Transcript ISChpt3-local-local

Integrated Science
Unit 1, Chapter 3
Unit One: Forces and Motion
Chapter 3 Forces and Motion
3.1
Force, Mass and Acceleration
3.2
Weight, Gravity and Friction
3.3
Equilibrium, Action and Reaction
3.1 Force, Mass and Acceleration

Unless you apply force, things
tend to keep on doing what
they were doing in the first
place.

Force causes an object to
accelerate, while the object’s
mass resists acceleration.

For every action, there is an
equal and opposite reaction.
Force
Every object continues in a state of rest, or motion, unless force is
applied to change things.
Force is an action that has the ability to change motion (Push or
Pull)
Commonly used units: Pounds and Newtons
1 newton = 0.228 pounds
1 pound = 4.448 newtons
Difference between force and mass
Force is a push or pulling action that can change motion, whereas
mass is the amount of “stuff” or matter in an object
Weight is the force acting upon an object due to the influence of
gravity
• On the moon… your mass would be the same but your weight
would be different.
WEIGHT IS A MEASURE OF FORCE… MASS IS NOT
Mass and Inertia
Inertia: Property of an object to resist changing its state of motion
Newton’s first law of motion is also known as the law of inertia
The amount of inertia an object has depends on its mass
Mass is a measure of the inertia of an object
Greater mass = Greater inertia
3.1 Newton's Second Law
acceleration (m/sec2)
a=F
m
force (newtons, N)
mass (kg)
The acceleration of an object is directly proportional to the
force acting on it, and inversely proportional to its mass
Force causes acceleration
Mass resists acceleration
Balanced
and
unbalanced
forces
B
The motion of an object depends on the total of all forces action
on the object.
We call the total of all forces the net force
When forces on an object are balanced, the net force is zero and
we say that the object is at equilibrium.
3.1 Force, Mass, and Acceleration
Key Question:
What is the relationship between force, mass
and acceleration?
*Read text section 3.1
BEFORE Investigation 3.1
3.2 Gravity

What is gravity?
— Gravity is a force.
— Gravity depends on mass.
— Gravity accelerates objects.
 Gravity on earth accelerates
at a rate of 9.8 m/s2
3.2 Gravity
 If
you were on Mars, your
force/mass balance would
have to be adjusted.
 The
planet is smaller than
Earth and therefore Mars’s
gravity is weaker.
Weight vs. Mass
 Weight
is the force created by gravity on
objects
 Your
mass is constant throughout the
universe, but your weight changes depending
on what planet you are on.
3.2 Weight
If you know the mass of an object, you can calculate its weight
using Newton’s second law
Weight force (N)
Fw = mg
gravity (9.8 m/sec2)
mass (kg)
3.2 Weight and Galileo
A
legend has it that,
around 1587, Galileo
dropped two balls from
the Leaning Tower of
Pisa to see which would
fall faster.
Weight and Mass
 We
tend to use mass and weight
interchangeably in our everyday lives
 BUT…
 Weight
in physics…
is never measured in grams and mass
is never measured in pounds or newtons.
3.2 Gravity
 The
attractive force
from gravity between
objects of ordinary
mass is incredibly
small.
3.2 Gravity
 You
feel weight
because the mass of
Earth is large enough
to create significant
gravity forces.
3.2 Newton's Law of Universal
Gravitation
 The
force of attraction between two objects is
directly related to the masses of the objects
and inversely related to the square of the
distance between them.
3.2 Newton's Law of Universal
Gravitation
gravity (9.8 m/sec2)
mass 1 (kg)
Force (N)
F = G m1m2
R2
mass 2 (kg)
distance (m)
between m1 and m2
3.2 Weight, Gravity and Friction
Key Question:
How does increasing the mass of the car
affect its acceleration?
*Read text section 3.2
BEFORE Investigation 3.2
3.2 Friction
 Friction
is a force that always opposes motion.

What is left is
often called the
net force.

What is the net
force acting on
the car?

Friction reduces
acceleration
Types of Friction

Air Friction: air around moving objects creates opposing
friction

Sliding Friction: when two surfaces rub against each other

Viscous Friction: Objects that move in liquids

Rolling Friction: One object rolling over another

Objects that continuously rub against each other will cause
wear
3.3 Equilibrium, Action and Reaction
Key Question:
What is Newton's third law of motion?
*Read text section 3.3
AFTER Investigation 3.3
3.3 Equilibrium, Action and Reaction

Momentum
—
explains why the speed and the direction of
motion are related to the mass of the object
3.3 Momentum
Law of Conservation of Momentum: As long as interacting objects
are not influenced by outside forces, their momentum before the
interaction will equal their momentum after the interaction.
Momentum (kg-m/sec)
P = mv
velocity (m/sec)
mass (kg)