Download Newton`s 2nd Law

Forces
Chapter 6.1
What You Already Learned
• Velocity is the ratio of the change in
position of an object over a period of
time.
• Acceleration describes a change in
velocity of an object.
Acceleration
• What may cause an object to
experience a change in velocity?
–
•A
is a
or
.
–A
may cause an object to speed up.
–A
may cause an object to slow down.
–A
may also cause an object to
change direction.
Types of Forces
•
•
•
•
•
Force
• Force is a
quantity, therefore it has
both
and
.
• The SI unit for force is the
.
– The
1N =
–1
accelerate a
–1
sized apple.
is a derived unit where:
is the amount of force required to
mass
.
is about the weight of a medium
Contact vs. Long Range
• Some forces act over distances while
others act only when two objects are in
contact with one another.
–
exist when two
objects are in contact with one another.
– Long-range (
) forces act over
distances without a need for
.
forces
and
are long-range forces.
The System and Environment
• The object of interest is called the
.
• The area around the object is called the
.
FN
Fg
Diagram
FN
FN = Force of
Desk on Book
System
=
Fg
and the
Fg = Force of
Gravity on Book
diagrams provide a means by which all the
acting on a system can be summarized and accounted for,
determined.
Newton’s
Law
• Newton determined that the
an object is
the
the object.
applied to move it and
to the
a
Where F = Force in Newtons
m=
of the object
a=
of the object
of
to
of
Newton’s 2nd Law and Force vs.
Acceleration
• How does acceleration relate to force?
– As the force on an object
, the
rate of acceleration will
.
– i.e. Bigger
= Bigger
m2
m1
Acceleration
Note: The slope of the line
in a F vs. a graph will
of the
___ > ___ equal the
object.
The greater the slope, the
greater the
Newton’s 2nd Law and
velocity vs. time (constant force)
• Newton’s 2nd Law indicates that a constant
force applied to an object will cause the
speed to change at a
• i.e.
is
.
Note: Remember from
kinematics that the slope
of the line is equal to
.
Time
Newton’s 2nd Law
/ Different Mass
• If the applied force to a system is held constant and
the mass is increased from m1 to m2, what would a
velocity vs. time graph look like?
m1
Force is
Constant
m2
___ > ___
Time
=
Acceleration.
Newton’s 2nd Law
/ Different Force
• If the mass is held constant and the force applied to
a system is increased from F1 to F2, what would a
velocity vs. time graph look like?
F2
Mass is
Constant
F1
___ > ___
Time
=
Acceleration.
Example: What is the rate of
acceleration?
• Two people are pushing a stalled car. The mass of
the car is 1850 kg. One person applies a 275 N force
while the other applies a 395 N force. A third force
of 560 N acts in in the opposite direction compared
to the two people. What is the acceleration of the
car?
Diagram the problem
Fnet =
State the Known and Unknowns
• What is known?
– Mass (m) =
kg
– Force of person #1 (N) =
– Force of person#2 (N) =
– Opposing Force (N) =
• What is not known?
– Acceleration (a) = ?
N
N
N
Perform Calculations
• Fnet =
Where:
o Fnet =
• Substitute for Fnet and solve for a:
o
=
+
oa=
oa=
oa=
o If there was no opposing force, how would the rate
.
of acceleration change?
o What do you think is the source of the opposing
.
force?
Key Ideas
• Force is a
quantity that is a measure of the
magnitude of a push or pull in Newtons.
• Forces exist as
or
.
• Use
to represent
forces when problem solving.
• Newton’s 2nd Law of Motion states that the rate of
acceleration of an object is
to the
applied and
to
its mass.
– A constant force applied to an object will cause it to accelerate
at a
.
– As force
,
increases.
– As mass
,
decreases.