The Law of Inertia
•Objects at rest remain at rest unless acted
upon by an outside force.
•Objects in motion will remain in motion
unless acted upon by an outside force.
What causes the tire to
stop rolling?
Inertia is dependent on mass.
The larger the mass, the more inertia it has.
I’m not
moving till you
explain this
report card!
A force is a push or pull.
Forces can change the motion of an object.
Force is measured in Newtons.
(appropriately named!)
One Newton = the amount of force needed to accelerate
1 kilogram by 1 meter per second squared.
1 kg
a = 1 m/s2
Weight – is a force.
It is the force of gravity acting on an object’s mass.
Weight (in newtons) = mass x acceleration due to gravity.
example: 1 kg x 9.8 m/s2= 9.8 Newtons
Calculate your weight in Newtons!
Friction – is a force caused by objects touching
that opposes motion, but is necessary.
Friction helps
us stand
Friction helps go
forward and stop
Friction helps
us walk and run
Force is a vector quantity.
100 Newtons
The length of the arrow shows the strength of the force.
The direction shows the direction that the force is applied
Force vectors are added and subtracted just as velocity
vectors are…
The sum of the force vectors (the resultant) is called
the net force.
If the net force is zero, there is no movement. The
system is at equilibrium. = The forces are balanced.
An unbalanced force results
in movement in the direction
of the larger force.
Consider the forces on this box:
Fn = Normal force (always acts
perpendicular to the surface)
Fw = weight of the box
If the box has a force applied to it, there are two additional forces.
Fn = Normal force (always acts
perpendicular to the surface)
Ff = Friction force
Fa = Applied force
Fw = weight of the box
Coefficient of Friction
Each combination of surfaces that come in contact with
each other have their own value for friction. This is called
the coefficient of friction.
The coefficient of friction () can be calculated by:
 = Ff
Fn
The
box problem
is placedinvolving
on a smooth
wooden table. A force of 14 N
Example
friction:
is necessary to to keep the box moving at a constant velocity.
If the box has a weight of 40 N…
1. Draw the forces effecting the box.
2. What is the coefficient of friction?
This 52 N saucer sled is pulled across a cement sidewalk
with a force of 18 N.
What is the coefficient of friction between
the sled and the sidewalk?
The 52 N saucer sled is now placed on snow and a 450 N
boy sits on it. The coefficient of friction for the snow and
metal sled is 0.012.
What force is necessary to pull the sled at constant speed?
(Hint: the applied force is equal, but opposite direction to the
force of friction.)
Newton’s second law explains the relationship
between the net force on an object, it’s mass
and the acceleration that results from the force:
Let’s look at the relationships:
If mass is constant, the larger the force on an object, the
faster the objects accelerates.
Large force = mass x large acceleration
Small force
= mass x small acceleration
This is a direct proportion.
Let’s look at the relationships:
If the force is constant, the larger the mass of an object, the
slower the object accelerates.
force = small mass x large acceleration
force = large mass x small acceleration
This is an indirect or inverse proportion.
Let’s look at the relationships:
If the acceleration is constant, the larger the mass of an object,
the stronger the force must be to change the motion.
small force
= small mass x acceleration
large force = large mass x acceleration
This is a direct proportion.
Solving problems related to Newton’s second law:
The basic equation is:
Force = mass x acceleration
or F = ma
Try this:
Rearrange the variables so that you can solve for
acceleration or mass.
a=f/m
m=f/a
For Practice:
Example 1
How much force must a 30,000 kg jet plane develop in order
to achieve an acceleration of 1.5 m/s/s?
Example 2
What acceleration is produced by a force of 500 N applied
to a 1000 kg car?
Example 3
If a grocery cart is pushed with a force of 75 N, and the
cart accelerates from rest at a rate of 1.5 m/s/s, what is the mass
of the cart?
What is the weight of the cart?