1) Speed: the rate at which an object moves or the rate of change of positionIt depends on distance and time.

2) Average speed is found by dividing the total distance by the total time.

Speed= distance time

3) Constant means “does not change” so constant speed is speed that does not change.

Like cruise control in a car!!

Since speed is a ratio of distance over time, the units for speed are distance units over time units.

3.2 Speed vs. time graphs

• The position vs. time graph has position on the y-axis and time on the x-axis.

Which runner has the fastest constant speed?

4) Velocity is speed in a direction.Ex: 55 m/s NORTH

If you change speed, OR you change direction, you’ve changed velocity

Let’s check:

Which of the following are velocity and which are speed?

a) 25 m/s forwardb) 1500 km/hc) 55 m/h southd) 35 m/s up

5) Acceleration is the rate of change of speed –

it is the change in speed divided by the change in time.

acceleration can be an INCREASE in velocity or a DECREASE in velocity OR a change in direction

6) If there is no change in speed,

but there is a change in direction

acceleration has occurred.

3.3 Acceleration and direction

• A car driving around a curve at a constant speed is accelerating because its direction is changing.

7) If the value for acceleration is positive, the object is speeding up. If the value is negative the object is slowing down.(also called deceleration)

You do the math:1) What is the speed of an object that

travels 40 meters in 2 seconds?

2) What is the speed of an object that travels 50 meters in 100 seconds?

3) How fast is a car accelerating when it speeds up from 50 mph to 60 mph in ten seconds?

3.3 Acceleration and motion graphs• The position vs. time graph shows acceleration

more clearly.• This graph is a curve when there is

acceleration.

8) An object is in free fall if it is accelerating due to the force of gravity and no other forces are acting on it.

Objects in free fall on Earth accelerate downward.(that means they get faster as they fall)

9) Falling objects increase their speed by 9.8 m/s every second, or 9.8 m/s2 ;

this is their acceleration due to gravity.

10) A projectile is an object moving under the influence of only gravity.

A moving soccer ball is an example of a projectile.

EXTRAS!

3.2 The position vs. time graph

• Position vs. time data tells you the runner’s position at different points in time.

• The runner is at 50 meters after 10 sec., 100 meters after 20 sec. and 150 meters at 30 sec.

3.2 Graphs show relationships

• A good way to show a relationship between two variables is to use a graph.

• A graph makes it easy to see if changes in one variable cause changes in the other variable (the effect).

3.2 Slope

• You can use position vs. time graphs to quickly compare the speeds of different objects.

A steeper line on a position vs. time graph means a faster speed.

3.2 Speed vs. time graphs

These graphs each show the same event.What differences do you notice?

Do these graphs display the same data?