Forces and Motion

1
Forces and Motion
2
Force

• FORCE is a push or a pull applied to an object
  that will cause it to start moving, stop moving
  or change its speed or direction
• Demonstration

3
Force

• Force Mass x Acceleration
• F MA
• Force is measured in Newtons (N) which is one
  kilogram meter per second squared
• N kg x m/s2

4
Newtons First Law (law of inertia)

• MASS is the measure of the amount of matter in an
  object
• measured in grams (g)
• or kilograms (kg)

5
Newtons First Law (law of inertia)

• WEIGHT is a measure of the force of gravity on
  the mass of an object
• measured in Newtons (N)

6
Force

• But weight, whats my mass?
• Please do not confuse the two.
• Weight is determined by the acceleration due to
  gravity.
• If you were on another planet that had less
  gravity, you would weigh less.

7
NET FORCE

• In order for motion to occur, the net force must
  be gt0

10 N

20 N
10 N

10 N
10 N
0 N

10 N
20 N
10 N
8
THE EQUILIBRIUM RULE
Scales pushing up
Examples of Mechanical Equilibrium
Normal up
Weight down

• Computer setting on a table
• Weighing yourself on a set of scales
• Hanging from a tree
• Car parked on an incline

Tree pulling up
Weight down
Normal
Friction

Weight down
Weight down
9
The Equilibrium Rule

• SF0

10
SUPPORT FORCE

• In the first example of mechanical equilibrium
  the table supplied a force upward that was called
  the normal force. It is a support force.
• Consider the second example of mechanical
  equilibrium. The scales supply a support force
  on the man.

11
EQUILIBRIUM OF MOVING THINGS

• Equilibrium is a state of no change.
• If an object moves in a straight line with no
  change in speed or direction, it is in
  equilibrium.

Examples Driving at constant velocity
Normal up
Air resistance
Air Resistance
Force from road
Weight down
Terminal velocity in parachuting
Weight down
12
What do you weigh?

• Weight on Other Planets

13
Force Problems

• Lets start with an easy one, your weight.
• Remember that weight is relative your mass
  isnt changing (the amount of matter in you) but
  you weigh different amounts because of gravity
• Gravitys acceleration is 9.8m/s2
• On earth you take your weight to be what it is

14
Force Problems

• If you lived on another planet, such as mars for
  example, the acceleration due to gravity is
  3.8m/s2
• In order to find out weight, we use the following
  formula
• w m x g

15
Force Problems

• Since gravity is a force (pulling you towards the
  center of the planet) this is technically a force
  problem
• My weight on earth is 185lbs, or 84kg (just
  divide your weight by 2.2)
• That means that if we stick my weight in and we
  know the acceleration due to gravity here on
  earth, we can find out my mass

16
Force Problems

• w m x g
• 84kg x m/s2 m x 9.8m/s2
• M 8.57kg
• Kg x m/s2.. Thats also called a Newton!
• So my mass is 8.57kg.

17
Force Problems

• But what if we were on another planet?
• Well, we use w m x g
• W 8.57kg x 3.8m/s2
• W 32.57N
• As a reminder, weight is m x g, so it equals a kg
  m/s2, or a N.
• Mass is measured in kg.

18
Force Problems

• Ok, now you try one
• What would be your weight on Jupiter, where
  gravity is 22.88m/s2?
• What would be your weight on the sun, 274.4m/s2?
  Thats assuming you could stand on it

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And now for something completely different

• The Galaxy Song

20
Force Problems

• Ok, lets move on to earthly stuff.
• We remember that f ma
• What would be the force exerted by a truck with a
  mass of 1818kg accelerating at 15m/s2?
• f ma
• F 1818kg x 15m/s2
• F 750N

21
Force Problems

• If you accelerate a rocket with a mass of 300kg
  at Tabers face at 500m/s2 with what amount of
  force will it hit him?
• F ma
• F 300kg x 500m/s2
• F 150,000

22
More Practice

• Troy Polamalu, with a mass of 115kg, hits Adrian
  Peterson with a force of 2300N. With what
  acceleration does Troy hit Adrian? What force
  does Adrian exert on Troy?

• F ma
• 2300N 115kg x a
• A 2300N / 115kg
• A 20m/s

23
One More

• A 20g sparrow mistakes a pane of glass for air
  and slams into a window with a force of 2N. What
  is the birds acceleration?
• F ma
• 2N .02kg x a
• A 100m/s2 or 10gs!!

24
Oh yeah, one more.

• Suppose your car is parked on an incline of 10
  degrees. If the parking brake lets go and your
  car starts rolling, with what force are you going
  down the hill? What is your force on the ground?
  Assume the car weighs 1500kg.

25
Friction

• FRICTION is the force that acts in the opposite
  direction of the motion of the object

26
Types of Friction

• Static Friction Friction due to gravity when an
  object is at rest.
• Demonstration
• Sliding Friction Friction while an object is at
  motion.
• Example
• Rolling Friction Similar to sliding friction,
  but the object is on wheels or castors to reduce
  the sliding friction.
• Fluid Friction Friction through water or air
• Terminal Velocity

27
Types of Friction
28
Sliding Friction

• Ffriction µFnormal
• µ the coefficient of sliding friction (has no
  units)
• product of the friction b/w materials and amount
  of force
• 1. Ben is walking through the school cafeteria
  but does not realize that the person in front of
  him has just spilled his glass of chocolate milk.
  As Ben, who weighs 420 N, steps in the milk, the
  coefficient of sliding friction between Ben and
  the floor is suddenly reduced to 0.040. What is
  the sliding force of friction between Ben and the
  slippery floor?

29
Friction

• While redecorating her apartment, Kelly slowly
  pushes an 82 kg china cabinet across the wooden
  dining room floor, which resists motion with a
  force of 320 N. What is the coefficient of
  sliding friction between the china cabinet and
  the floor?
• 3. A rightward force is applied to a 10-kg
  object to move it across a rough surface at
  constant velocity. The coefficient of friction,
  µ, between the object and the surface is 0.2. Use
  the diagram to determine the gravitational force,
  normal force, applied force, frictional force,
  and net force.
• (Neglect air resistance.)

30
Terminal Velocity
31
Projectile Motion

• What is a projectile? Throw ball
• Projectiles near the surface of Earth follow a
  curved path
• This path is relatively simple when viewed from
  its horizontal and vertical component separately
• The vertical component is like the free fall
  motion we already covered
• The horizontal component is completely
  independent of the vertical component (roll ball)
• These two independent variables combined make a
  curved path!

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Projectile Motion
33
Projectile Motion
No Gravity
With Gravity
34
Projectile Motion
35
Horizontally Launched Projectile(initial speed
(vx) 25 m//s)

• Time Horizontal Displacement (x)
• 0s 0m
• 1s 25m
• 2s 50m
• 3s 75m
• 4s 100m
• 5s 125m
• Ts vxt

• Vertical Displacement (y)
• 0m
• 25m
• 50m
• 75m
• 100m
• 125m
• ½ gt2

36
Horizontally Launched Projectiles

• What will hit the ground first, a projectile
  launched horizontally, a projectile dropped
  straight down, or a project fired up?

37
The Plane and the Package
38
Projectile Motion

• Remember that nothing is accelerating the
  projectile after it leaves
• The only thing accelerating the projectile after
  launch is gravity
• The two vectors can be separated into the
  velocity at launch and the acceleration of gravity

39
Truck and Ball

• Imagine a pickup truck moving with a constant
  speed along a city street. In the course of its
  motion, a ball is projected straight upwards by a
  launcher located in the bed of the truck. Imagine
  as well that the ball does not encounter a
  significant amount of air resistance. What will
  be the path of the ball and where will it be
  located with respect to the pickup truck?

40
Fast-Moving ProjectilesSatellites

• What if a ball were thrown so fast that the
  curvature of Earth came into play?
• If the ball was thrown fast enough to exactly
  match the curvature of Earth, it would go into
  orbit
• Satellite a projectile moving fast enough to
  fall around Earth rather than into it (v 8
  km/s, or 18,000 mi/h)
• Due to air resistance, we launch our satellites
  into higher orbits so they will not burn up

41
Satellites
Launch Speed less than 8000 m/s Projectile falls to Earth                                               Launch Speed less than 8000 m/s Projectile falls to Earth                                              
Launch Speed equal to 8000 m/s Projectile orbits Earth - Circular Path                                               Launch Speed greater than 8000 m/s Projectile orbits Earth - Elliptical Path                                            
42
ARISTOTLE ON MOTION

• Aristotle attempted to understand motion by
  classification
• Two Classes
• Natural and Violent

43
Natural Motion

• Natural motion depended on nature of the object.
• Examples
• A rocks falls because it is heavy, a cloud floats
  because its light
• The falling speed of an object was supposed to be
  proportional to its weight.

44
Natural Motion

• Natural motion could be circular (perfect objects
  in perfect motion with no end).

45
Violent Motion

• Pushing or pulling forces imposed motion.
• Some motions were difficult to understand.
• Example the flight of an arrow
• There was a normal state of rest except for
  celestial bodies.

46
Aristotle

• Aristotle was unquestioned for 2000 years.
• Most thought that the Earth was the center of
  everything for it was in its normal state.
• No one could imagine a force that could move it.

47
GALILEO AND THE LEANING TOWER

• 17th Century scientist who supported Copernicus.
• He refuted many of Aristotle's ideas.
• Worked on falling object problem - used
  experiment.

48
GALILEO'S INCLINED PLANES

• Knocked down Aristotle's push or pull ideas.
• Rest was not a natural state.
• The concept of inertia was introduced.
• Galileo is sometimes referred to as the
• Father of Experimentation.

49
NEWTONS FIRST LAW OF MOTION

• Newton finished the overthrow of Aristotelian
  ideas.
• Law 1 (Law of Inertia)
• An object at rest will stay at rest and an object
  in motion will stay in motion unless acted upon
  by an outside force.

50
Newtons First Law (law of inertia)

• INERTIA is a property of an object that describes
  how hard it is to change the motion of the
  object
• More mass more inertia
• FMA

51
Newtons Second LawForce Causes Acceleration

• In order to make an object at rest move, it must
  accelerate
• Suppose you hit a hockey puck
• as it is struck it experiences acceleration, but
  as it travels off at constant velocity (assuming
  no friction) the puck is not accelerating
• if the puck is struck again, then it accelerates
  again the force the puck is hit with causes the
  acceleration
• Acceleration depends on net force
• to increase accelerationincrease net force
• double accelerationdouble the force
• Force Acceleration
• directly proportional

52
Newtons Second Law

• Acceleration depends on mass
• to decrease accelerationincrease mass
• to increase accelerationdecrease mass
• double the mass ½ the acceleration
• Acceleration 1/mass
• inversely proportional

53
Newtons Second Law

• Newton was the first to realize that acceleration
  produced when something is moved is determined by
  two things
• how hard or fast the object is pushed
• the mass of the object
• Newtons 2nd Law
• The acceleration of an object is directly
  proportional to the net force acting on the
  object and is inversely proportional to the
  objects mass

54
Newtons Second Law

• Second Law Video

55
Newtons Second Law

• a F/m or F ma
• Robert and Laura are studying across from each
  other at a wide table. Laura slides a 2.2 kg
  book toward Robert. If the net force acting on
  the book is 1.6 N to the right, what is the
  books acceleration?
• A F/m
• 1.6N / 2.2kg
• .73m/s2

56
Newtons Second Law

• 2. An applied force of 50 N is used to
  accelerate an object to the right across a
  frictional surface. The object encounters 10 N of
  friction. Use the diagram to determine the normal
  force, the net force, the mass, and the
  acceleration of the object. (Neglect air
  resistance.)
• 3. Rose is sledding down an ice-covered hill
  inclined at an angle of 15.0 with the
  horizontal. If Rose and the sled have a combined
  mass of 54.0 kg, what is the force pulling them
  down the hill?

57
Newtons 2nd Law Kinematics

• A 4.60 kg sled is pulled across a smooth ice
  surface. The force acting on the sled is of
  magnitude 6.20 N and points in a direction 35.0
  above the horizontal. If the sled starts at
  rest, what is its velocity after being pulled for
  1.15 s?
• V 1.265 m/s
• The fire alarm goes off, and a 97 kg fireman
  slides 3.0 m down a pole to the ground floor.
  Suppose the fireman starts from rest, slides with
  a constant acceleration, and reaches the ground
  floor in 1.2 s. What was the force exerted by
  the pole on the fireman?
• F 201.76N

58
Sample Problems

• Which exerts a greater force on a table a 1.7kg
  physics book lying flat or a 1.7kg physics book
  standing on end? Which applies a greater
  pressure? If each book measures .26m x .210m x
  .04m, calculate the pressure for both.
• Same
• Standing
• Flat 311N/m2 Standing 2.0x103 N/m2

59
Sample Problems

• A 1250kg slippery hippo slides down a mud-covered
  hill inclined at an angle of 18 degrees to the
  horizontal. If the coefficient of sliding
  friction between the hippo and the mud is .09
  what force of friction impedes the hippo? If the
  hill were steeper, how would this affect the
  coefficient of friction?
• 1068.75N

60
Sample Problems

• Mr. Micek loves to ride his motorcycle. Mr.
  Micek and his motorcycle have a combined mass of
  518kg. With what force must each tire push down
  on the ground to hold the bike up? If the contact
  pattern of the tire is 15cm x 35cm (for each
  wheel), what pressure do they exert? If Mr. Micek
  accelerates at 8.8m/s2 what force will he exert?
  If he parks his bike on a hill at an angle of 25
  degrees what must the force due to friction be to
  keep it there?
• 2590N 49.3kPa 4558.4N 2175.6N