1. History of the Laws of Motion
Chapter 12 Motion

2. Aristotle ~350 B.C.
He believed the natural state for all objects was at rest.
He believed all motion was caused by a force. If the force on the object stopped, the object would eventually stop.

3. Aristotle was WRONG!!!
Many people still think along the lines of Aristotle.
Objects do NOT “want” to be at rest.
If we can remove friction, objects will not stop.
On Earth a completely frictionless surface is impossible.

4. Galileo ~1600
Galileo argued a moving object would move forever, without friction.
He also argued that objects naturally resist CHANGE in motion
he called this inertia

5. Inertia inertia- an object’s resistance to change in motion.
NOT just an object’s resistance to stopping
but also to slowing down and speeding up.
Inertia is measured by its mass.
The more mass the more an object will resist a change in motion.

6. Sir Isaac Newton
Developed and explained three laws to explain how objects move.
He also explained other things including gravity and calculus.

7. Newton’s 1st Law of Motion
An object at rest tends to stay at rest, and an object in motion tends to stay in motion in a straight line at a constant speed unless it is acted on by a net outside force.
~an object is going to keep on doing what it is already doing unless something comes along and pushes it.

8. Newton’s First Law
What makes an object speed up, slow down, or change directions?
Objects change their state of motion only when a net force is applied.
This principle is Newton’s first law.

9. Newton’s First Law, continued
Objects tend to maintain their state of motion.
Inertia is related to an object’s mass.
inertia: the tendency of an object to resist a change in motion unless an outside force acts on the object
Seat belts and car seats provide protection.
When a car comes to a stop, your seat belt and the friction between you and the seat provide the unbalanced backward force that is needed to bring you to a stop as the car stops.

10. Examples
How can a magician pull a table cloth out from under the plates?
The plates have inertia, they will not move unless a force is applied to them.

11. Newton’s Second Law of Motion

12. Newton’s Second Law
What determines how much an object speeds up or slows down?
Net force is equal to mass times acceleration. The unbalanced force on an object determines how much an object speeds up or slows down.
This principle is Newton’s second law.
net force = mass × acceleration, or F = ma
Force is measured in newtons (N): 1 N = 1 kg × 1 m/s2

13. Newton’s Second Law, continued
For equal forces, a larger mass accelerates less.

14. Newton’s Second Law, continued
For equal masses, a greater force produces a greater acceleration.

15. And it states…
The acceleration of an object will be in the same direction and directly proportional to the net force applied, and the acceleration will be inversely proportional to the mass of the object.
So the harder you push something the faster it accelerates.
And the more massive an object the harder it is to accelerate.

16. Back to mass volume and weight
Mass is the amount of matter in an object (its inertia)
Volume is the size
Weight is the force of gravity on your body (mass and the acceleration due to gravity)
Don’t forget the difference between these!!

17. Math Skills Newton’s Second Law
Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the upward acceleration of the lion and stretcher is m/s2. What force is needed to produce this acceleration of the lion and the stretcher?
1. List the given and unknown values.
Given: mass, m = 175 kg
acceleration, a = m/s2
Unknown: force, F = ? N

18. Math Skills, continued 2. Write the equation for Newton’s second law.
force = mass  acceleration
F = ma
3. Insert the known values into the equation, and solve.
F = 175 kg  m/s2
F = 115 kg  m/s2
F = 115 N

19. Newton’s Second Law, continued
Newton’s second law can also be stated as follows:
The acceleration of an object is proportional to the net force on the object and inversely proportional to the object’s mass. a c e l r t i o n = f m s F

20. Weight and Mass How are weight and mass related?
Weight is equal to mass times free-fall acceleration.
weight: a measure of the gravitational force exerted on an object
weight = mass x free-fall acceleration, or w = mg

21. Weight and Mass, continued
Weight is measured in newtons.
Weight is different from mass.
mass = a measure of the amount of matter in an object
weight = the gravitational force an object experiences because of its mass

22. Chapter 12 section 2 Free fall

23. Law of Universal Gravitation
Why do objects fall to the ground when dropped?
All objects in the universe attract each other through the force of gravity.

24. Law of Universal Gravitation, continued
Newton’s law of universal gravitation gives the size of the gravitational force between two objects:
m1 and m2 are the masses of the two objects
d is the distance between the two objects
G is a constant

25. Law of Universal Gravitation, continued
All matter is affected by gravity.
Two objects, whether large or small, always have a gravitational force between them.
When something is very large, like Earth, the force is easy to detect.
Gravitational force increases as mass increases.
Gravitational force decreases as distance increases.

26. Law of Universal Gravitation, continued

27. Free Fall
What is the relationship between free-fall acceleration and mass?
In the absence of air resistance, all objects falling near Earth’s surface accelerate at the same rate regardless of their mass.
free fall: the motion of a body when only the force of gravity is acting on the body

28. What is free fall?
Free fall- objects falling unaffected by air resistance.
On Earth we can’t fall without air resistance, but for normal situations the numbers are realistic.
When things fall they constantly accelerate.
If we can ignore air resistance, all objects accelerate at the same rate.
9.8 m/s2 = g (acceleration due to gravity).
*at sea level
If you assume up is positive (which is common) g = -9.8 m/s2.

29. If everything accelerates at the same rate, does that mean everything falls at the same rate?
Even they have a different weight?
yes
Even if they are different sizes?
Even if they are different shapes?
not if you include air resistance
If you put it in a vacuum, then yes

30. Velocity and acceleration of a ball thrown up in the air neglecting air resistance
t = 2 s
For this problem
positive is upwards
negative is down
v = 0 m/s (at rest)
a = -9.8 m/s2
t =3 s
t = 1 s
v = negative Mid (downward)
a = -9.8 m/s2
v = Mid
a = -9.8 m/s2
t = 4 s
t=0 s
v =Maximum
a = -9.8 m/s2
v = Maximum negative
(Downward)
a = -9.8 m/s2

31. Things that increase air resistance.
shape- more surface area means more air resistance.
velocity- the faster you go, the more air resistance (this is why meteors burn up in the atmosphere)
The “thickness” of the air you go through (there is less air resistance higher up in the atmosphere)

32. Horizontal and vertical motion
Will objects drop as fast if they are going forward?
Yes they will.
No matter how fast they are going.

33. Motion Map
1 sec
2 sec
3 sec

34. Free Fall, continued Air resistance can balance weight.
terminal velocity: the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity
Astronauts in orbit are in free fall.

35. Terminal Velocity
Terminal velocity- the velocity at which the upward force of air resistance equals the downward force of gravity.
Once you reach this velocity you will no longer accelerate. (just stay at the same velocity)
Parachutes increase your surface area to increase your air resistance in order to reduce your terminal velocity so you don’t die when you hit the ground.

36. Free Fall and Weightlessness

37. International Space Station
A joint project from the United States and Russia started in 1998 began construction on the International Space Station.
The first crew arrived in 2000.
It is a lab orbiting the planet where crews work and do research.
It is the 4th Space Station to orbit the Earth.

38. Pictures

39. More Pictures

40. Other Space Stations 1971 Salyut-1 (Soviet Union)
1973 Skylab (United States)
1986 Mir (Soviet Union)
None of these are operational anymore.

41. Weightlessness
People aboard the space station appear to be weightless (They float around like there is no gravity)
But they are not out of Earth’s gravitational field. (gravity is still pulling on them)

42. Freefall
The space station is actually in freefall around the planet.
That is what orbit is. (constant free fall around the planet)
It is moving so quickly forward, it clears the planet.

43. Frame of Reference
The “weightlessness” has to do with the frame of reference.
If everything is falling at the same rate, it appears as though it is floating with no gravity.
Similar to being on the Demon Drop at Cedar Point.

44. The Pitfall (Let’s watch the video..)
If you put a penny on your knee and ride the Demon Drop it will “float” in front of you, at least from your frame of reference.
Really it is falling at the same rate you are.
In the case of the space station everything is falling with you (air included), so you can’t tell you are falling.

45. Projectile Motion Why does a projectile follow a curved path?
Projectile motion has two components—horizontal and vertical. When the two motions are combined, they form a curved path.
projectile motion: the curved path that an object follows when thrown, launched, or otherwise projected near the surface of Earth

46. Projectile Motion, continued
Projectile motion has a horizontal component.
After you have thrown a ball, no horizontal forces are acting on the ball (if air resistance is ignored). So, the horizontal component of velocity of the ball is constant after the ball leaves your hand.
Projectile motion also has a vertical component.
When you throw a ball, gravity pulls it downward, which gives the ball vertical motion. In the absence of air resistance, gravity on Earth pulls objects that are in projectile motion downward with an acceleration of 9.8 m/s2, just as it pulls down all falling objects.

47. Projectile Motion, continued
Orbiting is projectile motion.

48. Newton’s Third Law

49. And it states…
For every (action) force there is an equal (reaction) force in the opposite direction on the object that applied the force by whatever the force was applied to.
shortened- for every action there is an equal and opposite reaction.
this has nothing to do with human nature or philosophy!!

50. It means… If you punch someone in the face…
It does not mean they will stand up and punch you back!!! (they could just sit there and cry, or run away!)
It means their face hits your hand with the same force your hand hit their face!
Which is why so many guys get broken hands in a fight!!

51. Major misconception
The action and reaction force are NOT on the same object!
action is on the object the force is applied
reaction is on what applied the force
If you kick a ball…
action force on the ball from your foot
reaction force on your foot from the ball

52. Force Diagram Force on the ball from These forces are your foot
equal in magnitude
(strength)
Causing
a change in
motion sending
it forward
And opposite in
direction.
Causing a change
in motion slowing
down your foot
Force on
your foot
from the ball

53. All Motion Depends On This Law
How do you walk?
You push on the ground backwards… the ground pushes you forward
How do you swim?
You push the water back, the water pushes you forward

54. Motion in Outer Space It’s a vacuum- there is nothing there
No air of any kind
If you want to move you better bring something with you to throw out the back!!
Space ships bring oxygen and hydrogen, and ignite them in the back of their ships
Which sends water vapor flying in the opposite direction and pushes them forward
They call it a burn on the movies (and in mission control)

55. Misconception Doesn’t this cancel out all forces?!?!
Shouldn’t there always be no change in motion?
NO!!
The forces are on different objects, so they can’t cancel each other out!

56. Conservation of Momentum
What is the total momentum after objects collide?
The total momentum of two or more objects after a collision is the same as it was before the collision. In other words, the total amount of momentum in an isolated system is conserved.
This principle is the law of conservation of momentum.

57. What is momentum p = m v momentum- how much inertia is in motion
It is how difficult it is to stop an object
it is mass and velocity
momentum = mass x velocity
p = m v
momentum’s symbol is p, units are kg m/s

58. Law of Conservation of momentum
The net momentum of a system cannot change.
Initial momentum equals the final momentum.
Momentum can be transferred from one object to another though.

59. continued
If both objects were at rest to start, the forward momentum of what was pushed will be equal to the backward momentum of what pushed it.
Momentum can be transferred to the Earth.

60. Everyday uses of the word momentum fit the definition
A fast moving car has more momentum than a slow moving car
A truck moving at the speed as a car has more momentum than the car
Something that is not moving has no momentum regardless of its mass

61. Quick problems
If the students who dropped my class have a mass of 220 kg and they ran at 3.4 m/s to the guidance office, what is their momentum?
If Joe throws his .12 kg calculator down at 5.1 m/s what is its momentum?

62. Momentum, continued
momentum: quantity defined as the product of the mass and velocity of an object
SI units of momentum = kilograms times meters per second (kg•m/s).
Momentum and velocity are in the same direction.
Momentum increases as mass and velocity increase.
Force is related to change in momentum.
As the period of time of the momentum’s change becomes longer, the force needed to cause this change in momentum becomes smaller.

63. Math Skills
Momentum Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley toward the pins.