Download Newton`s Second Law

Chapter 4
Forces
Forces and Interaction
• Force – a “push or pull”
• Contact Force – you physically push on a
wall
• Long-range Force – like magnets or gravity
• Force – a vector quantity. Has what two
parts.
• SI Unit – the Newton abbreviated as “N”
4 Types Forces
• Gravitational forces
• The Earth pulls and holds the moon in orbit
• The moon pulls and causes tide changes
• Electromagnetic forces
• due to electric charges, both static and moving.
• Strong Nuclear Forces
• holds particles in the nucleus together. (Strongest
of the 4)
• Weak Nuclear Forces
• Radioactive decay
Newton’s 1st Law
• An object at rest will stay at rest unless a force acts on it.
• Here, the girl is at rest until acted on by the force imposed
by the cannonball.
Newton’s 1st Law
• An object at rest will stay at rest unless a force
acts on it.
• Here, the skateboard/log combination is at rest
until acted on by the force imposed by the
rocket.
Newton’s 1st Law
• An object in motion will continue in a straight line until
acted on by some “outside” force.
• Consider a baseball that is pitched to a batter. What
force causes the ball to change direction?
Hit
Newton’s 1st Law
• An object in motion will continue in a straight line until
acted on by some “outside” force.
• Consider the space shuttle turning while in space flight.
What produces the forces?
Newton’s 1st law
• An object in motion will continue in a straight line
until acted on by some “outside” force.
• Consider the space shuttle when landing. What
produces the forces?
Multimedia
• The car and the wall
• The motorcyclist
• The truck and the ladder
Newton’s 1st Law
Newton’s 2nd Law
• Acceleration is directly proportional to the
magnitude of the net force.
• Acceleration is inversely proportional to the
mass of the object.
Newton’s 2nd Law
• Acceleration depends on both mass and
the net force
Fnet
a
m
F  ma
Acceleration depends on net force
• A force of 10N accelerates the box
Acceleration depends on net force
• A force of 20N accelerates the box twice
as fast
Acceleration depends on mass
• A force of 10N accelerates the box
Acceleration depends on mass
• A force of 10N accelerates the smaller box
faster
Weight and Mass
Mass:
A measure of the amount of matter in an object has.
Symbol (m)
Unit (kg)
Weight:
The gravitational force exerted on a body
Symbol (W)
Unit (N Newton)
W  mg
Weight
A Motor cycle weights 2450N. What is its mass?
W  mg
m W / g
2450 N
m
9.8 sm2
m  250kg
Newton’s 2nd Law
Newton’s Second Law – Non-Equilibrium
Situation
• Two 50g masses are placed 40cm on either side of a fulcrum.
What is the Net force?
• If left mass slides 20cm right, what happens?
• If the left mass slides 20cm right and grows to 100g, what type
of situation is this?
Newton’s 3rd Law
Newton’s 3rd Law
• When one object exerts a force on a
second object, the second exerts a force
on the first that is equal in magnitude but
opposite in direction.
• For every action there is an equal and
opposite re-action.
• Action – Reaction force pair
Newton’s 3rd Law
Example
• A book rests on a table
• The force from the weight
of the book pushes down.
• The table provides a
supportive force up.
N  mg
• Normal force (N) is
perpendicular to the
surface
W  mg
Free-body diagrams
Draw the free-body diagram for a book is at
rest on a table top.
Free-body diagrams
A girl is suspended motionless from a bar which
hangs from the ceiling by two ropes. A free-body
diagram for this situation looks like this:
Ftension
Ftension
Fgravity
Free-body diagrams WS 5b #3
An egg is free-falling from a nest in a tree.
Neglect air resistance. A free-body diagram
for this situation looks like this:
Fgravity
A rightward force is applied to a book in order to
move it across a desk at constant velocity.
Consider frictional forces. Neglect air resistance. A
free-body diagram for this situation looks like this:
Fnormal
F friction
Fgravity
Fapp
An egg is free-falling from a nest in a tree.
Neglect air resistance. A free-body diagram
for this situation looks like this:
A college student rests a backpack upon his
shoulder. The pack is suspended motionless
by one strap from one shoulder. A free-body
diagram for this situation looks like this:
A skydiver is descending with a constant
velocity. Consider air resistance. A free-body
diagram for this situation looks like this:
A force is applied to the right to drag a sled
across loosely-packed snow with a rightward
acceleration. A free-boy diagram for this
situation looks like this
A football is moving upwards towards its
peak after having been booted by the
punter. A free-body diagram for this situation
looks like this:
A car is coasting to the right and slowing
down. A free-body diagram for this situation
looks like this:
Net Force
• If there is no movement
–The net force is zero
• If there is no acceleration
–The net force is zero
• If there is acceleration
–The net force is not zero.
Free Body diagrams
• If the net forces are zero, the FBD is
balanced
Examples of zero net force?
• A book on a desk
• A sky diver with a parachute open
• A bike moving at a constant speed.
Free Body diagrams
• If the net forces are NOT zero, the FBD is
NOT balanced
Examples of non zero net force
• A book sliding across a desk
• A sky diver falling without a parachute
Net force: The vector sum of all forces
5N
5N
5N
5N
10N





5N
5N
10N
10N
5N





10N
0N
15N
5N
5N
Equilibrium or Non-equilibrium
• When the helicopter is on
•
•
•
•
the ground, what state is
it in?
Why isn’t it moving?
In order to get it moving,
what would we need?
What state is it in when it
is hovering in place?
In order to get it to move,
what would we need?
Net Force – Equilibrium in Motion
• What forces are acting on the train while it is moving in a
straight line at a constant speed?
• Engine Force (FE) – Force applied to propel the train along the
tracks.
• Opposition Force (Fo) – friction between the tracks, wind
resistance, etc. that attempts to slow the train down.
• What is the acceleration of the train? Negative, Zero, or Positive.
• Is this an equilibrium ornon-equilibrium situation?

FO
FE
Net Force – Non-Equilibrium
• Describe what happens in the animation below.
• What forces were acting on the train?
• Engine Force (FE) – Force applied to propel the train along the tracks.
• Opposition Force (Fo) – friction between the tracks, wind resistance,
etc. that attempts to slow the train down.
• Which force was larger?
• What is the acceleration of the train? Negative, Zero, or Positive.
• Is this an equilibrium or non-equilibrium situation?

FO

FE
Net Force – Non-Equilibrium
• Describe what happens in the animation below.
• What forces were acting on the train?
• Opposition Force (Fo) – friction between the tracks, wind resistance,
etc. that attempts to slow the train down.
• What is the acceleration of the train? Negative, Zero, or Positive.
• Is this an equilibrium or non-equilibrium situation?

FO
Net Force
• If there is no movement
–The net force is zero
• If there is no acceleration
–The net force is zero
• If there is acceleration
–The net force is not zero.
Newton’s Second Law – Non-Equilibrium
  Situation
 

F  F1  F2  F3...  ma
• The result of a Non-Equilibrium situation is that the
body, will begin to move.
F1
F  F1  ma
F1
F2
F  F1  F2  ma
Newton’s laws review
• Newton’s First Law - An object remains at
rest, or in uniform motion in a straight line,
unless it is compelled to change by an externally
imposed force.
• Newton’s first law describes an Equilibrium
Situation.
• An Equilibrium Situation is one in which the
acceleration of a body is equal to zero.
Newton’s laws review
• Newton’s Second Law – If there is a non-zero
net force on a body, then it will accelerate.
• Newton’s Second Law describes a Nonequilibrium Situation.
• A Non-equilibrium Situation is one in which the
acceleration of a body is not equal to zero.
Newton’s laws review
• Newton’s Third Law - for every action force
there is an equal, but opposite, reaction force.
• Newton’s Third Law says forces must come
in pairs.
• Paired force internal to a system have a Net
Force of zero.
Weight and Mass
Mass:
A measure of the amount of matter in an object has.
Symbol (m)
Unit (kg)
Weight:
The gravitational force exerted on a body
Symbol (W)
Unit (N Newton)
W  mg
Weight
A Motor cycle weights 2450N. What is its mass?
W  mg
m W / g
2450 N
m
9.8 sm2
m  250kg
Newton’s Second Law – Non-Equilibrium
  Situation
 

F  F1  F2  F3...  ma
• Two 50g masses are placed 40cm on either side of a fulcrum.
What is the Net force?
• If left mass slides 20cm right, what happens?
• If the left mass slides 20cm right and grows to 100g, what type
of situation is this?
Friction
• Friction is a force like any other force
• Friction acts on materials that are in
contact with each other
• Friction slows down motion
• Forces due to friction are always in the
opposite direction of the motion.
Friction
•
The force of friction depends on
1) The force pushing down (weight)
2) The surface materials in contact with
each other
Friction
Ff
N
force of
Normal
friction
force
 coeffiecient of friction
Ff   N
Friction Example
• A horizontal force of 30N pushes a 12kg crate across
a floor at a constant velocity. Find the coefficient of
N
sliding friction.
Ff
Ff
F
a
Ff   N

N
30 N
W

12kg (9.8 sm2 )
Fnet  F  F
  .25
app
Fapp  Ff
f
Friction e.g.
• A 4kg block has a
coefficient of friction of
.22 .
• What is the force of
friction for the 4kg block?
• What is the Fnet?
• What is the acceleration?
Fnet  ma
a  2.8 sm2
Fapp=20N
4kg
Ff   FN
Ff  .22(4kg )(9.8 sm2 )
F f  8.6 N
Fnet  Fapp  Ff
Fnet  20 N  8.6 N  11.3N
N
Ff
Fa
W
With out friction
Galileo On Motion
With friction
With out friction
With out friction