Transcript Newton`s Second Law of Motion Chapter 5 Force and Acceleration

Newton’s Second Law of
Motion
Chapter 5
Force and Acceleration
Force causes Acceleration
• Remember:
change in velocity
acceleration =
time interval
Acceleration is any change in velocity.
5.1 FORCE CAUSES
ACCELERATION
There is always MORE than
one force acting on an object
at one time.
Net Forces
Definition:
The combination of all the
forces that act on an object
is the NET FORCE.
The NET FORCE accelerates
objects.
~ means “is proportional to”
= means “is equal to”
ACCELERATION ~ NET FORCE
Examples of Proportion
Proportion means that as one
changes, the other changes at the
same rate.
EX:
The length of your spine is
PROPORTIONAL to your height.
Think about a grocery cart…
When the cart is empty, it is easy
to push.
The cart easily accelerates with
simple pushing.
What about when the cart is full
and the mass is greater?
The greater the mass of the cart,
the harder you have to push.
What is different with the 2 carts?
MASS
5.2 Mass Resists Acceleration
The more mass an object
has, the less acceleration
you can get –
using the same force.
In other words…
Acceleration is INVERSELY
proportional to mass, or…
ACCELERATION ~ 1 / MASS
5.3 Newton’s Second Law
“The acceleration produced by a
net force on an object is directly
proportional to the magnitude of
the net force, is in the same
direction as the net force, and is
inversely proportional to the mass
of the body.”
5.3 Newton’s Second Law
ACCELERATION ~
However, . . .
NET FORCE
MASS
If we use these units:
Net force measured in Newtons
Mass measured in kilograms
Acceleration measured in m/s/s
we can also say…
Acceleration =
Net Force
Mass
5.3 Newton’s Second Law
Using variables, we get the
equation:
A =
F
m
Rearranging the equation, we
can also say:
Force = (mass) X (acceleration)
or
F = ma
But remember…
You must convert your data
into the following units:
kg for mass
m/s/s for acceleration
N for net force
5.4 Friction
Definition:
…the force that acts to resist
the relative motion
(or attempted motion) of
objects or materials that are
in contact.
Fluids
If an object is a “fluid” it
is a liquid or gas.
Fluids can flow.
Air resistance
Air resistance is any friction
acting on something
moving through the air.
5.4 Friction
• Free body diagram – a
diagram showing all the
forces acting on an object.
5.5 Applying Force - Pressure
Definition:
…the force per unit of surface
area where the force is
perpendicular to the surface.
Pressure is measured in PASCALS.
In other words…
Pressure =
Force
Area of application
--or—
Pressure =
F (in Newtons)
A (in meters2)
Pressure
Measured in “Newtons per
Square Meter”
Or
N
2
m
N
2
m
= PASCALS or Pa
5.5 Pressure
• Two books laying on a table:
–One is upright; the other is
face down
Do they have the same force?
Do they have the same pressure?
(Fig. 5.7)
5.6 Free Fall Explained
• Free fall = falling free of air
resistance
• Acceleration is CONSTANT
does not change)
• Remember: g = 10 m/s2
(it
Ratios
• Ratios are “in proportion” to each
other.
•
F
M
F
=
M
Example of a Ratios
12
24
=
6
12
=
2
4
=
1
2
This is why a 10 kg cannon ball
flying out of a cannon has
proportionate
force (weight) / mass
as a 1 kg baseball flying tossed by
a child. Example page 48
5.7 Falling and Air Resistance
TERMINAL SPEED – the speed at
which the acceleration of a falling
object terminates because friction
balances the weight.
TERMINAL VELOCITY – terminal
speed together with the direction of
motion
(down for falling objects)
5.7 Falling and Air Resistance
• A lighter object will meet
terminal speed sooner than a
heavier object.
• A heavier object will continue to
accelerate until it reaches a
higher terminal speed.
Chapter 5 Key Terms
Air resistance
Fluid
Free-body
diagram
Inversely
Newton’s
Second Law
Pascal
Pressure
Terminal speed
Terminal
velocity