Physics:acceleration and force Galileo s Investigation What

1
(No Transcript)
2
Galileos Investigation
Engagement

• What are the similarities and differences between
  a basketball and a tennis ball?

3
Galileos Investigation
Engagement

• What will happen if the balls are released from
  the same height above the floor?
• Try to use Physics vocabulary in your response.

4
Galileos Investigation
Engagement

• Whats going on here, why did they move?
• How would you describe the motion of the balls?
• Path
• Speed
• Again, try to use Physics vocabulary in your
  response.

5
Beautiful Bounces HO 3.1
Exploration

• Materials per group Motion probe, laptop,
  bouncing ball, Handout 3.1 Exploration Guide,
  ring stand
• Position the probe on the ring stand 1.5 m from
  the floor with the sensor facing downward
• Start collecting data and release the ball so
  that it bounces underneath the probe
• Graph the motion of the bouncing ball and repeat
  as necessary to get a pretty graph

6
Beautiful Bounces HO 3.1
Exploration

• Examine the distance vs. time graph carefully
• On your Exploration Guide
• Sketch your position vs. time graph from the
  bouncing basketball
• Label the position of the floor and the motion
  probe
• Mark important points of the balls motion
• Discuss your findings as a class

7
Force of Gravity Pull or Push?
Explanation

• Force can be described as a push or pull one
  object exerts on another.
• One specific force is weight. Weight the force
  of gravity on an object.
• But what causes gravity?
• The presence of mass

8
Newtons Universal Law of Gravitation HO 3.2
Explanation

• Masses attract one another about their centers
  with a force proportional to the product of their
  masses and inversely proportional to the square
  of the distance between them (whew!).
• Try to write that in an equation form.

9
Newtons First Law
Explanation

• Objects require a force to change their velocity.
• Another term for this property of matter is
  inertia
• What is the principle property of an object that
  determines its inertia?
• mass

10
Newtons First Law Example
Explanation

• If we apply the same force (push) to a bus and a
  bicycle, which object will experience a greater
  change in velocity?

11
Newtons Second Law
Explanation
SF ma

• Force causes a change in motion
• Change in motion means change in velocity
• Change in velocity means???
• A change in velocity occurs in a certain amount
  of time.
• AHA! Thats a rate.

12
Acceleration is a Rate
Explanation

• The speedometer in your car indicates what
  property of motion?
• What happens to the needle if you speed up or
  slow down?
• The speed of the needle is a measure of your
  acceleration (in a straight line).
• Acceleration is the rate of change in velocity.

13
Newtons Second Law Example 1
Explanation
SF ma

• Important note the acceleration vector is always
  in the same direction as the force vector.
• If we apply a 100N force to a wagon full of rocks
  initially at rest with a mass of 50 kg, what is
  its acceleration?
• What does this answer tell us?
• What is the velocity of the object after 1
  second?
• 2 seconds?
• 5 seconds?

100 N
2 m/s2
14
Newtons Second Law Example 2
Explanation

• What happens when the acceleration vector points
  opposite to the velocity vector?

30 m/s
F
-5m/s2
How long will it take Mighty Mikey to stop the
train?
15
Newtons Third Law
Explanation

• No single isolated force can exist
• Equal and Opposite forces exist only between two
  objects

WRONG!
Push on water
16
Newtons Third Law Example
Explanation

• You push on the water
• The water pushes back on you with an equal force
  in the opposite direction
• So why do you move?

Push on swimmer
Push on water
CORRECT!
17
Acceleration Due to Gravity
Explanation

• The Tennis Ball and Basketball hit the ground at
  the same time because they accelerated at the
  same rate (10 m/s2).
• What force changed their velocities?...Weight.
• Weight mass X gravity (Wmg).

WbbgtWtb
18
Acceleration Due to Gravity
Explanation
F weight
F weight
m mass
m mass
F/ma
F/m a
a g regardless of mass!
19
Is there a Starbucks in the Leaning Tower of
Pisa?
Elaboration

• Observe the instructor drop a new set of objects.
• As a class, discuss the similarities and
  differences in their motion.
• Why is this different?

20
Paper Drop Challenge HO 3.3
Elaboration

• Materials per group 2 sheets of paper,
  stopwatch, 8 paperclips, 30cm of tape, scissors,
  meter stick.
• Have Materials Manager distribute supplies.
• You have 5 minutes for this activity.
• Make one whole piece of paper drop to the floor
  as fast as possible from a height of 2 meters and
  the other drop as slowly as possible from the
  same height. (Do not attach the scissors or meter
  stick to your paper.)
• GO!

21
Paper Drop Challenge
Elaboration

• As a group, describe your strategy for making the
  piece of paper
• Drop the slowest
• Drop the fastest
• Make a table of your drop times for comparison
  with the class
• Explain why your two pieces of paper accelerate
  at different rates.

22
Free Body Diagram HO 3.4
Explanation

• A free body diagram is a pictorial representation
  of all forces acting on an object.
• We consider the object to be isolated from the
  physical system and choose to examine only the
  forces directly acting ON the object, not forces
  applied BY the object.

23
Free Body Diagram Example
Explanation
Label the forces acting on the block.
24
Free Body Diagram Example
Explanation
Force on the block by the ground
Force on the block by Antonio
Force on the block by the ground
Force on the block by the rope.
Force on the block by the earth
25
Free Body Diagram Example 2
Explanation

• Forces on coffee filter before release (balanced
  forces)

Support from my hand
Weight
26
Free Body Diagram Example 2
Explanation

• Forces on coffee filter immediately after release
  (unbalanced force ? acceleration)

Weight
27
Free Body Diagram Example 2
Explanation

• Forces on coffee filter while falling

Air resistance
What factors influence the magnitude of the force
of air resistance?
Weight
28
So Why Different Times?
Explanation

• Force due to gravity is different for the two
  objects
• Their masses are different and therefore they
  have different inertias
• They do not accelerate at the same rate WHY???
• We must examine ALL the forces acting on the
  objects in order to determine the Net Force.

29
Net Force
Explanation
SF

• If you earn 2000/mo but taxes, ins, retirement
  and other reductions total 350/mo then your
• Gross Pay 2000/mo
• Net Pay 1650/mo

Net Force
30
Net Force
Explanation

• The force of air resistance is greater for the
  basketball than the coffee filter.
• However, the force of air resistance is
  negligible compared to the weight of the
  basketball, therefore the Net Force is toward the
  floor.

Air resistance
Weight
31
So Why Different Times?
Explanation

• The force of air resistance is also acting on the
  coffee filter and is comparable in size to the
  weight.
• In fact, at some point, the force of air
  resistance is equal to the weight of the coffee
  filter.
• When this is the case
• There is NO net force
• Therefore there is no acceleration
• Therefore the object moves at a constant speed
• This equilibrium condition is called terminal
  velocity

32
Sky Diver Physics HO 3.5
Evaluation

• A skydiver jumped out of a plane 1 sec ago.
• Label the forces on the skydiver
• After some time, the skydiver has zero
  acceleration
• What does this indicate about her speed?
• What is the name for this equilibrium condition
  of a falling object?
• Label the forces on the skydiver
• What factors influence the force of air
  resistance on the skydiver?
• BONUS What is the terminal velocity of a human?

33
Summary
Summary

• Force
• Gravity Weight
• Newtons Laws
• Air resistance
• Free Body Diagrams
• Net Force
• So what?
• This applies to transportation, moving objects,
  sports, etc.

34
Reading Assignments HO 3.6
Homework

• NSES
• p. 123, 126-127
• 149, 154-155 176-181
• BSL
• 87-92
• Integrated Science
• Forces p. 26-27, Falling Objectsp. 29-31, Laws
  of Motionp. 32-38

35
Concepts and Questions
Homework

• Integrated Science, Ch. 2
• Applying the Concepts
• 1-6, 8-9
• Questions for Thought
• 1, 5, 8