# MOTION: Describing and Measuring Motion

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MOTION: Describing and Measuring Motion
CHAPTER 10

Measuring Motion Motion is described as a change in position
An object is considered in motion when observed in relation to a REFERENCE POINT. Describe motions with reference terms such as: north, south, east, west, up, down, right, left, etc.

Frame of Reference The object or point from which movement is determined Any object can appear to be in motion or at rest depending on the frame of reference of the observer. Motion can only be determined when there is a frame of reference Earth is the most commonly used frame of reference

Frame of Reference and Relative Motion
Relative Motion is based on the person describing the motion based on their frame of reference. Example: When you are standing on the ground, that is your frame of reference. Anything you see, watch, or measure will be compared to the reference point of the ground.

Frame of Reference Frame of reference for 2 girls…
Red Dress Girl uses the moving vehicle As her frame of reference and is not Moving Yellow Dress girl uses her frame of Reference as a stationary position to See the other girl in motion. Frame of Reference for the Skydivers Plane – in motion away from them Themselves – not in motion The ground – in motion towards them

Example 1: Sitting at your desk, how fast are you moving?
Relative to the ground: ZERO Relative to the sun: 2.97 X104 m/s! An observer standing on the sun would say you are moving at 2.97 X104 m/s

Example 2: While sitting at a red light a bus stops in the lane next to you…as you are daydreaming looking at the side of the bus…you all of a sudden feel like you are rolling backwards Then you realize it is the bus moving forwards when you take into account a stationary frame of reference. This is your brain “confused”

Did you know? Motion Sickness is caused by your brain getting two different sets of information about your body’s motion based on its frame of reference, the information from your eyes and the information from your inner ear To help with motion sickness, try to look forward at a point far in the distance and stay focused on that.

Distance vs. Displacement
Vector (amount and direction) Direction DOES matter! The path does NOT matter! The straight-line distance (and direction) the object has travelled from its starting point. In the Daytona 500, the cars experience zero displacement (they start where they end). Scalar (amount only) Direction does NOT matter. The actual path matters! The total distance traveled. In the Daytona 500, the cars travel a distance of 500 miles.

Distance vs. Displacement
What is the overall distance traveled in the picture below? What is the overall displacement in the picture below? Leg 1 Distance = ______ Leg 1 Displacement = _____ Leg 2 Distance = ______ Leg 2 Displacement = _____ Leg 3 Distance = ______ Leg 3 Displacement = _____ Final Distance = ______ Final Displacement = _____ Leg 2 Leg 1 START Leg 3 End

Distance vs. Displacement

Vector Diagrams The objects motion is represented with an arrow in the direction traveling. The size of the arrow indicates the relative speed of that object.

Practice with Frames of Reference / Distance and Displacement
Worksheet as a class fun with mapping … check out Santa’s route to your house!

A little lesson through music
Speed and Velocity A little lesson through music

SPEED SPEED – the rate of change in position or rate of motion
INSTANTANEOUS SPEED – Rate of motion at any given instant (ex: speedometer in a car) CONSTANT SPEED – A speed that does not vary (ex: cruise control) Usain Bolt races at ESPN video clip Usain Bolt Olympics

Constant Speed vs. Instantaneous Speed….Let’s graph

CALCULATING SPEED When we calculate speed, we are calculating the average speed traveled. AVERAGE SPEED – a measure of total distance traveled divided by total time of travel SPEED (v) = DISTANCE (d) TIME (t) UNITS FOR SPEED ---- m/s

DISTANCE-TIME GRAPH The distance covered by an object is noted at regular intervals of time.

Measuring Speed Sport Science - John Wall Sport Science - Lebron James
Sport Science - Ndamukong Suh

VELOCITY VELOCITY – Describes both speed AND direction.
Velocity can change even if the speed of the object does not change. Calculating Velocity: Velocity = Displacement / time Answers will include direction

Velocity – Time Graph A velocity – time graph can show acceleration.

Distance-Time Graph vs. Velocity Time Graph
Shows velocity of object, time. Can calculate Acceleration Shows Motion, distance traveled, and time. Can calculate speed.

ACCELERATION ACCELERATION – The rate of change of velocity.
Acceleration can be a change in speed OR direction Circular motion is a constant acceleration Example: blades of a fan

Acceleration…Let’s graph

Constant Velocity = Zero Acceleration

Positive Velocity = Positive Acceleration

CALCULATING ACCELERATION
Divide the change in velocity by the time interval Acceleration (a) = (final velocity – initial velocity) time interval a = (vf – vi) t * Unit for Acceleration: m/s2 (plus direction)

Acceleration Practice
Natalie accelerates her skateboard along a straight path from 0 m/s to 4.0 m/s in 2.5 seconds. Find her average acceleration. a = m/s – 0 m/s 2.5 s a = 1.6 m/s2 along her path * Practice: p. 328 # 2-5

Example: Positive Acceleration
Acceleration = (30 m/s – 0 m/s)/10 s a = 3 m/s/s The car’s acceleration is increasing 3 m/s2

Examples: Negative Acceleration
a = (0m/s – 20 m/s)/10 s a = - 2 m/s/s The cars acceleration is decreasing at 2 m/s2

MOTION AND FORCE FORCE – A push or a pull one object puts on another.
BALANCED FORCE – Forces that are equal in size and opposite in direction. UNBALANCED FORCE (NET FORCE) – Forces that are unequal in size…Cause a change in motion. Unbalanced forces always change the velocity of the object.

Balanced vs. Unbalanced Forces
Balanced force pushing on each other Balanced Force opposite in direction - pulling Unbalanced forces….showing displacement of the object Unblalanced Forces – adding up to displace the object in one direction

Forces that Act on Objects
1. Normal Force – A support force when one object is in contact with a stable object (ex: Book on a table) 2. Tension Force – A force transmitted through a string, rope, or cable 3. Gravitational Force – The force of attraction to a very large object in the universe 4. Friction Force – The force that opposes motion between two surfaces.

Normal Force vs. Gravity Force

Tension Force

Friction Force

FRICTION FRICTION – The force that opposes motion between two surfaces that are touching each other. 2 TYPES OF FRICTION: STATIC FRICTION – The friction between surfaces that are stationary. KINETIC FRICTION – The friction between moving surfaces. Kinetic Friction can be SLIDING or ROLLING

Increasing or Decreasing Friction
Increase Friction by using rough surfaces EX: Cleaning surfaces with a rough sponge Decrease Friction by adding a liquid Ex: Oil, WD40

Gravity and Force

Gravity is a Field Force
All objects in the universe have a gravitational force. Larger objects exert more force than smaller objects.

Calculating Gravitational Force
Gravitational force (weight) of an objects is proportional to its mass Force (gravity) = (mass) (gravity) Acceleration due to Gravity (g) is equal to 9.8 m/s2

Newton’s Second Law An unbalanced force acting on an object causes that object to change speed or direction Acceleration = Force / Mass

Action - Reaction Every action has an equal and opposite reaction.
Example: The truck hits the sign therefore the sign hits the truck with an equal force in the opposite direction.