PHYSICS

Units:Analyzing MotionWork and Energy

WavesElectricity and Magnetism

TN Standards - Motion

• CLE.3202.3.1: Investigate the relationships among speed, position, time, velocity, and acceleration

• CLE.3202.Inq.2: Design and conduct scientific investigations to explore new phenomena, verify, previous results, test how well a theory predicts, and compare opposing theories

• CLE.3202.Inq.4: Apply qualitative and quantitative measures to analyze data and draw conclusions that are free of bias

• CLE.3202.Inq.6: Communicate and defend scientific findings

Chapter 11 - Motion

• Sections:

• 1 - Measuring Motion• 2 - Acceleration• 3 - Motion and Force

1 - Measuring Motion

• Key Questions:

• How is a frame of reference used to describe motion?

• What is the difference between speed and velocity?• What do you need to know to find the speed of an

object?• How can you study speed by using graphs?

Frame of Reference

• Observe an object in relation to other objects that stay in place, called reference points.

• A frame of reference is used to describe the motion of an object relative to these reference points.

• The trees in the background in Figure 1

Distance

• Distance measures the path taken

• To measure distance, you measure the length of the path that the object took.

• Displacement measures only the difference between the final and starting positions

Speed & Velocity• Speed – how fast an object moved

• Velocity – gives us both speed and direction

• Velocity is described relative to a reference point

• Up and right are positive; left and down are negative ( Cartesian Coordinate system )

+x-x+y

-y

Motion• When an object changes position with respect to

a frame of reference, the object is in motion

D

S t

• Speed: Distance travelled in a certain time period

Calculating Speed

• Need two quantities – distance travelled & time

• Average speed – distance divided by time interval

• Instantaneous speed– speed at a given time– very small time interval– Speedometer in car

D

S t

Motion• When an object changes position with respect to

a frame of reference, the object is in motion

D

V t

• Velocity: Displacement in a certain time period

Demonstration

• Slope of a distance vs. time graph equals speed

• Run a 6 m length ( in class? )

• Have your partner time you

• What is your speed?

Graphing Motion

• Sometimes easier to analyze motion on graphs

2 - Acceleration

• Key Questions:

• What changes when an object accelerates?• How do you calculate the acceleration of an

object moving in a straight line?• How can a graph be used to find acceleration?

Velocity Changes upon Acceleration

• Speed can change, but so can direction

• Notice speed is changing with each second

Changes in Direction

• Uniform circular motion has centripetal acceleration

• Sources: friction, tension

• Demonstration

Circular Motion Demonstration

• Foam ball with string attached to it

• Swing ball with hand

• Diagram features of motion

• What happens when I let go of the string?

2 - Acceleration

• Key Questions:

• What changes when an object accelerates?• How do you calculate the acceleration of an

object moving in a straight line?• How can a graph be used to find acceleration?

BELLWORK

• What changes when acceleration occurs?

2 - Acceleration

• Key Questions:

• What changes when an object accelerates?• How do you calculate the acceleration of an

object moving in a straight line?• How can a graph be used to find acceleration?

Calculating Acceleration•

V

A t

Calculating Acceleration• Acceleration is the rate at which velocity changes

• Acceleration is negative when slowing down

Graphing Accelerated Motion

• The slope of a straight line on a speed vs. time graph is equal to the acceleration.– Constant Acceleration

• Can also be seen from a distance vs. time graph

BELLWORK

• How do we calculate average acceleration?

3 - Motion and Forces

• Key Questions:

• What do scientists identify as the fundamental forces of nature?

• What happens when there is a net force acting on an object?

• What force always opposes motion?• Why is friction sometimes necessary?

Fundamental Forces• A Force is defined as any action that can change

the state of motion of an object– Gravity, electromagnetic force, strong nuclear force,

and the weak nuclear force• Fundamental forces vary in strength• Forces can act through contact or at a distance

Balanced and Unbalanced Forces• Whenever there is a net force acting on an object,

the object accelerates in direction of the net force• Balanced forces do not change motion• Unbalanced forces do not cancel each other

Friction• Is a force

• Always opposes motion

• Static friction – occurs between stationary surfaces• Rolling friction

• Kinetic friction – occurs between moving surfaces– Sliding friction ( usually greater than rolling fiction )

Friction and Motion• Friction is necessary for many everyday tasks to

function properly– Non-stick skillets– Cars would not move without friction!!

• Can increase helpful friction or decrease unwanted friction– Sand on roads make them less slippery– Lubrication reduces friction

Friction and Balance of Forces

• Cars do not or do move based on friction– Constant speed– Acceleration– No Slipping

Demonstration – Spring Scale• Drag wood and measure the force needed to

overcome friction ( various scenarios ):

• 1] single block of wood, wood-side down

• 2] single block of wood, sandpaper-side down

• 3] add another block to scenario #2

• 4] add something else to blocks ( your choice )

Questions

• What two factors appear to affect friction?

• What would make this experiment more conclusive ( or how would you be more confident in your data ) ?

Demonstration – Spring Scale

• Drag wood and measure the force needed to overcome friction

• Friction depends on force applied between surfaces but also surface roughness

• Rock keeps paper from being blown away by wind

• More weight on block more friction

• Rougher surface more friction