Transcript Phys Sci Chapter 3 notes

Chapter 3
Forces
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Newton’s 1st Law

The Law of Inertia: An object in motion will
stay in motion unless acted on by another
force. An object at rest, will stay at rest
unless acted on by another force.
Sec. 1 Force, Mass & Acceleration
For any object, the greater the force
applied to it, the greater its
acceleration.
 The acceleration of an object depends
on its mass as well.
 Force, mass, and
acceleration are
connected.

Newton’s 2nd Law


Newton’s 2nd law of motion—the net
force on an object causes the object to
accelerate in the direction of the net
force.
Equation:
•
acceleration = net force
mass
•
a = F/m
-orF = ma
•
F = ma
F = 1000kg x .05m/s2
F = 50 kg m/s2
Units
The SI unit for mass is the kilogram
(kg)
 The unit for acceleration is meters
per second squared (m/s2)
 So the unit for force is kg x m/s2
 The kg x m/s2 is called the newton (N)

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Friction


Friction—the force that opposes motion
between 2 surfaces that are touching each
other.
The amount of friction depends on 2 factors:
1.
2.


The kinds of surfaces
The force pressing the surfaces together
The areas where bumps on the surfaces stick
together are the source of friction.
The stronger the force, the more friction.
Types of Friction



Static Friction—the friction between
2 surfaces that are NOT moving past
each other.
Sliding Friction—the force that
opposes the motion of 2 surfaces
sliding past each other.
Rolling Friction—the friction between
a rolling object and its surface.
Air Resistance



When an object falls, it is pulled
downward by gravity.
Air resistance—a force that acts on
objects as they fall through the air.
The amount of resistance on an object
depends on the speed, size, and shape
of the object.
–
A feather will fall more slowly than an
apple
Terminal Velocity



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As an object falls, it accelerates.
The force of air resistance increases with
speed until it becomes large enough to cancel
out the force of gravity.
Then the forces on the object are balanced.
Terminal velocity—the highest velocity a falling
object will reach.
Terminal Velocity
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Sec. 2 Gravity

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The Law of Gravitation—states that any 2
masses exert an attractive force on each
other.
The amount of attractive force depends on
the mass and distance between them.
Gravity is a long-range force.
All stars in a galaxy exert a gravitational
force on each other.
–
This is what gives a galaxy its shape.
Gravitational Acceleration
Near Earth’s surface, the
acceleration due to gravity is
9.8 m/s2.
 When an object falls towards
earth it is called “free fall”
 To calculate the force of an
object in free fall

–
F = m x 9.8 m/s2.
Weight

Weight—the gravitational force
exerted on an object
–

Represented by the symbol W.
You can use Newton’s 2nd Law (F=ma) to
calculate weight
–
–
Weight = mass x acceleration due to
gravity.
W = m x 9.8 m/s2
Weight and Mass

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Weight and mass are not the same.
Weight is a force—mass is the amount of
matter.
Weight and mass ARE related
The more mass an object has, the more it will
weigh in the same location.
Weight is usually determined for Earth.
An object will have a different weight on the
moon.
Projectile Motion

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If a ball is thrown, it does not always
travel in straight lines—they curve
downward.
Anything thrown or shot is called a
projectile.
Projectiles curve because of Earth’s
gravitational pull and their own inertia.
They have both horizontal and vertical
velocities.
Horizontal and Vertical Motion
When you throw a ball, you give it
horizontal motion.
 After you let go, gravity starts to
pull it downward—giving it vertical
motion.
 The ball appears to travel in a curve,
even though the horizontal and
vertical motions are independent of
each other.

Centripetal Force


Centripetal acceleration—acceleration toward
the center of a curved or circular path.
Centripetal Force—a force directed toward
the center of a circle for an object moving in
circular motion.
Centripetal
force keeps
these riders
moving in a
circle.
Sec. 3 Newton’s
rd
3
Law
 Newton’s
3rd law of motion—
when an object exerts a force on
a second object the second one
exerts a force on the first that
is equal in size and opposite in
direction.
–
To every action there is an equal and
opposite reaction.
Action & Reaction



When you jump on a trampoline, you exert a
force downward; the trampoline exerts an
equal force upward—sending you back in the
air.
If forces are equal, how does anything ever
happen?
Action-reaction forces are acting on different
objects!
–
They are equal, but they are not balanced
Momentum


Momentum—the product of an object’s mass
and its velocity.
Represented by the symbol p
–
–



Momentum = mass x velocity
p=mxv
The unit for momentum is kg m/s
An archer’s arrow can have a large momentum because
it has a high velocity even with a small mass.
An elephant may have a low velocity, but has a large
momentum because of its large mass.
Newton’s 2nd Law and Momentum

The force of changing momentum:
–



F = (mvf – mvi) / t
Law of conservation of momentum—states
that total momentum is neither created or
destroyed, but it is transferred in collisions.
Example: when you hit a cue ball for the first
time in a game of pool, it has all of the
momentum. When it strikes the other pool
balls, it transfers some of its momentum to
them.
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