Kinematics – Defining Motion

http://www.aplusphysics.com/courses/honors/kinematics/honors_kinematics.html

Unit #2 Kinematics

Objectives and Learning Targets

Understand the difference between distance and displacement and between speed and velocity.

Calculate distance, displacement, speed, velocity, and acceleration.

Solve problems involving average speed and average velocity.

Unit #2 Kinematics

Defining Motion

Position - refers to an objects location at any given point in time. Position is a vector, and its magnitude is given by the symbol x.

Distance – is how far an object travels (in one dimension at a type) from its initial position. Distance is a scalar. It has a magnitude, or size, only. The basic unit of distance is the meter (m).

Unit #2 Kinematics

Sample Problem #1 On a sunny afternoon, a deer walks 1300

meters east to a creek for a drink. The deer then walks 500 meters west to the berry patch for dinner, before running 300 meters west when startled by a loud raccoon. What distance did the deer travel?

Unit #2 Kinematics

Sample Problem #1 On a sunny afternoon, a deer walks 1300 meters

east to a creek for a drink. The deer then walks 500 meters west to the berry patch for dinner, before running 300 meters west when startled by a loud raccoon. What distance did the deer travel?

Answer: The deer traveled 1300m +

500m + 300m, for a total distance

traveled of 2100m.

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Displacement Different from distance, displacement – is how far

an object is from its starting point, or its change in position.

The vector quantity displacement Δx = (x-x0) describes how far an object is from its starting point, and the direction of the displacement vector points from the starting point to the finishing point.

Like distance, the units of displacement are meters (m).

Unit #2 Kinematics

Sample Problem #2 A deer walks 1300 m east to a creek

for a drink. The deer then walked 500 m west to the berry patch for dinner, before running 300 m west when startled by a loud raccoon. What is the deer’s displacement?

Unit #2 Kinematics

Sample Problem #2 A deer walks 1300 m east to a creek

for a drink. The deer then walked 500 m west to the berry patch for dinner, before running 300 m west when startled by a loud raccoon. What is the deer’s displacement?

Answer: The deer’s displacement was 500m east.

Unit #2 Kinematics

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Sample Problem #3

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Sample Problem #3

Speed and Velocity

Knowing only an object's distance and displacement doesn't tell the whole story. Going back to the deer example, there's a significant difference in the picture of the deer's afternoon if the deer's travels occurred over 5 minutes 300 seconds) as opposed to over 50 minutes (3000 seconds).

How exactly does the picture change? In order to answer that question, you'll need to understand some new concepts – average speed and average velocity.

Average speed, given the symbol , is defined as distance traveled divided by time, and it tells you the rate at which an object's distance traveled changes. When applying the formula, you must make sure that x is used to represent distance traveled.

Unit #2 Kinematics

Sample Problem #4 A deer walks 1300 m east to a creek for a

drink. The deer then walked 500 m west to the berry patch for dinner, before running 300 m west when startled by a loud raccoon. What is the deer's average speed if the entire trip took 600 seconds (10 minutes)?

Unit #2 Kinematics

Sample Problem #4 A deer walks 1300 m east to a creek for a

drink. The deer then walked 500 m west to the berry patch for dinner, before running 300 m west when startled by a loud raccoon. What is the deer's average speed if the entire trip took 600 seconds (10 minutes)?

Answer:

Unit #2 Kinematics

Average Velocity

Average velocity, also given the symbol , is defined as displacement, or change in position, over time. It tells you the rate at which an object's displacement, or position, changes. To calculate the average velocity, you divide the displacement by time (remember it’s a vector)

Unit #2 Kinematics

Sample Problem #6 A deer walks 1300 m east to a creek for a drink. The deer then

walked 500 m west to the berry patch for dinner, before running 300 m west when startled by a loud raccoon. What is the deer's average velocity if the entire trip took 600 seconds (10 minutes)?

Unit #2 Kinematics

Sample Problem #6 A deer walks 1300 m east to a creek for a drink. The deer then

walked 500 m west to the berry patch for dinner, before running 300 m west when startled by a loud raccoon. What is the deer's average velocity if the entire trip took 600 seconds (10 minutes)?

Answer:

Unit #2 Kinematics

Avg. Speed vs. Avg. Velocity Notice how the answers for each are vastly

different, the main reason is because distance and speed are scalars; while displacement and velocity are vectors.

A good way to memorize is …

Speed = Scalar

Velocity = Vector

Unit #2 Kinematics

Sample Problem #7 Chuck the hungry squirrel travels 4m east

and 3m north in search of an acorn. The entire trip takes him 20 seconds. Find: Chuck’s distance traveled, Chuck’s displacement, Chuck’s average speed, and Chuck’s average velocity.

Unit #2 Kinematics

Sample Problem #7 Chuck the hungry squirrel travels 4m east

and 3m north in search of an acorn. The entire trip takes him 20 seconds. Find: Chuck’s distance traveled, Chuck’s displacement, Chuck’s average speed, and Chuck’s average velocity.

Answer:

Unit #2 Kinematics

Acceleration What would happen if velocity never changed?

Objects would move at the same speed and direction having the same kinetic energy and momentum.

Acceleration – the rate at which the velocity of an object changes

Unit #2 Kinematics

Acceleration This indicates that the change in velocity divided by the time interval gives you

the acceleration

Acceleration is a vector – it has a direction

the units of acceleration are meters per second per second, or [m/s2]

the units mean is that velocity changes at the rate of one meter per second, every second

an object starting at rest and accelerating at 2 m/s2 would be moving at 2 m/s after one second, 4 m/s after two seconds, 6 m/s after 3 seconds, and so on

Special note is the symbolism for v. The delta symbol ( ) indicates a change in a quantity, which is always the initial quantity subtracted from the final quantity. For example:

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Sample Problem #8 Monty the Monkey accelerates uniformly

from rest to a velocity of 9 m/s in a time span of 3 seconds. Calculate Monty's acceleration.

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Sample Problem #8 Monty the Monkey accelerates uniformly

from rest to a velocity of 9 m/s in a time span of 3 seconds. Calculate Monty's acceleration.

Answer:

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Rearranging Acceleration The definition of acceleration can be rearranged to

provide a relationship between velocity, acceleration and time as follows:

Unit #2 Kinematics

Sample Problem #9 The instant before a batter hits a 0.14-

kilogram baseball, the velocity of the ball is 45 meters per second west. The instant after the batter hits the ball, the ball's velocity is 35 meters per second east. The bat and ball are in contact for 1.0×10-2 second. Determine the magnitude and direction of the average acceleration of the baseball while it is in contact with the bat.

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Sample Problem #9 Answer:

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+ and - Accelerations Because acceleration is a vector and has direction, it's important

to realize that positive and negative values for acceleration indicate direction only.

Take a look at some examples…

http://www.aplusphysics.com/courses/honors/kinematics/honors_motion.html

Be careful with + and – vectors. A + accelerations does not always mean moving to the right, or a – mean moving left

See Phet examples

Unit #2 Kinematics