Physics 218, Lecture VII1 Physics 218 Lecture 7 Dr. David Toback.
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Physics 218, Lecture VII 1
Physics 218Lecture 7
Dr. David Toback
Physics 218, Lecture VII 2
Physics 218, Lecture VII 3
Before we begin
• Exam Next Tuesday– Only on topics through Chapter 3 on
Syllabus• Not circular motion!
• Today’s lecture not on the exam• However, I’ll try to make it a good
review
Physics 218, Lecture VII 4
Overview of Chapter 4
• Where we’re going and why–Dynamics vs. Kinematics
• Force• Newton’s Laws of Motion• Mass• Normal Force• Example problems
Physics 218, Lecture VII 5
Where we’re going and why
Moving from: “How things move” Kinematics To: “Why things move that way” Dynamics• Why do you care?
– You know how to calculate how fast you need to accelerate a car if you want it to go from 0 to 60mi/hr in 6 sec.
– What you need to know is how hard the engine needs to work to do it. What FORCE you need.
• This builds on all the kinematics, vectors and calculus we’ve done before.
• We’ll develop the concepts, then learn to solve the problems.
Physics 218, Lecture VII 6
Newton’s Laws
• Next we’ll do Newton’s laws• You shouldn’t memorize them, rather you
need to be able to understand and use them• Don’t write them down from the
transparencies, they’re in your book.• We’re going to translate them into English• Big picture:
Force
Physics 218, Lecture VII 7
Before we start
What is a Force?
• Examples:– Push
– Pull
– Slap
– Gravity
– Others?
Physics 218, Lecture VII 8
Newton’s First Law
“Every body continues in it’s state of rest or of uniform speed in a
straight line unless acted on by a non-zero net force”
Physics 218, Lecture VII 9
Translate that into Engligh:Force
To cause an acceleration requires a Force
or If there is an acceleration, there
must be a Force
Force is a VectorAdd up all the forces (vectors) to find the Net force
Physics 218, Lecture VII 10
Newton’s First Law
• Example of non-zero net forces? – Friction: Causing a rolling ball to stop
– Gravity making a ball fall• (Net force causes acceleration in the
negative direction)
• Example of zero net force– Car sitting on the pavement
• No acceleration, must be no net force
Physics 218, Lecture VII 11
Newton’s Second Law
“The acceleration of an object is directly proportional to the net
force acting on it and is inversely proportional to its mass. The
direction of the acceleration is in the direction of the net force
action on the object.”
Physics 218, Lecture VII 12
Newton’s Second Law
• Translate this into English:– Net forces cause
the velocity to change
If I have a force, what is my acceleration? Need the mass.
gmWWeight
ma F ,ma F
am F
:EquationVector
yyxx
Physics 218, Lecture VII 13
Force to stop a car
What constant net force is required to bring a car of mass m to rest from a speed of V within a distance of XF?
X = 0 X = XF
V0 = V V = 0
Physics 218, Lecture VII 14
Getting to Newton’s Third Law
• How does one apply a force?
• Applying a force requires another object!–A hammer exerts a force on a nail
Physics 218, Lecture VII 15
Newton’s Third Law
“Whenever one object exerts a force on a second object, the second
exerts an equal and opposite force”
OR
“To every action there is an equal and opposite reaction”
Physics 218, Lecture VII 16
Example
Skater pushes on the wall. The wall pushes back, that’s why she flies off with some non-zero speed.
Physics 218, Lecture VII 17
Examples
Force exerted by the person on the ground is equal and opposite to the force exerted by the ground on the person. She pushes on the ground and the ground PUSHES her forward
PGGP F- F
Physics 218, Lecture VII 18
Normal Force
Consider a pen on a table:– What about Gravity? Didn’t we just say that a
pen near the earth would be accelerating due to gravity?
– What is the acceleration?– What is the Force?– What keeps the pen from accelerating?
Clearly there is a force which keeps it from accelerating.
Call this the “normal” force!
Physics 218, Lecture VII 19
Moving towards Problems
We’re now done with the concepts for this chapter
• This is where the separation between the good students and bad students starts to get noticeable.
• “I understand the concepts, but I can’t do the problems”
• Just to be clear: The concepts were only the first half of the lecture. No points on the exams for knowing “concepts you can’t apply”
Physics 218, Lecture VII 20
Free Body Diagrams
This is the trick: 1. Draw all the forces on an object!2. Then do the summing!3. Sum all the forces in the X direction and the Y
direction separately!!!
• This is a GREAT way to pick up partial credit on an exam problem.
• If you don’t know how to draw a force diagram, you don’t know how to solve the problem.
Physics 218, Lecture VII 21
The weight of a boxA box with mass m is resting on a
smooth (frictionless) horizontal table.
a) Determine the weight of the box and the normal force
b) Push down on it with a force of FP. What is the normal force?
c) Pull up on it with a force of FP such that it is still sitting on the table. What is the normal force?
d) Pull up on it with a force such that it leaves the table and starts rising. What is the normal force?
Physics 218, Lecture VII 22
Pulling a box
FP
A box with mass m is pulled along a frictionless horizontal surface with a force FP at angle as given in the figure. Assume it does not leave the surface.
a)What is the acceleration of the box?
b)What is the normal force?
Physics 218, Lecture VII 23
Box on an inclined planeA box with mass m is placed on a frictionless incline
with angle and is allowed to slide down.
a) What is the normal force?
b) What is the acceleration of the box?
Physics 218, Lecture VII 24
2 boxes connected with a stringTwo boxes with masses m1 and m2 are placed on a
frictionless horizontal surface and pulled with a Force FP. Assume the string between doesn’t stretch.
a)What is the acceleration of the boxes?
b)What is the tension of the strings between the boxes?
Physics 218, Lecture VII 25
Atwood Machine
Two boxes with masses m1 and m2 are placed around a pulley with m2 >m1
a) What is the acceleration of the boxes?
b) What is the tension of the strings between the boxes?
Ignore the mass of the pulley, rope and any friction. Assume the rope doesn’t stretch.
Physics 218, Lecture VII 26
Next Time
• No Exam Thursday
• Thursday: Make sure you read
– Section 3-9: Uniform Circular motion
– Chapter 5: More force and friction
– There will be a quiz.
• Finish HW 3 to study for Exam
• Labs/Rec/Web quizzes as usual
Physics 218, Lecture VII 27
How to study for the exam
• Do all the HW problems
• 4 Problems. Dominated by Chapter 3, almost nothing from Chapter 1.
• Make sure you can do all the HW problems easily and by yourself with no numbers
• Concentrate on the “hardest” problems. Those will be the most like the ones on the exam.
Physics 218, Lecture VII 28
Example 1
Force to accelerate a fast car
Estimate the net force needed to accelerate a 1000kg car at ½g
Physics 218, Lecture VII 29
Hockey Puck
• Which of these three best represents a hockey puck in the real world?
Physics 218, Lecture VII 30
Example 4-16
Physics 218, Lecture VII 31
Before we begin
• Exam Thursday– Formula sheet to be passed out is already on
web– Here at 8:00AM– Only on topics through Chapter 3 on
Syllabus• Not circular motion!
• Today’s lecture not on the exam• However, I’ll try to make it a good
review
Physics 218, Lecture VII 32
Mass vs. Weight• Mass is a property of the body
– Aside: What is mass is a fundamental question at the forefront of physics today. Particle physics! String theory!
• Weight is a Force and depends on the mass
What is the difference between:– Your mass on the moon and the earth?– Your weight on the moon and the earth?– Your weight in a space ship and the earth?
j mg- mg F W Weight earth on the weight of Definition
G
Physics 218, Lecture VII 33
Next Time• Exam Thursday: Here during regular time
– Bring a calculator and ruler– Formula sheet will be provided (same as on the
web)– Will cover:
• Chapter 1 (1-6)• Calculus 1• Chapter 2 (1-7)• Chapter 3 (1-8, 10)
• Finish HW 3 to study for Exam• Labs/Rec/Web quizzes as usual
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