DESCRIBING MOTIONDESCRIBING MOTION

Is your table Is your table moving ?moving ?Is your table Is your table moving ?moving ?

YES!

A reference point is needed to describe motion. Thus, your table is moving if your reference point is the sun, but not if its the earth.

A c h a n g e i n A c h a n g e i n p o s i t i o n r e l a t p o s i t i o n r e l a t i v e i v e t o a t o a r e f e r e n r e f e r e n c e c e p o i n t .p o i n t .

Frame of reference-any spot you are doing your measurement from as long as it is not accelerating.

Motion is determined by your particular frame of reference. This is what is meant by relative motion. The motion is relative to (or depends upon) your frame of reference.

For a simple example, consider two people standing, facing each other on either side of a North-South street. A car drives past them heading South. For the person facing East, the car was moving toward the right. However, for the person facing West, the car was moving toward the left. This discrepancy is due to the fact that the two people used two different frames of reference from which to investigate this system.

Frames of ReferenceFrames of Reference

Object or point from which motion Object or point from which motion is determinedis determined

Most common is the Most common is the

earthearth Motion is a changeMotion is a change

in in positionposition relativerelative to to

a frame of referencea frame of reference

REVIEW

EXAMPLESEXAMPLESExample 1: Standing on a back of a motionless pickup Example 1: Standing on a back of a motionless pickup

truck throwing apples forward at 15 m/s.truck throwing apples forward at 15 m/s.

Example 2: Truck moving at 20 m/s Example 2: Truck moving at 20 m/s

Observer on the truck: apple going forward at 15m/s

Observer on the street: apple moving forward at 15m/sObserver on the truck: apple going forward at 15m/s

Observer on the street: apple moving forward at 35m/s Example 3: Sitting at your desk, how fast are you Example 3: Sitting at your desk, how fast are you

moving?moving?Relative to ground: motionless

Relative to the sun: 178,200 mi/hr

DID YOU KNOW OR DO YOU DID YOU KNOW OR DO YOU CARE?CARE?

Frames of reference and relative motion is Frames of reference and relative motion is actually the reason that people get car sick. actually the reason that people get car sick. Your brain is getting two different sets of Your brain is getting two different sets of information about your body’s motion that information about your body’s motion that might not exactly agree with each other; might not exactly agree with each other; information from your eyes, and information from your eyes, and information from your inner ear. Some information from your inner ear. Some people are more sensitive to these people are more sensitive to these differences, which causes them to feel car differences, which causes them to feel car sick as they watch the road “whiz” by. If sick as they watch the road “whiz” by. If you are prone to getting car sickness, try to you are prone to getting car sickness, try to look forward at a point far in the distance look forward at a point far in the distance and stay focused on that.and stay focused on that.

If you are standing in one place, and If you are standing in one place, and your friend walks by you, are you your friend walks by you, are you moving relative to your friend?moving relative to your friend?– Is your friend moving relative to you?Is your friend moving relative to you?– Is either of you moving relative to Is either of you moving relative to

the earth?the earth?

You are moving relative to your friend, and your friend You are moving relative to your friend, and your friend is moving relative to you!is moving relative to you!You are not moving relative to the earth, but your friend You are not moving relative to the earth, but your friend is. You are both moving relative to the sun!is. You are both moving relative to the sun!

If you and your friend are walking down If you and your friend are walking down the hall together at the same speed, in the hall together at the same speed, in the same direction, are you moving the same direction, are you moving relative to your friend?relative to your friend?

Is your friend moving relative to you?Is your friend moving relative to you?Are either of you moving relative to the Are either of you moving relative to the earth?earth?

You are NOT moving relative to your You are NOT moving relative to your friend, and your friend is NOT moving friend, and your friend is NOT moving relative to you. You both are moving relative to you. You both are moving relative to the earthrelative to the earth

DISTANCEDISTANCEVS.VS.

DISPLACEMENTDISPLACEMENT

When moving from one position to another we When moving from one position to another we move a distance. DISTANCE is the length of a path move a distance. DISTANCE is the length of a path between two points.between two points.

DISPLACMENT is the direction from the starting point and the length of a straight line from the starting point to the ending point

Displacement along a straight lineDisplacement along a straight line

Car A: had a distance and displacement of 6km

Car B: went a distance of 6km but had a displacement of just 2km

Use the diagram to determine the resulting displacement and Use the diagram to determine the resulting displacement and the distance traveled by the skier during these three minutes. the distance traveled by the skier during these three minutes. Note: He goes from A to B, then B to C, etc.Note: He goes from A to B, then B to C, etc.

Distance is 420m and displacement is 140mDistance is 420m and displacement is 140m

ScalarScalar

a quantity described by magnitude only

examples include:

time, length, speed, temperature, mass, energy

VectorVectora quantity described by magnitude and direction

examples include:

velocity, displacement, force, momentum, electric and magnetic fields

They are represented graphically as arrows.

The length of the arrow correspondsto the magnitude of the vector.The direction the arrow points

is the vector direction.

Examples include:A = 20 m/s at 35° NE B = 120 lb at 60° SE

C = 5.8 mph/s west

VECTORS

What’s your Vector Victor?What’s your Vector Victor?

A car travels 6 miles East and then 8 miles A car travels 6 miles East and then 8 miles North.North.

Determine the distance traveled by the car. Determine the distance traveled by the car. Draw and describe the displacement vector Draw and describe the displacement vector

of the car.of the car.

6 mi E

8 mi N10

mi

53o N of E

So the car’s displacement is 10 miles 53o N of E

Vector AdditionVector Additionvectors may be added graphically or analytically

TriangleTriangle ( (HeadHead--toto--TailTail) ) MethodMethod1. Draw the first vector with the proper length and orientation.

2. Draw the second vector with the proper length and orientation originating from the head of the first vector.

3. The resultant vector is the vector originating at the tail of the first vector and terminating at the head of the second vector.

4. Measure the length and orientation angle of the resultant.

VECTORS AND SCALARSVECTORS AND SCALARSREVIEWREVIEW

Vectors have both magnitude and Vectors have both magnitude and direction.direction.

Vectors can be represented by arrows. Vectors can be represented by arrows. The length of the arrow represents the The length of the arrow represents the magnitude, whereas the point of the magnitude, whereas the point of the arrow represents the direction.arrow represents the direction.

A resultant vector is the “vector sum” of A resultant vector is the “vector sum” of two or more vectors. Think of it as the two or more vectors. Think of it as the displacementdisplacement

Most all Physics quantities can be Most all Physics quantities can be described as either a vector or scalardescribed as either a vector or scalar

Distance is a scalar quantity Distance is a scalar quantity and tells you only the and tells you only the

magnitude (number sum only) magnitude (number sum only) of a path taken.of a path taken.

Displacement is the shortest Displacement is the shortest distance between the starting distance between the starting

point and the end point. point and the end point.

Distance vs. DisplacementDistance vs. Displacement

?

?

displacement

distance

The track racing Granny’s go three The track racing Granny’s go three laps around the 1 yard track.laps around the 1 yard track.

What distance did the Granny’s go?What displacement did the Granny’s experience

3 yards

0 yards--They went in a circle. Starting and stopping in the same place

means 0 displacement