Chapter 2 - Motion

Distance vs. Displacement An important part of describing the motion of an object is to describe how far it has moved, which is distance. The SI unit of length or distance is the meter (m). Longer distances are measured in kilometers (km).

Distance vs. Displacement Displacement is the distance and direction of an object's change in position from the starting point.

Displacement The runner’s distance is 80 meters. The runner’s displacement is 20 m north.

THINK! Write an example in your note-taker of a time when your distance was different than your displacement. Be specific!

Speed Speed is the distance an object travels per unit of time. The speed of an object can be calculated from this equation: SI unit for speed = m/s

Average Speed vs. Instantaneous Speed Average speed describes speed of motion when speed is changing. Average speed is the total distance traveled divided by the total time of travel.

THINK! A cross-country runner travels the first kilometer of a race in 10 minutes. She runs the next 2 kilometers in 14 minutes. What is her speed after the first kilometer? S = d ÷ t = 1 km ÷ 10 min 0.10 km/min What is her average speed at the end of the race? S = total distance ÷ total time S = 3 km ÷ 24 min 0.125 km/min

Instantaneous Speed Instantaneous speed is the speed at a given point in time. The speed shown on a speedometer is the instantaneous speed.

Distance vs. Time Graph On a distance-time graph, the time is plotted on the x-axis and the distance on the y-axis. The slope of a line on a distance-time graph is equal to the speed. The steeper the slope, the faster the speed.

THINK! On your note-taker draw 3 sketches of distance-time graphs For a slow moving object For a fast moving object For an object not moving

Velocity Velocity describes the speed and direction of a moving object. When you describe an object’s velocity you need to include the speed and direction!

Think about it! An escalator is moving down at a rate of 2 m/s. You run down the escalator at a rate of 3 m/s. What is your velocity? 2 + 3 = 5 m/s down

Think about it! An escalator is moving down at a rate of 4 m/s. You run up the escalator at a rate of 3 m/s. What is your velocity? 4 - 3 = 1 m/s down

Acceleration, Speed and Velocity Acceleration is the rate of change of velocity. When the velocity of an object changes, the object is accelerating. Three ways to change velocity or accelerate: Speed up Slow down Change direction

Think about it! The people on this ride are moving at a constant speed, but they are still accelerating…… HOW? WHY?

Positive Acceleration 2 types of Acceleration Positive Acceleration

Negative Acceleration 2 types of Acceleration Negative Acceleration

Calculating Acceleration To calculate the acceleration of an object, the change in velocity is divided by the length of time interval over which the change occurred.

Let’s Practice… Suppose a jet airliner starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 s. Initial velocity = 0 m/s Final velocity = 80 m/s Time to change = 20 s

More Practice (Calculating Negative Acceleration) Now imagine that a skateboarder is moving in a straight line at a constant speed of 3 m/s and comes to a stop in 2 s. a= vf- vi = 0-3 = -1.5 m/s2 t 2

That’s all for now! Let’s go calculate your speed and acceleration….Are you faster than a cheetah? A snake? A 3-toed sloth?