Title: Forces and Newton
1- Forces and Newtons Laws of Motion
2- Force
- A push or a pull.
- Force is not a thing in itself, but rather an
interaction between two objects.
3Examples
- A Flag being blown
- by the wind
- A jet engine propelling an airplane
4Fact or Fiction?
- Motion at a constant velocity requires a force.
- If you apply a constant force to an object, it
will move with a constant velocity. - If no forces act on it, a moving object will
eventually stop. - Inertia is a force.
5Forces
- Push or a pull
- Vector quantity
- Contact forces- arise from physical contact with
each other - Field (Noncontact) forces- force exerted even
when not in direct contact (ex. Gravity,
electrical force, magnetic forces)
6Weight VS Mass
http//www.pioneernet.net/curtis/wile_e/inline/wil
e_gravity.gif
7Mass
- the amount of matter in an object
- a constant property
- a measure of the inertia
- measured in kilograms (kg)
8Weight
- the force upon an object due to gravity
- weight mass ? accel. due to gravity
- Fw mg
- measured in Newtons (N)
9- The weight of a 10 kg brick is...
- A) 98 N
- B) 10 kg
- C) 9.8 kg
- D) 10 N
- E) 98 kg
10Mass and Weight
- On the Moon, the force of gravity is only 1/6 as
strong as on the Earth. - In space you are practically weightless but your
mass remains unchanged. - Your mass does not depend on where your are.
- e.g. Earth, Moon, or space
11Tension
- Tension (T)- force that acts through a solid
object such as a rope or chain directed along
the rope and pulls equally on the objects on
either end. - .
12Normal Force
- Normal Force (Fn)- contact force that always acts
perpendicular to the surfaces that are touching
13Free-Body Diagram
- Free-body diagrams are drawings used to show the
relative magnitude and direction of all forces
acting upon an object in a given situation.
14Basic Force Diagrams
A Car on a Level Surface All forces on the car
are vertical, so no horizontal force can be
generated.
A Car on a Banked Turn The normal force on the
car due to the road is no longer vertical, so a
component of the normal force acts in the
horizontal direction.
Normal force (N) - contact pressure that pushes
surfaces together. It is easier to drag a light
chair across a room than a heavy table. This is
because the weight of the table exerts a higher
normal force.
15A book is at rest on a table
16A girl is suspended motionless from a bar which
hangs from the ceiling by two ropes.
17An egg is free-falling from a nest in a tree.
Neglect air resistance.
18A flying squirrel is gliding (no wing flaps) from
a tree to the ground at constant velocity.
Consider air resistance.
19A rightward force is applied to a book in order
to move it across a desk with a rightward
acceleration. Consider frictional forces. Neglect
air resistance.
20Real Life Application
21More real life
http//www.ux1.eiu.edu/cfadd/1150/04Nwtn/appl.htm
l
22Another Force Diagram
The chandelier is separated from the chain for
interpretation purposes only!
http//www.ux1.eiu.edu/cfadd/1150/04Nwtn/appl.htm
l
23Fish in an Elevator
24Upward Acceleration
- Fnet FT FW or
- Fnet FT mg
- Fnet FT - mg ma
- FT ma mg
- FT m (g a)
25Downward Acceleration
- Fnet FT - Fw
- or
- Fnet FT mg
- Fnet FT - mg m ( - a )
- FT mg - ma
- FT m (g - a)
26Roller Coaster
http//science.howstuffworks.com/roller-coaster-sa
fety-harness-broke.htm
27Bring in the angles-a bit more complex ?
http//www.ux1.eiu.edu/cfadd/1150/04Nwtn/appl.htm
l
28Forces are Vectors too!
- Consider the tow truck at the right. If the
tensional force in the cable is 1000 N and if the
cable makes a 60-degree angle with the
horizontal, then what is the vertical component
of force which lifts the car off the ground? - 866 N.
29Box on an Incline
http//img.sparknotes.com/figures/1/174f9bfb530794
0e1bb1c2ddac88fd8d/problem1_5.gif
30Net Force
Balanced No Net Force Fnet 0 N
- Determined by combining ALL forces acting on an
object. - Zero net force zero acceleration
- If there is a net force, there will be an
acceleration.
Unbalanced Net Force Fnet ? 0 N
http//4.bp.blogspot.com/_fDHmeCjB-bk/SS4hRNbLwaI/
AAAAAAAAAHw/gx0p9HIDS1o/s1600-h/12.gif
31Balanced Forces (zero net)
32Unbalanced Forces (non-zero net)
Imagine pushing your text book so that it would
slide across the desk
33Equilibrium
- Balance
- In regards to motion an object has constant
velocity if it is in equilibrium. - NO acceleration.
- Types
- Static Equilibrium
- Dynamic Equilibrium
34Equilibrium- net force is equal to zero
35Static Equilibrium
- Velocity is zero
- Examples
Scales pushing up
Weighing yourself on a set of scales
Weight down
Car parked on an incline
Normal
Friction
Weight down
36- Dynamic Equilibrium
- Velocity is nonzero and constant
- Examples
- Driving at constant velocity
Normal up
Friction
Force from road
Air resistance
Weight down
Terminal velocity when parachuting
Weight down
37Friction
- The force that opposes the motion between two
surfaces that are in contact. - Friction is the "evil monster" of all motion.
Regardless of which direction something moves in,
friction pulls it the other way. - Move something left, friction pulls right. Move
something up, friction pulls down. - It appears as if nature has given us friction to
stop us from moving anything. - Friction is actually a force that appears when
there is relative motion between two objects. - Although two objects might look smooth,
microscopically, they are very rough and jagged.
38Friction
- A force that opposes motion. Friction acts in a
direction opposite to the object's direction in
motion. Without friction, the object would
continue to move at a constant speed forever - Static Friction when object is at rest
- Kinetic Friction when object is moving
- Sliding Friction when two surfaces
slide one over the other - Rolling Friction when two surfaces
slide one over the other
39Science Friction
- Friction (Ff)- contact force between two surfaces
that always opposes motion - 2 types
- Static friction (fs) keeps an object from
moving (must be overcome to move an object at
rest - Kinetic friction (fk) acts when an
object is moving - Kinetic friction is weaker than static friction
- Fklt Fs
40Friction is not always a bad thing!
- Walking! Your foot pushes backwards and friction
opposes it by pushing it forward
41Static (starting) Friction
- The force that opposes the start of the motion.
- Static means stationary ( not moving).
42Kinetic (sliding) Friction
- The force between surfaces in relative motion
- For the same object, why is the force of kinetic
friction less than the force of starting friction?
43Coefficient of Friction
- A constant that depends on the two surfaces in
contact - Ff ?FN
- ?(mu).coefficient of friction
Coefficient of Friction - Friction on an Incline
44Newtons First Law (1642-1727)
- The Law of Inertia
- A body remains at rest or moves in a straight
line at a constant speed unless acted upon by a
net force. - Objects do not accelerate unless a net force is
applied.
45Objects at Rest Remain at Rest
http//talesfromechocanyon.blogspot.com/2007_03_02
_archive.html
46Objects in Motion Remain Motion, unless
http//talesfromechocanyon.blogspot.com/2007_03_02
_archive.html
47Acted Upon by a NET Force
http//talesfromechocanyon.blogspot.com/2007_03_02
_archive.html
48Newtons 1st Law of Motion
http//quest.nasa.gov/space/teachers/rockets/image
s/hand_and_ball.gif
49Newtons Second Law
- The accel. of an object is directly proportional
to the net force acting on the object, and
inversely proportional to the mass of the object.
50Newtons Second law of Motion mathematically
Net Force (mass)(accel) Fnet ma
51NEWTON'S 2nd LAW OF MOTION
a
F
F
a
F
a
F
a
F
a
F
a
52When the acceleration is g we have Free Fall
m
2m
F
2F
53Terminal Velocity
Net Force
Acceleration g
Velocity 0 but motion is about to begin
mg
F
Acceleration lt g
v increasing downward
mg
F
Acceleration ltlt g
v still increasing downward just not as rapidly
as before
mg
F
Acceleration 0
Terminal velocity
mg
54Falling with Air Resistance
- Air resistance increases with speed and increased
cross-sectional area.
55Terminal Velocity
- When falling the force of air resistance becomes
large enough to balance the force of gravity. - At this instant in time, there is no net force
the object stops accelerating (see D below)
terminal velocity has been reached.
56- Example
- Light and heavy parachutists
57Practice
http//www.freewebs.com/mathphyedu/14.bmp
- A 0.02 kg bullet shot horizontally with a speed
of 340 m/s enters a 1.2 kg apple that is 8.5 cm
in diameter. If the bullet exits the apple with a
perceived speed of 320 m/s, how much force was
applied to the bullet by the apple? What was the
acceleration of the apple?
58Fact or Fiction??
- When you kick a can, the can kicks you back with
the same force? - When we jump the floor pushes us upward.
- It is possible to get yourself off the floor or
chair without an outside force being exerted on
your body
59Newtons Third Law
- Action-Reaction
- When one object exerts a force on another object,
the second object exerts a force of equal
strength in the opposite direction on the first
object.
Example of Newtons 3rd Law http//bp3.blogger.co
m/_vQA3QRUnk3M/RrgyfrRniPI/AAAAAAAAAMs/DYlhM7pDeI8
/s1600-h/newton
60Newtons Third Law of Motion
For every action, there is always an equal
(magnitude) and opposite (direction) reaction.
action or reaction refers to
force.
Action/Reaction forces do NOT act on the SAME
object!
61Reaction road pushes on tire
Action tire pushes on road
62Reaction gases push on rocket
Action rocket pushes on gases
63Action- Reaction Forces
- Do Action-Reaction forces cancel each other?
- No, they are acting on different objects. Forces
can only be added together when they are acting
on the same object.
http//quest.nasa.gov/space/teachers/rockets/princ
iples.html
64Action earth pulls on kid
Reaction kid pulls on earth
65Newtons 3rd Law of Motion
Action Force A guy is pushing a ball leftwards
Reaction Force the ball is pushing the guy
rightwards
66- Horse pulls on cartcart pulls on horse. How
does the cart ever move forward? - Use Newtons 2nd and 3rd laws of motion to
explain.
67Identify at least six pairs of action-reaction
force pairs in the following diagram
68Tug-a-war
- If Fido and Rover play tug-a war, how do the
pulls of the dogs compare? - If each dog pulls with 50 N of force, what is the
tension force in the middle of the rope (between
the dogs)?
69- While driving, Mrs. Ingle observed a bug striking
the windshield of her car. The bug hits the
windshield and the windshield hits the bug. Which
of the two forces is greater the force on the
bug or the force on the windshield? EXPLAIN!
70Newtons Cradle
http//www.celsum.com/newton.htm
71Putting Newton's Laws of Motion Together
- An unbalanced force must be exerted for a rocket
to lift off from a launch pad or for a craft in
space to change speed or direction (First Law). - The amount of thrust (force) produced by a rocket
engine will be determined by the rate at which
the mass of the rocket fuel burns and the speed
of the gas escaping the rocket (Second Law). - The reaction, or motion, of the rocket is equal
to and in the opposite direction of the action,
or thrust, from the engine (Third Law).
http//www.spacetoday.org/images/Rockets/ArianeRoc
kets/Ariane5LaunchArianespace.jpg
http//quest.nasa.gov/space/teachers/rockets/princ
iples.html