Unit 3

Acceleration from gravity is 9.8 meters/sec/sec (SI) or 32 feet/sec/sec (English)

That means that the speed of a falling object increases by about 10 m/sec every second that it falls.

So after 2 seconds, an object is traveling 20m/sec

After 8 seconds it is traveling about 80 m/sec

The distance an object falls can be determined by the formula

Distance = ½ gravity (time)² D= ½ g t²

A rock falls for 4.5 seconds before it hits the ground, how far did it fall?

D = ½ g t² Gravity is 9.8 m/sec/sec or round to 10 to make it easier.

½ (10) (4.5)² 5m/sec² x 20.25 sec² = 101.25 meters.

Speed is a scalar – only needs strength.

Velocity is a vector quantity - needs to show the direction

60 MPH South 400 rpm clockwise

Linear - goes in a straight line: trains, cars, etc.

Rotational - goes in a circle: drill, flywheel, etc.

Average speed = total distance/total time

Displacement

timespeed

You travel 780 miles in 12 hours. Find your average speed.

Average speed = total distance/total time

780 miles/12 hours = 65 miles/hr or 65 MPH

P =per

You travel 200 miles in 4 hours, and then 210 miles in 3 hours. What is your average speed for the entire trip?

Total distance/total time

410 miles/7 hours = 58.57 MPH

Distance = the actual distance traveled

Displacement = the distance on the shortest line between where you started and where you ended up.

Four cities all lie along a straight line as shown in the diagram. A delivery driver departs from City B, drives to City D, and then drives to City A.

1. What distance does the driver cover during the described trip? If it takes him one hour, what is his average speed?

7 + 9 + 9 +7 +5 = 37 miles so 37 MPH

2. What is the driver’s displacement at the end of the described trip? If it takes him 1 hour, what is his average velocity?

5 miles 5 miles/hour (west or left)

Acceleration = Final velocity (vf) – Initial velocity (vi)/time

vf-vi/t or ∆v/t

Tells how much faster you go every second.

Units in m/sec sec

m/sec/sec or m/sec²

You are jogging at 2m/sec. You start to run and reach 10m/sec in 4 seconds. What is your acceleration?

vf – vi/t

10m/sec – 2m/sec divided by 4 sec = 2m/sec/sec or 2m/sec²

Production rate = number produced/time

100 washers come off a conveyor belt in 10 minutes. Find the production rate.

100 washers/10 minutes = 10 washers/min

Angular speed (rotational). There are 6.28 radians in one revolution.

Common units for angular speed are: rev/min (rpm) rad/sec

A flywheel goes 180 revs in 10 minutes. Find angular speed in revs/min and rad/sec.

To find rad/sec: first change minutes to seconds (divide by 60) then times by 6.28 to change rev to radians.

180 rev/10 min = 18 rev/min (rpm)

18 rev/min divided by 60 sec = 0.3 rev/sec x 6.28 rad/rev. = 1.88 rad/sec.