Friday

Friday

What are we going to do?

• Record data for 2 runs more than minimum block length

• Conversions

• Homework!

• Review

• Homework Handout #1

• Motion Detectors

Main point

• How to accurately describe the motion of objects

MPV’s

• I will be recording data to see who has moved their car past the minimum distance twice

• Get your papers out, complete all calculations

• Task two job

Task 2 (Due Wednesday)

• The results must by typed or printed on an index card (3x5 or 4x6) in a table that includes:

– For each run: overall displacement/ distance/ time/ velocity

– The correct unit value for each number– The mean of the 3 velocity times– Discuss at least 2 specific reasons why the values may

not be absolutely correct (sources of error)

A drawing or photo of the car used to gather evidence is to be attached to the other side of the index card

Conversions

• If you are converting to a smaller unit, multiply the measurement to get a bigger number.

• If you are converting to a larger unit, divide the measurement to get a smaller number

Conversion Factors we use :

• 1 meter = 100 centimeters

• 1 meter = 1000 millimeters

• 1 kilometer = 1000 meters

• Interchange meter for, seconds, grams, Newtons, Joules…

Time conversions

• 1 minute = 60 seconds

• 1 hour = 60 minutes

• 1 day = 24 hours

• 1 year = 365 days (approximation)

Example problem

• Convert 1.85 meters into centimeters

• Since meters are larger than centimeters, we are converting to a smaller unit

• We multiply 1.85 by the converting factor (100) = 185 cm

Mathematically:

• 1.85 m x 100 cm = 185 cm 1 m

Example, you try:

• Convert 645 seconds into the same number of minutes

Answer

• Since we are converting to a larger unit, we divide by factor

• 645 sec / 60 = 10.75 min

Another way to do the solution

• Dividing is the same as multiplying by the inverse

• 645 / 60 = 645 x 1/60

• 645 sec x 1 min/ 60 sec = 10.75 min

Try these

• 456.3 mm (millimeters) = _____ m

• 456.3 /1000 = .4563 m

• 8 days = ______ min

• 8 x (24) x (60) = 11520 min

Page 17

• Write 550 millimeters as meters– Convert to bigger, divide– 550/1000 = .550 m

• Write 3.5 seconds as milliseconds– Convert to smaller, multiply– 3.5 x 1000 = 3500 millseconds

Average velocity

• Displacement / time

• Units are m/s

• V = d / t

Book Problems

• Find the velocity in m/s of a swimmer who swims 110 m toward the shore in 72 s

• Answer v = d/t = 110 m/ 72 s = 1.5 m/s

Problem 2

• Find the velocity in m/s of a baseball thrown 38 m from third base to first base in 1.7s.

• Answer v = d/t = 38 / 1.7 = 22 m/s

Problem 3

• Calculate the displacement in meters a cyclist would travel in 5.00 h at an average velocity of 12.0 km/h to the southwest.

• Answer:– Rearrange equation v = d/t to d = vt– D = vt = 12.0 km/h x 5.00 h = 60 km– Convert 60 km into meters

• Convert to smaller units means multiply– 60 km (1000 m/1 km) = 60,000 m

Manipulating equations

• V = d / t

• Vt = dt / t t’s cancel on right

• Vt = d same as d = vt

Ex: problems

• A) How long will it take you to travel a length 307 m at 6.4 m/s?

• Ans: v = d/t 6.4 = 307 /t t =307/6.4 = 48s

• If a car travels 150 blocks in 34.3 seconds, what is it’s average velocity in m/s.

• Ans: convert blocks to meters- 150 x .305 = 45.8m

• Then calc. vel. 45.8m /34.3s = 1.34 m/s

Try this

• If you can go 30 meters in 2.0 second, what is your average velocity in m/s?

• Answer: 30/2 = 15 m/s• What is your average velocity in km/h?• Ans: to convert to km, you divide 30 by

1000 = .03• to convert to hours, you divide 2 by

3600 = .000555• Divide .03/ .000555 = 54.05

Or

• To convert from m/s to km/h multiply by 3.6

• To convert from km/h to m/s divide by 3.6

Due Monday

• Homework Handout #1

• You should get a copy from me in class

• There is a copy on website

Motion Detectors

• Gather much more information on motion during any given time period

• Uses sound to measure position

• Produces a visual report on motion information ( A graph)