2.1 Describing Motion

DESCRIBING MOTION

Is your table moving ? Y

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

change in position rel ative to a reference point

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.

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

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 Reference REVIEW

Object or point from which motion is determined ■ Most common is the earth ■ Motion is a change in position relative to a frame of reference ■

EXAMPLES Example 1: Standing on a back of a motionless pickup truck throwing apples forward at 15 m/s. Observer on the truck: apple going forward at 15m/s Observer on the street: apple moving forward at Example 15m/s 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 Example 3: Sitting at your desk, how fast are you 35m/s moving? Relative to ground: motionless Relative to the sun: 178,200 mi/hr

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

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If you are standing in one place, and your friend walks by you, are you moving relative to your friend? – Is your friend moving relative to you? – Is either of you moving relative to theareearth? You moving relative to your friend, and your friend is moving relative to you! You are not moving relative to the earth, but your friend is. You are both moving relative to the sun!

If you and your friend are walking down the hall together at the same speed, in the same direction, are you moving relative to your friend? Is your friend moving relative to you? Are either of you moving relative to the earth? You are NOT moving relative to your friend, and your friend is NOT moving relative to you. You both are moving relative to the earth

DISTANCE VS. DISPLACEMENT When moving from one position to another we move a distance. DISTANCE is the length of a path 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 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 the distance traveled by the skier during these three minutes. Note: He goes from A to B, then B to C, etc. Distance is 420m and displacement is 140m

Scalar a quantity described by magnitude only examples include:

time, length, speed, temperature, mass, energy

Vector a quantity described by magnitude and direction examples include:

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

VECTORS They are represented graphically as arrows. The length of the arrow corresponds to the magnitude of the vector. The direction the arrow points is the vector direction.

mples include: A = 20 m/s at 35° NE

C = 5.8 mph/s west

B = 120 lb at 60° SE

What’s your Vector Victor?

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

i

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A car travels 6 miles East and then 8 miles North. Determine the distance traveled by the car. Draw and describe the displacement vector of the car.

10  m

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53o N of E 6 mi E

8 mi N

Vector Addition

vectors may be added graphically or analytically

Triangle (Head-to-Tail) Method 1. 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 SCALARS REVIEW Vectors have both magnitude and direction. Vectors can be represented by arrows. The length of the arrow represents the magnitude, whereas the point of the arrow represents the direction. A resultant vector is the “vector sum” of two or more vectors. Think of it as the displacement Most all Physics quantities can be described as either a vector or scalar

Distance is a scalar quantity and tells you only the magnitude (number sum only) of a path taken.

Displacement is the shortest distance between the starting point and the end point.

Distance vs. Displacement ? displacement

? distance

The track racing Granny’s go three laps around the 1 yard track. What distance did the Granny’s 3 yards go? What displacement did the Granny’s 0 experience --They went in a circle. Starting and stopping in theyards same place means

0 displacement