Download Forces, Laws of Motion & Momentum ppt

1
What is force?
 When two or more forces act on an
object at the same time, the forces
combine to form the net force
Forces on an object that are equal in
size and opposite in direction are
called balanced forces
Force is the action which has the
ability to change motion.
• Objects need a force applied to them in order
to change their motion.
• A force is also called a
push or a pull.
Sir Isaac Newton described
three Laws that govern
motion.
3
What is force?
 Forces don’t always balance out:
Force is measured in Newtons (N) in the
metric system.
1 N = 1 kgm/s2
The English unit for force is a pound (lb).
4.48 N = 1 lb
Forces are vector quantities and can
be denoted with arrows because you
can push in specific direction.
5
Force diagrams: Four forces can
act on an object
Fn = normal force.
This is the force that holds
atoms together and makes
things solid.
Ff= frictional
force
Fa= applied
force
This is a force that
happens whenever
two surfaces touch.
This force will
always oppose the
applied force.
This is the force
that is being
applied to the
object. It always
comes out of the
center of mass
Weight= Fg= Gravitational force
This is the force created because of
gravity pulling on the mass of the object.
6
Football Physics
• Why are the most massive players
lineman? Why are the least massive
players receivers?
Receivers want to change their motion, lineman
want to hold their ground
What if forces are balanced?
Newton’s First Law of Motion
1. An object at rest stays at rest or an object in
motion stays in motion (constant velocity)
unless acted on by an unbalanced (net)
force.
• It is also known as “the law of inertia”
•
Inertia: an objects ability to resist a change in
motion.
Example: The man pushed the cart in order
to make it roll.
The bigger the cart the
harder the man must push
8
Newton’s First Law of Motion
An object at rest will stay at rest
and an object in motion will continue
in motion unless acted upon by an
unbalanced force.
nd
Newton’s 2
Law of Motion
• Which wagon is going
to be easier to pull?
Which wagon needs a
greater force to pull
it?
• How can you increase
the acceleration of
wagon A?
A
B
Force on different masses
• Which one will be harder to push will the
same force?
What if forces are unbalanced?
Newton’s Second Law of Motion
1. If an unbalanced (net) force is applied to a
mass it will accelerate.
•
The mathematical relationship between acceleration
and mass is directly proportional and inversely
proportional to force. A math formula can be written:
F=ma
F = force
m = mass
a = acceleration
What is mass?
The amount of stuff
(atoms) in an object
12
Newton’s Second Law
• So if you double the force what
happens to the acceleration?
• It also doubles
• If you apply the same force to twice
the mass what happens to
acceleration?
• Results in half the acceleration
The 2nd law says….
• The more mass an object
has, the more force it’s
going to take to get it to
accelerate.
You try!
Example: The man pushed the cart with 10 N of force to
make it accelerate 20 m/s2. What was the mass of the cart?
See if you can calculate the mass.
F = 10 N
a = 20 m/s2
m= ?
m=F
a
m = 10 N (kgm/s2) = 0.5 kg
20 m/s2
F
m a
15
What if forces are unbalanced?
Tracker A is pulling on the 10 kg block with a force of 50 N
and tracker B is pulling on the block with 20 N. How fast is
the block accelerating and in what direction?
F = 30 N
Remember your vector rules
m = 10 kg
Net Force= 50 N – 20 N= 30 N
a=
a=F
m
?
= 30N
= 3 m/s2
10kg
16
Remember what happens if all four forces are
balanced?
Fn
Ff
Fa
Fg
The object is standing still or moving at a constant velocity!
17
Weight and Gravity
• Gravity is the rate at which all things fall to the earth.
– It is always 9.8 m/s2
Notice that gravity is an
acceleration value
• Weight is a measure of how gravity pulls on your mass
to create a downward force.
– Your weight can be calculated just like any other force is
calculated.
Fw= weight
Fw= mg
m = mass
g = gravity
g = 9.8 m/s2
Air resistance is friction
for falling objects. Air
All things fall at this rate
resistance is why it
no matter what their
seems like things fall at
size!!!!
18
different rates.
Remember the things that effect
gravity are:
1. The size of the mass.
– The bigger the mass the greater the gravitational
pull.
2. The distance between the masses.
– The farther apart two masses, the less
gravitational force is felt between
them.
19
Weight vs. Mass
• Your mass is constant
because it is a measure
of the amount of atoms in
your body.
• Your weight will change if
the gravity of the planet
changes.
Since Mars is a smaller
planet, it will have a
smaller gravity and pull
down on a mass with
less force.
20
Mass vs. Weight
Mass is...
• A measure of the
amount of matter in
an object
• Always constant no
matter where in the
universe
• Measured with a
balance
• Expressed in kg
Weight is...
• A measure of the
gravitational force on
an object
• Varies depending on
where you are in the
universe
• Measured with a
spring scale
• Expressed in newtons
(N)
Newton’s 3rd Law of
Motion
• For every
action,
there is an
equal but
opposite
reaction
Forces come in pairs
Newton’s Third Law of Motion
1. For every action there is an equal and opposite
reaction.
• These forces always occur in pairs and are often
called action-reaction forces.
• Remember; if forces are equal, what is happening to
the object? They are standing still or moving at
Action:
The boy threw the
ball forward with a
certain amount of
force
constant velocity.
Reaction:
The boy rolled
backward with the
same amount of
force
23
Momentum: An object’s tendency
to continue with a certain motion
• Momentum has to do with collisions and
what happens to motion when objects
collide.
• The Law of conservation of momentum
states that the amount of momentum after
a collision must equal the amount of
momentum before the collision if no
outside forces act on the objects.
24
• You can calculate momentum using this formula:
p = momentum
p = mv
m = mass
v = velocity
25