MOTION:Speed and Velocity

Chapter 8 Section 1 Part 1

MotionMotion

• Motion is just a change in position.Motion is just a change in position.

• How do we know that an object is How do we know that an object is moving?moving?– An object is moving if its position An object is moving if its position

changes against some background changes against some background that stays the same.that stays the same.

– We call this background a We call this background a reference reference frameframe..

Speed

• Speed describes how fast an object moves.

• An object that covers equal distances in equal amounts of time is moving at constant speed.

• Most objects do not move at constant speed. Instead, we will work with a quantity called average speed.

• To find average speed, you must measure two quantities:

– The distance traveled by an object– The time it takes to travel that distance

• To calculate average speed, divide the distance of the object by the time it takes to travel that distance.

• Speed (v) = distance (d) / time (t)

d

v t

Here is a little bug

located at 0 cm at 0 seconds.

10 seconds later,

the little bug is located at 50 cm.

• The distance traveled by the little bug = 50 cm

• The time it took the little bug to travel that distance = 10 seconds

• The speed of the little bug is just his distance divided by how long it took him.

• Little bug’s speed = 50 cm/10 s

= 5 cm/s

• Speed only tells you how fast something is moving.

• Speed is always a positive number.

• The SI unit for speed is m/s. Speed is often also measured in other units such as km/h or mi/h.

Comparing Speed Units

Mi/h Km/h m/s

12 20 6

25 40 11

37 60 17

50 80 22

62 100 28

75 120 33

Velocity

• Velocity is both speed and direction.

• Velocity is how fast and which way.

• The direction of motion can be described in various ways such as: – North, south, east and west– Along the water, toward the house, out of town

etc…– Positive or negative along a line of motion

• NOTE: Velocity will always be considered positive in the direction of motion in our book!

Example: (Page 255)

• Metal stakes are sometimes placed in glaciers to help measure a glacier’s movement. For several days in 1936, Alaska’s Black Rapids glacier surged as swiftly as 89m per day down the valley. Find the glacier’s velocity in meters per second. Remember, velocity includes the direction of motion.

• List the given and unknown values.– Time, t = 1 day = 86,400 s– Distance, d = 89 m– Velocity, v = ? m/s down the valley

• Write the equation for velocity.

• v = d / t

d

v t

• Insert the known values and solve.

v = d = 89m = .0010 m/s down t 86,400 s the valley

• Now try the Practice problems at the bottom of page 255 # 1-4.

• Hints: #3. 12 km/h = 12,000 m/h #4. 2.6 km = 2,600 m

Remember to use your velocity triangle!

1) Find the velocity in meters per second of a swimmer who swims exactly 110 m toward the shore in 72 s.

2) Find the velocity in meters per second of a baseball thrown 38 m from third base to first base in 1.7 s.

3) Calculate the distance in meters a cyclist would travel in 5.00 hours at an average velocity of 12.0 km/h to the southwest.

4) Calculate the time in seconds an Olympic skier would take to finish a 2.6 km race at an average velocity of 28 m/s downhill.

• So… are speed and velocity the same?

• A car traveling at constant speed turns the corner.– Does the speed change?

•The speed does not change, it remains constant.

– Does the velocity change?•The velocity changes because the

direction changes.

Last Thoughts…