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Some Upfront Definitions…PowerPoint Presentation

Some Upfront Definitions…

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Some Upfront Definitions…

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- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

- Scalars – a quantity that has only a magnitude (with unit).
- Ex: mass, temperature, speed

- Ex: force, velocity, acceleration.

- Mass is not weight, but is related to weight. Unit: kg

- Weight is not mass, but is related to mass

- Speed is a scaler quantity. Unit: m/s

Tyler Junior CollegePhysics 1405Elementary Physics

NEWTON’S 1st LAW--INERTIA

- aka – the law of inertia
- “Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.”

- At rest
- Constant speed in a straight line
- Accelerated motion caused by forces

ConcepTest

If an object is moving, then it must be experiencing a net force.

- True
- False

ConcepTest

Which of the following is NOT in one of Newton’s three states of motion?

- fish on a merry-go-round
- truck driving in a straight line but speeding up
- astronaut weightless in space
- all the above
- none of the above

- Objects at rest experience no net force.
- Objects moving in a straight line at constant speed experience no net force.
- Objects that move in any other way are experiencing a net force.
- When a net force stops acting, the object must begin to move in either a straight line at constant speed or it must come to rest.

ConcepTest

Which of the following is always true?

- If an object is moving, it must be experiencing a net force.
- If an object is moving, it must have constant velocity
- If an object is at rest, then it must be experiencing no net force.
- All the above
- None of the above

block

force

- A Force is a push or a pull
- Forces have both magnitude & direction
- Something with both magnitude & direction is called a ______
- All Forces are Vectors
- Vectors are usually symbolized as arrows.

- Vectors are usually drawn as arrows
- Vectors can be added
- The sum of vectors is called the resultant

+

=

5N

10N

5N

5N

5N

10N

5N

5N

- Net Force is the resultant of all forces.

What is the net force acting on this object?

a. 0 N

b. 10 N, right

c. 5N, down

d. 10 N, left

e. 5N, up

10N

15N

25N

5N

15N

5N

5N

5N

- Vectors in non-parallel directions can be added by the parallelogram method.

resultant

- Vectors can also be broken down into perpendicular component parts.

resultant

ConcepTest

resultant

2 vectors

Where is the

resultant?

parallelogram

What is the net force on a cart pulled to the left with a 100 Newton force & to the right with a 40 Newton force?

a. 60 N to the right

b. 60 N to the left

c. 40 N to the right

d. 0 N

e. None of the above

What will be the direction of the net force? Assume all vectors are equal magnitude.

a. straight down

b. down-right

c. down-left

d. right

e. up-right

Tyler Junior CollegePhysics 1405Elementary Physics

Linear Motion

- Instantaneous speed – how fast something moves at any particular instant in time.
- Average speed – how fast something moves on average.

Driving to Dallas (about 100 miles) requires about 2 hours.

What is the average speed?

What is the instantaneous speed?

- Velocity is the rate of change of position.
- Velocity is a vector quantity!
- Velocity is speed in a given direction.
- Unit: m/s, but must include direction info.

- Constant speed does not necessarily mean constant velocity.
- Constant direction does not necessarily mean constant velocity.
- However, constant velocity does mean constant speed in a constant direction.
- Also, velocity can have positive or negative values.

A truck drives in a steady circle with an unchanging speed. The truck has…

a. constant velocity

b. constant direction

c. constant position

d. constant speed

e. None of the above

If an object has constant velocity then…

a. no net force is acting on the object

b. the object must be accelerating

c. the object is experiencing a net force

d. All the above

e. None of the above

- …is the rate of change, or change per second, of velocity.
- Acceleration occurs whenever there is an increase or a decrease or a direction changein velocity.
- Acceleration has odd looking units

- Note: acceleration is initially a confusing subject.
- It is complex because it is a rate of a rate.
- Acceleration is not velocity, nor is it even a change in velocity. Acceleration is the rate at which velocity itself changes.

- Since acceleration is the rate at which velocity changes,
- then constant acceleration means the velocity changes at a constant rate.
- A constant rate of change can be represented as a line on a graph.

In this graph

a. velocity is constant

b. acceleration is zero

c. velocity is not changing

d. acceleration is constant

e. all the above

velocity

time

In this graph

a. velocity is constant

b. acceleration is zero

c. velocity is not changing

d. acceleration is constant

e. all the above

velocity

time

Graphing

Constant velocity

At rest (special case of constant velocity)

Constant acceleration

velocity

position

time

time

acceleration

time

velocity

position

time

time

acceleration

time

velocity

position

time

time

acceleration

time

This graph best represents which of the following?

a. Object at rest

b. Object moving with constant velocity

c. Object moving with constant acceleration

d. All the above

position

time

Draw the position vs time & velocity vs time graphs for an object in free fall.

velocity

position

time

time

A truck drives in a steady circle with an unchanging speed. Does the truck experience an acceleration?

a. yes

b. no

c. Not enough information to tell

A truck drives in a straight line while slowing to a stop. Does the truck experience an acceleration?

a. yes

b. no

c. Not enough information to tell

- If position is constant, then velocity is zero.
- If position changes then velocity is nonzero.
- If speed or direction changes then velocity changes.
- If speed and direction are constant, then velocity is constant.
- If velocity is zero or constant, then acceleration is zero.
- If velocity changes then acceleration is nonzero.
- If velocity decreases then acceleration is negative.

- Does a falling object accelerate?
- Galileo used inclined planes to slow falling objects.
- Galileo found that when air resistance is small enough to neglect, all objects fall with the same unchanging acceleration.
- During each second of fall, objects gain a speed of 10 m/s.
- In other words, the acceleration due to gravity for a freely falling object is about 10 m/s2

- Galileo discovered that all falling objects experience an acceleration, g, of about 10 m/s2
- So…you can figure out how fast any object falls (ignoring air resistance)…
- And…you can figure out how far an object falls (ignoring air resistance)…

It takes about 10 seconds for a rock dropped from a cliff to hit the ground. How high is the cliff?

a. 50 m

b. 100 m

c. 250 m

d. 500 m

A kicker can kick a ball about 125 m straight up. How long will the ball be in the air?

a. 5 sec

b. 10 sec

c. 15 sec

d. 20 sec

e. 25 sec

f. None of the above

A kicker can kick a ball about 125 m straight up. What is the velocity of the ball when it returns to the ground?

a. 50 m/s

b. 500 m/s

c. 100 m/s

d. 10 m/s

e. None of the above

A kicker can kick a ball about 125 m straight up. What is the velocity of the ball at the top?

a. 50 m/s

b. 500 m/s

c. 100 m/s

d. 10 m/s

e. None of the above

- Occurs when the sum of all forces equals zero.
- If ∑F≠0, then the object is “accelerating” and NOT in a state of mechanical equilibrium.

- On earth all objects experience a force due to Earth’s gravity.
- The acceleration due to this force is g.
- The force itself is called weight.
- Weight = mass x g
- Weight is a Force

- For an object on a surface in a state of M.E.

- Gravity sux!
- Gravity is an attractive force.
- All masses are attracted to each other by the force of gravity.
- You are held to earth by this force.
- If m1=earth’s mass, m2=your mass, r=radius of earth, then this equation reduced to…
- F = W = m2g Gm1/r2 = g

ConcepTest

WeightMass

slug

lb

USSI

kg

N

A man pushes a crate so that it slides across the floor with an unchanging velocity. Is the crate in mechanical equilibrium?

a. YESb. NO

A man pushes a crate so that it slides across the floor with an unchanging velocity. What is the net force acting on the crate?

a. Nonzero to the right, b. Nonzero to the left, c. Zero

An airplane moves along in straight, level flight at 500 miles/hour. Which is false?

- The plane is in mechanical equilibrium
- The plane is experiencing no net force
- The sum of the forces acting on the plane is zero
- The 1st law does not apply to the plane
- None, they are all true.

A 20kg object is at rest on a table top. What is the normal force?

- 20 lbs upward
- 20 kg upward
- 20 N downward
- 200 kg upward
- 200 N upward

Tyler Junior CollegePhysics 1405Elementary Physics

Newton’s 2nd Law

- “The acceleration of an object is directly proportional to the net force on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.”
- IOW:
- This can be rewritten:

- The weight of an object on Earth is
- This is nothing but

What is the weight of a 500 kg man?

- 500 lbs
- 500 N
- 5000 lbs
- 5000 N
- None of the above

A force is applied to a mass causing an acceleration. The force is then cut in half. How does the acceleration change?

- The acceleration remains the same
- The acceleration is doubled
- The acceleration is cut in half
- The acceleration increases by 4
- None of the above

A force is applied to a mass causing an acceleration. The mass is then cut in half. How does the acceleration change?

- The acceleration remains the same
- The acceleration is doubled
- The acceleration is cut in half
- The acceleration increases by 4
- None of the above

A 50kg mass experiences a net acceleration of 5m/s2. What is the net force the mass experiences?

- 250 N in the direction opposite the force
- 500 N in the direction opposite the force
- 500 N in the same direction as the force
- 250 N in the same direction as the force
- None of the above

- When surfaces touch, a force known as friction exists.
- The friction of sliding is somewhat less than the friction that builds up before sliding takes place.
- So, it is a bit easier to push something after you get it going.

- Static friction & Sliding friction
- Does NOT depend on speed
- Does NOT depend on area of contact

- Fluid friction (aka Drag)
- Does depend on speed & area of contact

If you push a crate with a force over a frictionless surface it will accelerate. If you then push the crate with an identical force over a surface with friction, how will the second acceleration compare to the first?

a. same

b. more

c. less

A woman pushes a 100kg crate with a 50N force across the floor at a constant speed and in a straight line. What is the force of friction?

a. 100 kg

b. 100 N

c. 50 N

d. 500 N

e. None of these

m

m

m

- Do all objects fall with the same acceleration?

- In reality, when an object falls, air resistance acts like a frictional force.
- Air Resistance = Fluid Friction
- Recall that fluid friction depends on speed and area of contact.
- This leads to the concept of Terminal Velocity.

- Terminal speed is the speed when acceleration stops. It occurs when the forces of weight and air resistance are balanced.

air resistance

When the forces are equal, the mass falls with constant velocity, terminal velocity, meaning no acceleration.

m

weight = mg

If you release a flat piece of paper and your textbook at the same time, which will reach the ground first?

Tyler Junior CollegePhysics 1405Elementary Physics

Newton’s 3rd Law

- A force is nothing by itself.
- Two objects are required.
- There is an “action” and a “reaction”.
- “You cannot push without being pushed.”
- Examples: hammering, book on tabletop, leaning against wall, rocket, etc.
- IOW: forces come in pairs

“Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.”

- IOW: forces come in pairs
- Which force is action and which is reaction doesn’t matter. Neither exists without the other!

- If forces are always equal and opposite, why don’t they always cancel?
- For the 3rd Law to be true you must define your system correctly.
- When action and reaction are internal to the system, they cancel and produce no acceleration.
- If defined properly, there are no external forces…all forces are internal to the system.
- Such a system is called an isolated system.

- Example of a firing tank. (what is the system?)
- Example of a rocket. (what is the system?)

ConcepTest

- An astronaut is stranded in space and wants to get back to her ship. All she has is pouch full of tools. How can she get back?

ConcepTest

- A bug strikes the windshield of her car. Which experiences the greater force, the car or the bug?a. bugb. carc. same

ConcepTest

A gun recoils when it fires a bullet. The acceleration of the recoiling gun is

- greater than the bullet
- smaller than the bullet
- same as the bullet
- not enough info to know

ConcepTest

- Suppose there are three astronauts outside a spaceship and two of them decide to play catch with the other. All three astronauts weigh the same on Earth and are equally strong. Describe the motion of the three astronauts as the game proceeds. How long will this game last?

Tyler Junior CollegePhysics 1405Elementary Physics

Energy Concepts

- What is Energy?
- Work
- Power
- Potential Energy
- Kinetic Energy
- Work-Energy Theorem
- Conservation of Energy

- …you can’t see it.
- …you can’t feel it.
- …you can’t taste it.
- Energy is difficult to define.
- Energy is abstract.
- Energy is an important concept to science.

- Kinetic Energy = energy of motion
- Potential Energy = stored energy
- Electric Energy = energy from electricity
- Heat Energy = energy from heat
- Chemical Energy
- Nuclear Energy
- Atomic Energy
- Mass is also a form of energy.

- Energy is best understood in terms of WORK.
- Work
- Has an explicit definition in physics.
- Work involves a Force acting on an object over a distance.

- Think of WORK as the amount of energy required to lift an object.
- The Force required to lift an object is equal to the weight of the object.
- NOTE: If object is moved perpendicular to the force then no Work is done.
- Units of Work
- Force (N)
- Distance (m)
- 1 Nm = 1Joule = 1J

- Work can be positive, negative, or zero.

ConcepTest

- How much work does it take to lift a 10 N rock 1 m in height?

1m

ConcepTest

- How much work is done moving the rock as shown?

- The previous example assumes there is no friction.
- If there were friction, then there are more forces applied to the rock than just the lifting force.

- Work is NOT a form of energy.
- Work is a way of transferring energy from one place to another place or from one form to another form.
- Kinetic Energy & Potential Energy are two examples of forms of energy.

ConcepTest

- Two bricks are lifted a distance of 1 m. Which requires more work?

1m

- The concept of Work says nothing about the amount of time required to do the work.
- The concept of Power considers the amount of time in which the work occurs.
- Power is the energy per unit of time.
- Twice the power mean twice the work in the same amount of time OR the same amount of work in half the time.

- The unit of Power is the Joule per Second, also known as the Watt.
- Another way to look at Power:
- 1 liter of fuel can do a certain amount of work; but the power produced when we burn it can be any amount, depending on how fast it is burned. It can operate a lawn mower for a half-hour or a jet engine at 3600 times the power for a half-second.

ConcepTest

- Two physics students, Will and Abel, are in the weightlifting room. Will lifts the 100-pound barbell over his head 10 times in one minute; Abel lifts the 100-pound barbell over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power?

ConcepTest

- If little Nellie Newton lifts her 40-kg body a distance of 0.25 meters in 2 seconds, then what is the power delivered by little Nellie's biceps?

- Potential Energy is the energy due to the position of an object. PE is stored energy.
- Examples:
Pile driverDrawn bow string

- An elevated object has potential energy due to its elevated position.
- This kind of PE is called Gravitational Potential Energy.
- Gravitational PE = Weight X Height

- A stretched spring has stored energy due to the stretch.

- The change in an object’s Potential Energy is equal to the amount of Work done on/by it.

ConcepTest

The same ball is used in each diagram. Fill in the values for A, B, C, D, and E.

ConcepTest

A person does 10 J of work while lifting a bale of hay. How much does the bale’s potential energy change?

- 10 J
- 100 J
- 50 J
- Answer depends on mass of the bale
- None of the above

ConcepTest

Object A has twice the mass of Object B. You lift both the same height. Compared to B, A has _______ the potential energy.

- just as much
- half
- twice
- answer depends on mass of A
- None of the above

- KE is a type of motion into which PE may be transformed.
- KE is the energy of motion.
KE = ½ x mass x velocity2

- The amount of KE an object has depends on the object’s mass and the object’s velocity.

ConcepTest

If the velocity of an object increases to three times what it originally was, how does the object’s kinetic energy change?

a. x2 (doubles)

b. x3 (triples)

c. x6

d. x9

e. x15

- Something is conserved if it does not change.
- For an isolated system, total energy is conserved, not just PE and not just KE, but TOTAL ENERGY.
“Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.”

ConcepTest

- For a swinging pendulum…
- When is KE greatest?
- When is PE greatest?
- When is KE least?
- When is PE least?