physical science ii
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Physical Science II. Motion. Motion. Why is being able to describe motion important?. Motion. Being able to accurately describe motion has been a vital part of our global culture. As a race, humans have made significant advantages in being able to describe location and motion. Examples - PowerPoint PPT PresentationTRANSCRIPT
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PHYSICAL SCIENCE IIMotion
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Motion• Why is being able to describe motion important?
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Motion• Being able to accurately describe motion has been a vital
part of our global culture.
• As a race, humans have made significant advantages in being able to describe location and motion.
• Examples• Using Landmarks• Magnetic Compass• Detailed Maps• Global Positioning System (GPS)• Odometers and Speedometers
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Motion• Describing motion can be tricky.• Take for example the following statement:
• “Everything is moving.”
• Do you agree with that statement? Why or why not?
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Frame of Reference• In order to accurately describe motions in the universe,
there needs to be a Frame of Reference.
• A Frame of Reference is simply a group of objects that are not moving with respect to one another.• Examples: The classroom, the floor, the ground, Hershey, Pa.
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Frame of Reference• Let us go back to the previous quote:
• “Everything is moving.”
• This statement can be right or wrong depending on your chosen frame of reference.
• If your frame of reference is this room, then certainly not all objects are moving.
• However, if your frame of reference is the sun, then this room along with everything inside is moving as the Earth rotates and revolves around the sun.
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Frame of Reference• Let us try another example:
• A man and a woman are seated inside a train that is driving from Harrisburg toward Philadelphia at 65mph.
• The woman believes they are not moving, while the man believes they are moving.
• Who is correct in this situation?
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Distance vs. Displacement• Assume for a minute that you are lost somewhere in
Pennsylvania.
• A helpful resident tells you that you are 50 kilometers from Hershey.
• With this information can you get back home?
• What other information would you need?
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Distance vs. Displacement• Imagine again you are lost somewhere in Pennsylvania.
• The same helpful resident instead tells you that you are 50 kilometers northwest of Hershey.
• Can you find your way home?
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Distance vs. Displacement• What we have just discovered is there are two types of
length measurements.
• The first scenario describes a measurement of distance.
• Distance is simply a measurement of length, it has no direction.
• Examples: • You are 50 kilometers from Hershey.• I am 3 meters from the door.• The athlete ran 5 kilometers.
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Distance vs. Displacement• The second scenario described a measurement of
displacement.
• Displacement is a length measurement that also has a specific direction.
• Examples:• You are 50 kilometers northwest of Hershey.• I am 3 meters to the south of the door.• The athlete ran 5 kilometers to the east.
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Distance vs. Displacement• We’ve identified the difference between distance and
displacement, but what is the importance of having two different types of length measurements?
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Speed vs. Velocity• In the previous section we looked at the difference
between distance and displacement and determined there are two distinct ways of measuring the length you’ve traveled.
• In addition to measuring how much length something has travelled, it is often important to know how fast an object is or was moving.
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Speed vs. Velocity• Imagine you are going to travel to the beach on vacation.• In your preparations before travel, you may want to know
things like:• Directions – What roads do I take?• Travel Time – How long will I be driving?
• Travel time is of course dependent on many factors:• How much traffic.• How far must you travel.• How many stops you make.• How fast can you drive.
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Speed vs. Velocity• The final bullet point “How fast can you drive.” is a
measurement of speed.
• Instantaneous Speed – a measurement of how fast you are moving at one particular moment.• Example: At 8:30 am we were traveling at 15 m/s (meters per
second)
• This measurement is only a snapshot in time, since at 8:31 you may have been traveling at 40 m/s.
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Speed vs. Velocity• Average Speed – a measurement of how much distance
you’ve traveled in a certain amount of time.• While driving to the beach our average speed was 20 m/s.
• To calculate average speed use the following equation:
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Speed vs. Velocity• Example Problems:
• A runner traveled 1000m in a time of 250s. What was the runner’s average speed?
• How long did it take a bullet to travel 300m if it had an average speed of 350m/s?
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Speed vs. Velocity• Sometimes knowing only how fast something travels is
not enough information to completely analyze a situation.• It may also be important to know which way an object is or
was moving.• Instantaneous Velocity – measures how fast an in what
direction and object is moving at one particular moment.• At 8:30 we were traveling 15m/s to the east.
• Again this measurement is only a snapshot in time.
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Speed vs. Velocity• Average Velocity – a measurement of how much
displacement occurred in a certain amount of time.• While driving to the beach our average velocity was 10m/s to the
southeast.
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Speed vs. Velocity• Example Problems
• A truck traveled 2000m north in 65s. What was the average velocity for this vehicle?
• A cheetah ran at an average velocity of 25m/s west for 15s. What was the displacement of the cheetah?
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Velocity Vectors• Sometimes the motion of an object involves more than
one velocity.
• When this happens it becomes necessary to find the resultant velocity.
• The resultant velocity is the sum of all the individual velocities of an object’s motion.
• When finding this sum it is important to consider the direction of the velocity as well as the numerical value.
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Velocity Vectors• This involves using vectors.
• A vector is simply an arrow that is used to represent velocity.
• The length of the tail indicates the size of the velocity and the arrow head points out the direction.
Tail
Head
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Velocity Vectors• When vectors are in the same direction, they are added
together.
• If velocity vectors are in opposite directions they are subtracted from each other.
• When velocities are at right angles to each other you must add them using the Pythagorean Theorem.
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Velocity Vectors• Examples:
• A river is flowing downstream at a speed of 2-m/s. If a person is floating in a raft on this river, how fast are they moving past the shoreline?
• A boat is traveling downstream at a speed of 10-m/s in a river that is flowing at a speed of 2-m/s. What is the resultant velocity of the boat with respect to the shoreline?
• A boat is traveling upstream at a speed of 10-m/s in a river that is flowing at a speed of 2-m/s. What is the resultant velocity of the boat with respect to the shoreline?
• While in a canoe, two people row with a velocity of 4-m/s north across a river that is flowing with a velocity of 3-m/s east. What is the resultant velocity of these two people?
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Displacement Vectors• Displacement can also be represented by a vector.
• Examples:• A girl rides her bike 200-m east, pauses for a moment and then
continues to ride another 50-m east. What was the resultant displacement of the girl?
• A person drives their car 20-mi to the north, then turns and drives 5-mi to the south. What is the resultant displacement of the person?
• A man drives his car 3-mi to the east, then turns left and drives 4-mi to the north. What is the resultant displacement of the man?
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Acceleration• In the previous section we discussed how most objects do
not travel at a constant velocity.
• When the velocity of an object is changing, the object is undergoing an acceleration.
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Acceleration• Acceleration – the rate at which velocity is changing.
• Three ways an object can experience an acceleration:• The object’s speed can increase.• The object’s speed can decrease.• The object can change direction.
• Changing any one of these three will cause the velocity of an object to change, but in many cases of acceleration both speed and direction will be altered.
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Acceleration• Mathematically, acceleration is defined as:
Acceleration is measured in meters per seconds squared:
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Acceleration• Example:
• A car experiences a change in velocity from 7-m/s to 12-m/s, which is caused by an acceleration of 2.5-m/s2. How long was the car accelerating?
• In a time period of 4 seconds a truck accelerates from rest to a velocity of 10-m/s. What was the acceleration experienced by this truck?
• While driving a car at a velocity of 20-m/s, a person spots a deer standing in the roadway. To avoid striking the animal the person applies the brakes and brings the car to rest. If the brakes caused the car to stop in 2 seconds, what was the vehicle’s acceleration?
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Gravitational Acceleration• The acceleration due to gravity on Earth is approximately
9.8-m/s2.• This acceleration affects all objects that are near the
surface of the planet Earth regardless of their size, shape or mass.
• Example: • Which object accelerates faster: a penny or a feather?
• In the absence of air resistance, both objects fall at the same rate!
• This shows that Earth’s gravitational acceleration affects all objects equally.