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Forces and Newton’s Laws
Physical Science
Unit 8
Newton’s Laws
• 1st Law: An object in rest will stay in rest and an
object in motion will stay in motion at a
constant velocity unless acted on by an outside
force.
• 2nd Law: Force equals mass times acceleration
• 3rd Law: For every action there is an equal and
opposite reaction.
• Law of Universal Gravitation: There is a
gravitational force between every two objects.
They pull on each other equally.
Fist of all… What is a force?????
• A push or a pull that has the ability to change
motion!
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Newton’s Law
(Law of Inertia)
• An object at rest will stay at
rest, and an object in motion
will stay in motion at constant
velocity, unless acted upon by
an unbalanced (outside) force.
Law of Inertia
• “An object will keep doing what it is doing.”
• An object resists changes in motion because
of its inertia. We have to apply a force to
change the motion.
Example: These pumpkins
will not move unless they
are acted on by an
unbalanced (outside) force.
Law of Inertia
• ALL objects have inertia.
• Since all objects have inertia, do they all have the
same tendency to resist change??
– NO!
– Inertia depends on the mass of the
object!!!!!!!!!!!!!!!!!!
• More Mass = More Inertia
• Example: It is easier to kick a soccer ball that has a mass of
2 kilograms than a bowling ball that has a mass of 12
kilograms.
Examples of Inertia…
• If we were to kick the bowling ball
and the soccer ball which one would
go farther?
The soccer
ball!!!
Newton’s 2nd Law
• The force needed to move an object is equal
to the object’s mass x its acceleration.
• Force = mass x acceleration (F = ma)
Measuring Force
• We measure force in Newtons (after Sir Issac
Newton, founder of all of Newton’s Laws).
• This can be abbreviated to “N”.
• Therefore…
– F = ma
• N = (kg)(m/s2)
• N = kg*m/s2
Practice…
• If the mass of a helicopter is 4,500 kg. and the net
force on it is 18,000 N, what is the helicopter’s
acceleration?
– F = ma
• F = 18,000 N
• m = 4,500 kg
• a=?
– 18,000 N = (4,500 kg)a
– a = 18,000 N / 4,500 kg
– a = 4 m/s2
Practice…
• What is the net force on a dragster with a mass
of 900 kg if its acceleration is 32.0 m/s2?
• Mike's car, which weighs 1,000 kg, is out of gas.
Mike is trying to push the car to a gas station,
and he makes the car go 0.05 m/s/s. Using
Newton's Second Law, you can compute how
much force Mike is applying to the car.
Friction
• The force that opposes sliding when two
objects are in contact.
Terminal Velocity
• As an object falls through the air,
gravity accelerates that object
• As an object accelerates, the
upward force of air resistance
increases
• Terminal Velocity: highest falling
speed an object will reach!
• F gravity = F air resistance
Gravity
• Attractive force between any
two objects
• Depends on the masses of the
objects and the distance
between them
• Acceleration due to gravity on
earth = 9.8 m/s2
Weight
• Gravity exerts a
force on all objects
• Weight is the force of gravity on objects
• Weight = mass x gravity ( W = m x g)
• Weight will change if acceleration due to gravity
changes!
Practice…
• On earth, what is the weight of a television
having a mass of 75 kg?
• M = 75 kg g = 9.8 m/s2 W=?
• W = (75 kg) (9.8 m/s2)= 735 kg x m/s2
735 N
Practice…
• Find the acceleration due to gravity on Mars if
a person with a mass of 60.0 kg weighs 22.2 N
on Mars?
Review… What is the difference
between weight and mass???
• Mass is the amount of matter in an object. This does
not change in an object as it moves away from the
earth.
• Weight is caused by gravity. Therefore, the greater the
distance, the less the force of attraction. The less the
distance the greater the force of attraction.
• Your weight is dependent upon the mass of the earth.
If you were able to go to the planet, Jupiter, your
weight would be 27 time greater, because the mass of
Jupiter is 27 times greater.
What causes “weightlessness”?
• Even in the space shuttle, there is a force of
attraction exerted by the earth on the shuttle
and its contents.
• When the space shuttle temporarily “falls”
toward the earth, the contents of the space
shuttle appear to be weightless, but in fact
they are falling with the space shuttle. This is
called “freefall”.
What is centripetal force?
• “centripetal” means to move toward the center.
• Centripetal acceleration is the acceleration of an
object toward the center of a curve or circular
path.
• Centripetal force is a force toward the center of a
circle which holds the moving object in its circular
path.
Newton’s 3rd Law
• When an object exerts a force on second
object, the second object exerts a force on the
first that is equal in size but opposite in
direction.
• A.K.A. action-reaction forces
• For every action there is an equal but opposite
reaction.