Ch 11 Motion

Please define these vocabulary words:

• Distance• Speed• Average speed• Instantaneous speed• Velocity• Acceleration• Free fall • Constant acceleration

11.1: Distance and Displacement

Distance…•length of a path between two points

•When an object moves in a straight line, the distance is the length of the line connecting the object’s starting point and its ending point.

Measuring Distance• The SI unit for measuring distance is the

meter (m). • For very large distances, it is more

common to make measurements in kilometers (km).

• Distances that are smaller than a meter are measured in centimeters (cm).

Displacement…• DIRECTION from the starting

point and the LENGTH of a straight line from the starting point to the ending point.

Distance vs Displacement• How are distance and displacement

different? –Distance=length –Displacement=direction and the

length

Warm Up 3/24/09Fill in the Blanks…

( write complete statements)

1. EVERYTHING is in constant _ _ _ _ _ _ .2. Distance is the _ _ _ _ _ _ between a starting point

and an ending point.3. Displacement involves the _ _ _ _ _ _ between two

points and the _ _ _ _ _ _ _ _ _ it is traveling.4. In order for an object to move, a _ _ _ _ _ has to

act upon it.5. The _ _ _ _ of an object affects how much force is

required to move it.

11.2: Speed and Velocity

Speed-The ratio of the distance an object moves to the amount of time the object moves.

-The SI unit of speed is meters per second (m/s).

-Two ways to express the speed of an object are average speed and instantaneous speed.

Formula used to Calculate Speed

Speed=distance/timeS=d/t

Distance (m)/Time (s)=Speed (m/s)

Average Speed vs Instantaneous Speed

• How are instantaneous speed and average speed different? –Average speed is computed for the

entire duration of a trip, and instantaneous speed is measured at a particular instant.

Average Speed• Average speed, is the total distance traveled,

d, divided by the time, t, it takes to travel that distance.

Instantaneous Speed-Instantaneous speed, v, is the rate at which an object is moving at a given moment in time.

-The speedometer in a car measures the car’s instantaneous speed.

-Note the scale markings are given both in km/h and miles per hour, mph.

Graphing Motion• Speed can also be determined by looking at a

distance-time graph.–The slope of a line on a distance-time graph

is the speed of the object.• Slope is the change in the vertical axis

value divided by the change in the horizontal axis value.• Steeper slope means faster speed.

Distance-Time Graphs

Distance-Time Graphs

Distance-Time Graphs

Velocity•Velocity is a description of

BOTH speed and direction of motion. • The unit for velocity is

meters per second.

Practice• A football field is about 100 m

long. If it takes 20 seconds to run its length, how fast (what speed) were they running?

Practice• The pitcher’s mound in baseball

is 85 m from the plate. It takes 4 seconds for a pitch to reach the plate. How fast is the pitch?

Practice• If you drive at 100 km/hr for 6

hours, how far will you go?

Practice• Every summer I drive to

Michigan. It is 3900 km to get there. If I average 100 km/hr, how much time will I spend driving?

Practice• If a delivery truck drives 537 miles in 7 hours, how fast was he driving?

11.3: Acceleration

Velocity• Velocity is a description of BOTH speed and direction of motion. • The unit for velocity is meters per second.

What is acceleration?• The rate at which velocity

changes.• In science, acceleration

applies to any change in an object’s velocity.

What is acceleration?• Acceleration can be caused by positive

(increasing) change in speed or by negative (decreasing) change in speed.

• Deceleration is an acceleration that slows an object’s speed.

• The unit for acceleration is meters per second squared (m/s2).

Free Fall• Free fall is the movement of an object

toward Earth solely because of gravity. • Objects falling near Earth’s surface

accelerate downward at a rate of 9.8 m/s2.

• Each second an object is in free fall, its velocity increases downward by 9.8 meters per second.

Constant Acceleration• Constant acceleration is a steady

change in velocity.• The velocity of an object moving

in a straight line changes at a constant rate when the object is experiencing constant acceleration.

Calculating Acceleration• You calculate acceleration for straight-line

motion by dividing the change in velocity by the total time.

• Acceleration is the rate at which velocity changes. Vi is the initial velocity, vf is the final velocity, and t is total time.

Calculating AccelerationIf the velocity increases, the acceleration is positive. If the velocity decreases, the acceleration is negative.

• If you are coasting downhill on a bicycle, your velocity increases, and your acceleration is positive.

• If you continue coasting on level ground, your velocity decreases, and your acceleration is negative.

Practice

• A parachute on a racing dragster opens and changes the speed of the car from 85 m/s to 45 m/s in a period of 4.5 seconds. What is the acceleration of the dragster?

Practice

• A car traveling at a speed of 30.0 m/s encounters an emergency and comes to a complete stop. How much time will it take for the car to stop if it decelerates at -4.0 m/s2?

Practice

• A helicopter’s speed increases from 25 m/s to 60 m/s in 5 seconds. What is the acceleration of this helicopter?

Practice

• After traveling for 6.0 seconds, a runner reaches a speed of 10 m/s. What is the runner’s acceleration?