CHAPTER 4 FORCES and the LAWS of
MOTION
THE NORMAL FORCE
a force that acts on a surface in a direction perpendicular to the
surface
Fn
THE NORMAL FORCE
the television is in equilibrium, so the normal force from the table must be equal in magnitude and opposite in direction to
the gravitational force exerted on the television.
Fn
THE NORMAL FORCEFn is ALWAYS perpendicular to the surface,
but NOT ALWAYS
opposite to the FgFn = mg ∙ cos
The Net ForceIs the vector sum of all forces acting on the body
• 4 forces acting on the car
• Add them as vectors
• The resultant is then the NET
FORCE• If the NET FORCE
is 0, then you know that the car
is travelling at constant velocity
(zero acceleration)
Free Body Diagrams (FBDs)
are diagrams used to show the relative magnitude and direction of all forces
acting upon an object in a given situation.
Free Body Diagrams (FBDs)
• 4 forces acting on this object, but there MIGHT be more or less forces
• Represent the object as a SQUARE or a CIRCLE• You MUST show all the forces acting on the object
Free Body Diagrams (FBDs)A puck is sliding on a
sheet of ice to the right
How many forces are acting on the puck?
Label all of them!
Fapp
Fgravity
Ffriction
FA
Free Body Diagrams (FBDs)A puck is sliding on a
sheet of ice to the right
Fapp
Fgravity
Ffriction
FA
If the object is moving, you MUST show
the direction of acceleration
Free Body Diagrams (FBDs)A puck is sliding on a
sheet of ice to the right
Fapp
Fgravity
Ffriction
FA
If the object is moving, you MUST show
the direction of acceleration
a
Interpreting FBD (Quantitative = with numbers)
NET FORCE??
Fapp
Fgravity
Ffriction
FAa
Interpreting FBD (Quantitative = with numbers)
NET FORCE??
25 N
40 N
10 N
40 N a Fnet = 0 N in
vertical direction
Fnet = + 15 N in
horizontal direction
Interpreting FBD (Quantitative = with numbers)
100 N
47 N
100 N
47 N a Fnet = 0 N in
vertical direction
Fnet = 0 N in
horizontal direction
If the Net Force is 0 (Fnet = 0 N), the object is in EQUILIBRIUM
4.1 The Newton’s FIRST Law:Tells us what happens when all existing forces acting on a body
are balancedIf the Net Force is 0 (Fnet = 0 N), the
object is in EQUILIBRIUM
100N
47 N
100 N
47 N If the forces acting on a body are
balanced, then the
acceleration of the body will be 0 m/s2
4.2 The Newton’s Second Law:
Tells us what happens when an unbalanced force acts on a
bodyIf the Net Force is NOT 0 (Fnet ≠ 0 N), the
object is NOT in EQUILIBRIUM
200N
47 N
100 N
47 N aIf an
UNBALENCED force acts on a body,
the body will ACCELARAT
E
4.2 The Newton’s Second Law:
Tells us what happens when an unbalanced force acts on a
bodyThe rate at which the body will
ACCELERATE depends DIRECTLY on the UNBALANCED FORCE (Fnet) and INVERSELY on the
mass of the body. ∝a ∝ Fnet
a ∝ 1/m
4.2 The Newton’s Second Law:
Tells us what happens when an unbalanced force acts on a
bodyThe rate at which the body will
ACCELERATE depends DIRECTLY on the UNBALANCED FORCE (Fnet) and INVERSELY on the
mass of the body. ∝
Fnet = a · m
a = Fnet / m
First Law vs. Second Law
A 45.0 N block is being pushed along a floor, where the coefficient of kinetic friction is 0.333. If a horizontal force of 25.0 N is applied, at what rate will
the block accelerate? 1. FBD2. Known?3. Unknown
?4. Formula(s
)
Fg = 45.0 N
µk = 0.333
Fapp = 25.0 N
Fk = ???
Fapp = 25.0 N
Fg = 45.0 N
Fk = ???
FN = ???
m = Fg/g
= 45.0/9.81
= 4.59 kg
Fnet = a · m
Fg = m · g
Fk = µk · FN
A 45.0 N block is being pushed along a floor, where the coefficient of kinetic friction is 0.333. If a horizontal force of 25.0 N is applied, at what rate will
the block accelerate? 1. FBD2. Known?3. Unknown
?4. Formula(s
)
ClassworkPage: 112Problems: all problems
22
QUIZOn Monday: 15 minutes
Problems: from 4.1 and 4.2
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