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Force
Chapter 6
Force
•Any push or
pull exerted on
an object
System
•The object with
the force
applied
Environment
•The world
surrounding the
object
Contact Force
•A force that acts
on an object by
touching it
Contact Force
•A baseball bat
striking a ball
Long-range
Force
•A force that acts
on an object w/o
touching it
Long-range
Force
•The force of
gravity
Agent
•Whatever is
causing the force
Inertia
•The resistance to
change
(in motion)
Equilibrium
•When the net
forces acting on
an object = zero
Force Vector
Diagram
•A Diagram showing the
vectors of all forces
acting on an object.
Force Vector Diagram
Force of
table on
the ball
Weight
on table
Draw Force Vector
Diagrams of:
1)A book on a desk
2)A book being pushed
across the desk
3)A book falling
Newton’s
Laws of
Motion
Newton’s
st
1
Law
An object will remain
at rest or in constant
straight-line motion if
the net force acting
on it is zero
Newton’s
st
1
Law
The velocity is
constant and
acceleration is zero
when the net force on
an object is zero
Newton’s
nd
2
Law
The acceleration of an
object is directly
proportioned to the
net force applied to it
Newton’s
a=
nd
2
Law
Fnet
m
Newton’s
nd
2
Law
Fnet = ma
Newton’s
rd
3
Law
For every action,
there is an equal &
opposite reaction
Newton’s
rd
3
Law
FA on B =
-FB on A
Drill:
•Write out
Newton’s Laws
of Motion
Two horizontal forces of
23.5 N & 16.5 N are acting
in the same direction on a
2.0 kg object. Calculate:
1) net Force on the object
2) its acceleration
Two horizontal forces of
23.5 N & 16.5 N are
acting in opposite
directions on a
2.0 kg object. Calculate:
1) net force on the object
2) its acceleration
Forces of 4.0 N west &
3.0 N north are acting on a
2.0 kg object.
Calculate:
1) net Force on the object
2) its acceleration
Calculate the
acceleration of a 1500
g object falling
towards Earth when
the Fair friction is 11.7 N.
List Newton’s
Laws of
Motion
Types of Forces
Friction
Normal
Spring
Tension
Thrust
Weight
Friction (Ff)
• The contact force that acts to
oppose sliding motion
between surfaces
• Its direction is parallel &
opposite the direction of
sliding
Normal (FN)
•The contact force exerted
by a surface on an object
•Its direction is
perpendicular & away
from the surface
Spring (Fsp)
• A restoring force, or the push
or pull a spring exerts on an
object
• Its direction is opposite the
displacement of an object at
the end of a spring
Tension (FT)
• The pull exerted by a string,
rope, or cable when attached to a
body & pulled taut
• Its direction away from the
object & parallel to the string at
the point of attachment
Thrust (Fthrust)
• A general term for the force
that moves rockets, planes, etc
• Its direction is the same
direction as the acceleration of
the object barring any resistive
forces
Weight (Fg)
• Force due the gravitational
attraction between two objects
like an object & the Earth
• Its direction is straight down
towards the center of the Earth
Drill: Name &
describe the 6
types of forces
Weight (Fg)
Weight = Fg = mag = mg
Fg = W = mg
When an object is
launched, the only
forces acting upon it
are the forces gravity
& air friction.
No net force is
required to keep an
object in motion.
Frictional forces
oppose motion.
Inertia is not a force,
but the resistance to
the change in motion
or momentum.
Air exerts huge &
balanced frictional
forces on an object.
When in motion, the
net Ff of air is large.
Terminal Velocity
•The constant velocity that
is reached when the force
of air friction of a falling
object equals its weight
Friction (Ff)
Kinetic frictional force
Ff, kinetic
Static frictional force
Ff, static
Draw Vector Force
Diagrams of:
1) a skydiver gaining
downward velocity
2) a skydiver at terminal
velocity
Draw Vector Force
Diagrams of:
3) a rope pulling a ball up at
constant velocity
4) a rope accelerating a ball
upwards
An object’s weight on
Earth is 490 N. Calculate:
1) its mass
2) its weight in the moon
2
where gmoon = 1.60 m/s
An 500.0 g object on an
unknown planet has a
weight of 250 N.
Calculate the acceleration
caused by the planet’s
gravity.
Static Ff
•The force exerted on
one surface by
another when there is
no relative motion
Kinetic Ff
•The force exerted on
one surface by
another when in
relative motion
Forces of 5.0 N
Drill: west, 9.0 N east,
& 3.0 N north act
upon a 15 kg object.
Calculate its
acceleration
Forces acting on an object:
FN = -W
FA > Ff
FN
Ff
Fg or Weight
Fapplied
Net Force (Fnet)
• Summation of all forces
acting on an object
• Resultant vector of all the
forces
Net Force (Fnet)
Fnet = ma
Net Force (Fnet)
Fnet = FA + FB
+ FC + etc
Static Ff
Ff, static = msFN
m
m is proportionality
constant called the
frictional coefficient
Kinetic Ff
Ff, kinetic = mkFN
A 25 N force is required to
pull a 50.0 N sled down the
road at a constant speed.
Calculate the sliding
frictional coefficient
between the sled & the
road.
A person & a sled have a
total weight of 490 N. The
sliding frictional coefficient
between the sled & the snow
is 0.10. Calculate the force
required to pull the sled at
constant speed.
Drill: Calculate the
acceleration of the sled if
the applied force pulling
on the sled is 299 N.
W = 490 N
m = 0.10
Calculate the force
required to pull a 500.0
g block with an
2
acceleration of 3.0 m/s .
m = 0.50
Periodic Motion
•Repetitive or
vibrational motion
like that of a spring,
swing or pendulum
Simple Harmonic
Motion
• Periodic motion in which
the restoring force is
directly proportional to
the displacement
Period (T)
•The time required
to complete one
full cycle of motion
Amplitude
•Maximum
displacement from
the zero point or
equilibrium
Pendulum Motion
Formula
l
T = 2p ---ag
Calculate the
period of a
pendulum with a
length of 49 cm:
Drill: Calculate the
length of the pendulum
of a grandfather clock
whose period is equal
1.0 second: C HW
Fundamental Forces
•Gravitational
•Electromagnetic
•Strong Nuclear
•Weak Nuclear
Calculate the force
required to pull a 150 g
block at a constant
velocity of
180 km/hr.
m = 0.20
A 9.8 kN car went from
0 to 25 m/s in 5.0 s. mK
between car & road =
0.20. Calculate the force
applied by the engine of
the car.
Drill: Calculate the force
required to start a 2.0 kg
block & its acceleration
when moving.
ms = 0.20, mk = 0.10
Calculate the force
required to start a 2.0 kg
block & calculate its
acceleration when
moving.
ms = 0.20, mk = 0.10
A 6.0 kg ball is attached
by a rope over a pulley
to a 4.0 kg ball.
1) Draw the problem.
2) Calculate each ball’s
acceleration
A 6.0 kg ball is attached
by a longrope over a
pulley to a 4.0 kg ball.
1) Calculate air friction
at max velocity
A 150 g baseball, was hit
& came to rest in 4.0 s
after going 100.0 m.
Calculate: vi, a, & Ff on
the ball.
A 50.0 kg box falls off a
0.49 km cliff.
1) Calculate vi, vf, a, & t.
2) Calculate Ff at
terminal velocity
A 10.0 kg box falls off a
0.49 km cliff & hits the
ground in 20.0 s.
1) Calculate vf & a.
2) Calculate Ff if air
friction is included
Calculate the force
required to pull a 250
g block at a constant
velocity of
360 km/hr. m = 0.30
Drill: Calculate the
force required to
accelerate a 1500 g
block along the floor
2
at 3.0 m/s .
m = 0.25
A 65 kg boy & a 35 kg
girl are in a tug-of-war
on ice. The girl’s
acceleration is 13
2
cm/s . Calculate the
boy’s acceleration.
Calculate the apparent
weight of a 50.0 kg
person on a scale on
an elevator descending
2
at 2.0 m/s .
Calculate the
apparent weight of a
50.0 kg person on a
scale on an elevator
2
ascending at 2.0 m/s .
Drill: Calculate the
period of the
pendulum on Big Ben
which is 4.9 m long.
Calculate the force
required to accelerate
a 10.0 kg block
straight up at
2
25 cm/s .
Calculate the force
required to accelerate
a 50.0 kg block
straight up over a
2
pulley at 5.0 m/s .
Calculate the
acceleration of a system
of a 55.0 kg block tied
to a 45.0 kg block
hanging over a pulley.
Calculate the frictional
coefficient of a 100.0
kg block if a 150 N
force causes it to
2
accelerate at 50.0 cm/s .
Drill: Calculate the
frictional coefficient
of a 10.0 kg block if a
98 N force causes it
to slide at 30.0 cm/s.
A 5.0 N force
accelerates a 1000.0 g
2
block at 45.0 cm/s .
Calculate mK.
Calculate the
acceleration of a system
of a 200.0 kg cart on a
plane tied to a 50.0 kg
block hanging over a
pulley.