push a friend on a sled. Your friend and the sled
together have a mass of 70 kg. If the net force on
the sled is 35 N, what is the sled’s acceleration?
Net Force = Fnet = 35 N
A = Fnet / m
Friend + Sled Mass = m = 70 kg
Want to know Acceleration = a = m/s2
A = 35 N / 70 kg = 0.5 N/kg = 0.5 kg m X 1 = 0.5 m/s2
S2
kg
- Force that opposes the sliding motion of 2
surfaces that are touching each other
Depends on Two Factors:
1. Type of Surfaces
2. Force pressing surfaces together
Caused By: Microwelds (bumps on the
surface of objects) which stick
together
1.Sticking
Friction
Surfaces STICK together. Increase the
force pushing objects together = Surface
bumps become closer = Increase Friction. A
force must be applied to break the sticking
2.Static
Friction
Frictional Force that prevents two surfaces
from sliding past each other
3.Sliding
Friction
Force that opposes the sliding motion
between two surfaces. Microwelds constantly
break and reform as you slide. Must apply a
Force to continue sliding.
4.Rolling
Friction
Friction between rolling objects and a
surface. Rolling Friction causes a train to
come to a stop.
-Opposes the motion of objects that
move through the air
-Causes objects to fall at different
accelerations and different speeds
-This is why a feather falls at a
slower speed than a bowling ball
-Object = Air resistance
-Depends on objects SIZE, SHAPE,
and SPEED
Term. Vel = 53m/s for humans, slows
to 5-10 m/s with open chute
-The highest speed a falling object will reach
-It is a constant velocity (speed) thus the net
force = 0 b/c air resistance balances force of
gravity, and acceleration = 0
-Depends on size, shape, and mass
-Ex: Open parachute v/s Closed parachute
(air resistance is greater for the opened one)
• Attractive force
between any 2 objects
• Force of gravity between
objects increases as the
objects’ mass increases or
as they get closer
together. Decreases as
distance increases
• Depends on Masses
of the objects and the
distance between the
objects
• Everything with
mass exerts the
force of gravity!
Why don’t you feel the force of gravity from your book? Why
don’t you feel the force of gravity from the sun? Why do you
feel the force of gravity from the Earth?
• Developed by Newton (1687)
• Long-range force (never reaches zero)
• G = universal gravitational constant
= 6.67 X 10^-11 m3/kg s2
• F = Force of gravity between any 2 objects
F = G (m1m2)
d2
• Acceleration due to gravity (w/out air
resistance) =
9.8 m/s2
F of gravity (N) = mass (kg) X acceleration of gravity (m/s2)
F = mg
• Review: Mass = amount of matter in an object
• You find your mass by standing on a scale
• Gravitational force exerted on an object =
object’s weight
Weight (N) = mass (kg) X acceleration of gravity (m/s2)
W = mg
• Weight changes as gravitational force on
objects change
Freefall = force of gravity stills acts on
you
Weightlessness = You feel weightless
because you no long exert a force down.
Everything in freefall falls with the same
acceleration so you seem to be floating.
• Projectile = anything thrown/shot through the
air
• Gravity causes projectiles to follow a curved path
• Horizontal and Vertical Motions
• Independent of each other
• Horizontal motion can be constant while
vertical motion is changing due to gravity
• Horizontal and Vertical Distance
•Objects travel same vertical distance in same
time even though one travels more horizontal
distance
Describes Action and Reaction
When one object exerts a
force on another object, the
second object exerts a force
on the first that is equal in
strength and opposite in
direction
“to every action there is an
equal and opposite reaction”
Action and Reaction
Forces are EQUAL
So how does anything ever
happen?? For instance
swimming, if you push down on
the water and the water
pushes back, why are you
propelled through the
water??
Action and Reaction Forces act on different
objects. Even though they are equal they
are NOT Balanced.
As a rocket burns fuel it produces hot gases.
The rocket exerts a force on the gasses and
causes them to escape out the back.
The gases in turn exert a force on the rocket
and cause it to…..???
• The product of an object’s mass and velocity
• Related to how much force is needed to
change an object’s motion
• Symbol: p
• Equation:
momentum (p) = mass (kg) X Velocity (m/s)
p = mv
So how can a football player increase his/her
momentum??
Run Faster or Increase his/her Mass
p=mXv
Review:
Acceleration
Newton’s Second Law
a = Vf – Vi
F=mXa
t
Put these two together and you write the 2nd law this
way:
F = (mvf – mvi)
t
mvf and mvi are both momentum (p)
So: You can calculate Net Force if you have momentum
Momentum can be transferred from one
object to another: it isn’t lost or created
Ex: Pool
You hit the cue ball into the
group of balls and they all begin
to move. The momentum the
cue ball had is transferred to
the other balls.
If a hockey puck is moving to the right and gets
hit from behind by a second hockey puck what
happens?
The hockey puck that got hit increases its
speed. It gains momentum from the second
puck.
If two hockey pucks collide going opposite
directions what happens?
They reverse their direction and move with the
same speed. Total momentum is Zero