Download It s the Law! - Sunnyvale ISD

October 22, 2012
Newton’s Physics
It s the Law!
Presentation 2003 R. McDermott
October 22, 2012
Sir Isaac Newton 1643 -­‐ 1727
• In 1671, at the age of 28, Newton’s mathema?cal genius led to his inven?on of calculus.
• Later, in 1687, Newton published his Principia, which basically invented the physics that we s?ll study today.
• He may very well be the greatest mathema?cian and scien?st of all ?me!
October 22, 2012
Force: Push or Pull
• Gravita?onal Force (Weight)
• Electromagne?c Forces
• Electric
• Magne?c
• Tension – String, Wire, Rope, Spring, Cable, Rigid Rod,
Muscular Contrac?on
• Compression -­‐ Spring, Rigid Rod
• Fric?on • Nuclear Forces
• Strong
• Weak
October 22, 2012
Classifying Forces:
• “Field” forces – “forces at a distance”
• Gravita?on: mass to mass
• Electric: charge to charge
• Magne?c: pole to pole
• Nuclear forces
• Strong: nucleon to nucleon
• Weak: quark to quark
• “Contact” forces – everything else
October 22, 2012
Newton 1
• When no net force exists, an object’s mo?on does not change.
• Fx = Fy = 0
• This is called “first condi?on for equilibrium”. • Unit – Newton
• Amount of force needed to accelerate one kg, 1 m/s2.
5
October 22, 2012
Free-­‐Body Diagrams
• FBD’s isolate an object.
• Decide what object is of interest
• Iden?fy forces on it:
• First look for field forces
• Then look for other objects that touch
• Draw object in as a box
• For each force, draw an arrow from the box in the correct direc?on
October 22, 2012
October 22, 2012
Diagram
• Weight:
Normal:
Friction:
F
N
Ff
FG
October 22, 2012
Surface Force -­‐ Normal
• When surfaces are compressed by outside forces, they “push back” with a “normal force”
• Normal means perpendicular and is usually labeled N
October 22, 2012
Surface Force -­‐ Fric?on
• If you adempt to slide rough surfaces across one another, fric?on will act opposite to your sliding adempt
• The force of fric?on is parallel to the surface at all ?mes and is usually labeled f
October 22, 2012
Horizontal Surface
October 22, 2012
Hanging at Rest
FT
Fg
October 22, 2012
Hanging at Rest
FT
Fg
October 22, 2012
Sitting at Rest
FN
Fg
October 22, 2012
Sitting at Rest
FN
Fg
October 22, 2012
Falling with constant speed.
Ff
Fg
October 22, 2012
Falling with constant speed.
Ff
Fg
October 22, 2012
Sliding along a frictionless surface at constant speed.
FN
Fg
October 22, 2012
Sliding along a frictionless surface at constant speed.
FN
Fg
October 22, 2012
Falling without air
resistance.
Fg
October 22, 2012
Falling without air
resistance.
Fg
October 22, 2012
Sliding to a halt over a
rough surface.
FN
Ff
Fg
October 22, 2012
Sliding to a halt over a
rough surface.
FN
Ff
Fg
October 22, 2012
Rising without air
resistance.
FA
Fg
October 22, 2012
Rising without air
resistance.
FA
Fg
October 22, 2012
Rising with
constant speed.
FT
Fg
October 22, 2012
Rising with
constant speed.
FT
Fg
October 22, 2012
FT
Fg
Accelerating UPWARD at 9.80m/s2.
October 22, 2012
FT
Fg
Accelerating UPWARD at 9.80m/s2.
October 22, 2012
Determining Net Force
• 1. A man is pulling on his dog with a force of 70.0 N directed at an angle of +30.0° to the horizontal. Find the x and y components of this force.
2. A gust of wind blows an apple from a tree.
As the apple falls, the gravitational force on
the apple is 2.25 N downward, and the force
of the wind on the apple is 1.05N to the right.
Find the magnitude and direction of the net
October 22, 2012
October 22, 2012
Newton’s 1st Law:
• “An object at rest tends to remain at rest unless acted upon by an outside force.”
• “An object in mo?on tends to remain in mo?on unless acted upon by an outside force.”
• Also known as the Law of Iner?a
October 22, 2012
Iner?a
• Since objects “resist” changes in their mo?on, this “resistance” was given a name -­‐ iner?a.
• Iner?a is directly related to the mass of an object
Massive objects like ships
have tremendous inertia.
October 22, 2012
1st Law Summarized:
• If you want to change what an object is doing, then you must apply a force to it.
• If an object’s mo?on is not changing, then there is no net force ac?ng on the object.
• In the absence of fric?on, an object moving at a constant speed in a straight line requires no force to keep moving!
October 22, 2012
Galileo vs. Aristotle
In our experience, objects must be pushed in order to keep
moving. So a force would be needed to have a constant
velocity. This is what Aristotle claimed in his in his series
of books entitled "Physics", written 2400 years ago.
But 400 years ago, another scientist and astronomer,
Galileo, proposed the following thought experiment
which revealed another perspective.
October 22, 2012
Thought Experiment
Imagine two perfectly smooth ramps connected together
by a perfectly smooth surface. If a ball is let go at the top
of the one ramp, what will happen?
October 22, 2012
Thought Experiment
Imagine two perfectly smooth ramps connected together
by a perfectly smooth surface. If a ball is let go at the top
of the one ramp, what will happen?
October 22, 2012
Thought Experiment
Imagine two perfectly smooth ramps connected together
by a perfectly smooth surface. If a ball is let go at the top
of the one ramp, what will happen?
October 22, 2012
Thought Experiment
Imagine two perfectly smooth ramps connected together
by a perfectly smooth surface. If a ball is let go at the top
of the one ramp, what will happen?
October 22, 2012
Thought Experiment
If a ball rolls down one ramp, it keeps
rolling up the other side until it
reaches the same height.
October 22, 2012
Thought Experiment
Now repeat that experiment, but make the second ramp
less steep.
What Will Happen?
October 22, 2012
Thought Experiment
Now repeat that experiment, but make the second ramp
less steep.
What Will Happen?
October 22, 2012
Thought Experiment
Now repeat that experiment, but make the second ramp
less steep.
What Will Happen?
October 22, 2012
Thought Experiment
Now repeat that experiment, but make the second ramp
less steep.
What Will Happen?
October 22, 2012
Thought Experiment
It will still keep rolling until it reaches the same height,
but it has to roll farther!
October 22, 2012
Thought Experiment
Finally, make the ramp flat.
Now what will happen?
October 22, 2012
Thought Experiment
Finally, make the ramp flat.
Now what will happen?
October 22, 2012
Thought Experiment
Finally, make the ramp flat.
Now what will happen?
October 22, 2012
Thought Experiment
Finally, make the ramp flat.
Now what will happen?
October 22, 2012
Thought Experiment
Finally, make the ramp flat.
Now what will happen?
October 22, 2012
Thought Experiment
Finally, make the ramp flat.
Now what will happen?
October 22, 2012
Thought Experiment
It will keep rolling forever, no
external force is necessary.
October 22, 2012
Newton's 2nd Law
October 22, 2012
October 22, 2012
A dog pulls on a pillow with a force of 5N at an angle of 37o
above the horizontal. Find the x and y components of this
force.
October 22, 2012
Joe Ponder, from North Carolina, once used his teeth to lift a pumpkin
with a mass of 275 kg. Suppose Ponder has a mass of 75 kg, and he
stands with each foot on a platform and lifts the pumpkin with an attached rope. If he holds the pumpkin above the ground between the
platforms, what is the force exerted on his feet? (Draw a free-body
dia- gram showing all of the forces present on Ponder.)
October 22, 2012
In 1994, Vladimir Kurlovich, from Belarus, set the record as the
world’s strongest weightlifter. He did this by lifting and holding
above his head a barbell whose mass was 253 kg. Kurlovich’s mass at
the time was roughly 133 kg. Draw a free-body diagram showing the
various forces in the problem. Calculate the normal force exerted
on each of Kurlovich’s feet during the time he was holding the
barbell.
October 22, 2012
The net force exerted by a woodpecker’s head when its beak strikes a
tree can be as large as 4.90 N, assuming that the bird’s head has a mass
of 50.0 g. Assume that two different muscles pull the woodpecker’s
head forward and downward, exerting a net force of 4.90 N. If the
forces ex- erted by the muscles are at right angles to each other and
the muscle that pulls the woodpecker’s head downward exerts a force
of 1.70 N, what is the magnitude of the force exerted by the other
muscle? Draw a free-body diagram showing the forces acting on the
woodpecker’s head.
October 22, 2012
A 1850 kg car starts from rest and reaches a velocity of 1.44 m/s in 2
seconds.
a. What is the acceleration of the car?
b. What is the force on the car during this time of acceleration?
October 22, 2012
A 40 kg wagon is towed up a hill inclined at 18.5o with respect to the horizontal.
The tow rope is parallel to the incline and exerts a force of 140 N on the wagon.
Assume that the wagon starts from rest at the bottom of the hill, and disregard
friction.
a. How fast is the wagon going after moving 30 m up the hill.
b. How fast is the wagon going after moving 80 m up the hill?
October 22, 2012
October 22, 2012
October 22, 2012
October 22, 2012
A 1850 kg car starts from rest and reaches a velocity of 1.44 m/s in 2
seconds.
a. What is the acceleration of the car?
b. What is the force on the car during this time of acceleration?
October 22, 2012
A 40 kg wagon is towed up a hill inclined at 18.5o with respect to the horizontal.
The tow rope is parallel to the incline and exerts a force of 140 N on the wagon.
Assume that the wagon starts from rest at the bottom of the hill, and disregard
friction.
a. How fast is the wagon going after moving 30 m up the hill.
b. How fast is the wagon going after moving 80 m up the hill?
October 22, 2012
October 22, 2012
October 22, 2012
120o
10N
October 22, 2012
151o
10N
October 22, 2012
October 22, 2012
October 22, 2012