Newton’s Laws
Targeted Skills for Newton’s Laws (Lecture
ONLY)
1. Identify and apply Newton’s Laws of Motion to
a variety of qualitative and quantitative
problems.
2. Identify: Gravitational Forces (Fg), Tension
Forces (FT) Normal Forces (FN) and Frictional
Forces (Ff).
3. Draw free body diagrams (FBD).
4. Analyze position versus time, velocity versus
time and acceleration versus time graphs for
regions of zero and non-zero net force.
5. Solve dynamics problems.
Newton’s First Law of
Motion
Describe the motion of an
arbitrary object setting in the
room.
Define Mass/Inertia
Inertia – A measure of a bodies resistance to a
change in motion.
Mass = Inertia
Mass – A measure of a bodies resistance to a
change in motion
Newton’s First Law of Motion
Describe the motion of an arbitrary object sitting
in the room.
What is required to change an object’s motion?
An UNBALANCED Force
Define Newton’s First Law of Motion
(Also known as Law of Inertia)
An object will remain in a state of constant motion
unless acted upon by an unbalanced force.
Newton’s Second Law of Motion
What is required to change an object’s motion?
An UNBALANCED Force
UNBALANCED Force = NET Force
NET Force = F (Sum of forces)
What results if an unbalanced force is applied to
an object?
ACCELERATION
Define Newton’s Second Law of Motion
The acceleration of an object is proportional to
the net force applied to the object and inversely
proportional to the object’s mass.
Newton’s Second Law of Motion
Equation of Newton’s Second Law of Motion
F = ma
Questions
How do we know if we have an unbalanced
force?
If there is an unbalanced force, in what direction
is it acting?
Answer
FREE-BODY DIAGRAM
A diagram of the object involved in a problem and
the forces exerted on the object.
Free-Body Diagram Construction
Horizontal / Vertical Scenarios
A jet plane is gliding at a constant elevation at a
constant velocity. Draw the Free-Body Diagram
of the forces acting on the plane.
NO air resistance.
A jet plane is flying at a constant elevation at a
constant velocity. Draw the Free-Body Diagram
of the forces acting on the plane.
Consider Air Resistance.
Free Body Diagram Construction
Horizontal / Vertical Scenarios
A jet plane is flying at a constant elevation with an
increasing velocity. Draw the Free-Body Diagram
of the forces acting on the plane.
Consider Air Resistance.
A jet plane is flying at a constant elevation with a
decreasing velocity. Draw the Free-Body
Diagram of the forces acting on the plane.
Consider Air Resistance.
Free Body Diagram Construction
Rules
1. Draw an arrow representing the weight of the
object.
2. Label the arrow Fg.
3. Draw additional arrows in the appropriate
directions to represent any forces acting
on the object. The length of the arrows
should be proportional to the quantity of
the force.
Free Body Diagram Construction
Rules
5. Label arrows with appropriate names, e.g.:
• Force of Gravity, Fg
• Tension, FT
• Normal, FN
• Friction, Ff
6. Remember, ONLY the arrows constitute the
free body diagram.
Free Body Diagram Worksheet
Example Problem 1
Two forces are applied to a 10 kg block. Calculate the net force
block on the block if F1 equals 15 N and F2 equals 30 N.
F2
F1
F = 30N –15 N
F = 15 N to right
What is the block’s acceleration?
F = ma
10 kg
Example Problem 1
Two forces are applied to a 10 kg block. Calculate the net force
block on the block if F1 equals 15 N and F2 equals 30 N.
F2
F1
F = 15 N to right
What is the block’s speed after 4
seconds if it was initially at rest?
G: F = 15 N to right
U:
vf = ________
E:
F=ma
S:
S:
10 kg
Example Problem 2
Fred and Wilma push a stalled car at constant velocity along
level ground. If Fred and Wilma push to the right with 395 N
and 275 N respectively, what is the magnitude of the opposing
force? Identify the opposing force.
F = 395N + 275 N + f = 0 N
Identify the opposing force.
G: constant velocity--acceleration = 0
U:
Ff = ______
E:
F=ma
S:
S:
Example #3
A dirt buggy has a mass of 575 kg. The buggy
uniformly accelerates from rest for 4 seconds and
travels 35 meters. What’s the buggy’s
acceleration?
G: m = 575 kg
vi = 0
U:
E:
S:
S:
t=4s
d = 35 m
acceleration
d = vit + ½at2
See Overhead
See Overhead
Example #3
A dirt buggy has a mass of 575 kg. The buggy
uniform accelerates from rest for 4 seconds and
travels 35 meters. How fast is the buggy traveling
after accelerating for 4 seconds?
G: m = 575 kg
vi = 0
U:
E:
S:
S:
t=4s
d= 35 m
velocity
vi = vf + at or vf2 = vi2 + 2ad
See Overhead
See Overhead
Example #3
A dirt buggy has a mass of 575 kg. The buggy
uniform accelerates from rest for 4 seconds and
travels 35 meters. What net force is applied to the
buggy?
G: m = 575 kg
vo = 0
U:
E:
S:
S:
t=4s
d = 35 m
net force
F = ma
See Overhead
See Overhead
Example #4
The maximum force a grocery sack can withstand
and not rip is 250N. If 20 kg of groceries are
lifted from the floor to the table with an
acceleration of 5 m/s, will the sack hold? if F1
equals 15 N and F2 equals 30 N.
G: m = 20 kg
a = 5 m/s2
F
max
U:
E:
S:
S:
= 250 N
F2= 30 N
F=______
F = ma + mg
See Overhead
See Overhead
Newton’s Third Law
Definition of Newton’s Third Law of Motion
When two bodies interact, the forces on the
bodies from each other are always equal in
magnitude and opposite in direction. These are
referred to as Action-Reaction pairs of forces.
Horse-Cart Problem
Draw ALL the forces acting on the horse, cart and
roadway.
Newton’s Third Law
Identify action-reaction pairs of forces.
Explain how the horse can move.
Newton’s Third Law
Explain how the horse-cart can move.
Behavior of Forces
Internal Forces – Come in pairs (action-reaction),
cancel one another, and can NOT accelerate an
object.
External (applied) Force – Individual forces which
may accelerate an object (F > 0).
All type of forces (Fg, FN, FT, Ff) can behave as
internal and external forces.