Linear Motion Chapter 2

Relative Motion Everything moves, even things at rest Everything moves, even things at rest Relative – regarded in relation to something else; depends on point of view, or frame of reference Relative – regarded in relation to something else; depends on point of view, or frame of reference A book at “rest”, relative to you, is actually moving at 30 km/s with respect to (w.r.t.) the sun, and even faster w.r.t the center of the galaxy A book at “rest”, relative to you, is actually moving at 30 km/s with respect to (w.r.t.) the sun, and even faster w.r.t the center of the galaxy Unless stated otherwise, when we discuss speeds of things in our environment, we mean speed w.r.t. the surface of Earth Unless stated otherwise, when we discuss speeds of things in our environment, we mean speed w.r.t. the surface of Earth Motion is relative! Motion is relative!

Motion is Relative!

Speed Speed – The measure of how fast something is moving; the rate at which distance is covered Speed – The measure of how fast something is moving; the rate at which distance is covered Instantaneous Speed – The speed at any instant in time Instantaneous Speed – The speed at any instant in time Average Speed – The total distance covered divided by the total time Average Speed – The total distance covered divided by the total time Speed = Δ Distance/Δ Time Measured in meters/second (m/s)

Average vs. Instantaneous Speed

Velocity Velocity – Speed in a given direction Velocity – Speed in a given direction Constant Velocity – Requires that both constant speed and constant direction must be maintained Constant Velocity – Requires that both constant speed and constant direction must be maintained Changing Velocity – Either the speed or the direction changes, the velocity changes Changing Velocity – Either the speed or the direction changes, the velocity changes Measured in meters/second (m/s) Measured in meters/second (m/s)

Acceleration Acceleration – The rate at which velocity is changing Acceleration – The rate at which velocity is changing Term applies to both increases and decreases in speed (difference between positive and negative acceleration) Term applies to both increases and decreases in speed (difference between positive and negative acceleration) Acceleration= Δ Velocity/Δ Time Measured in meters/second² (m/s²) Measured in meters/second² (m/s²)

Acceleration applies to changes in direction also; when the direction changes, the acceleration changes Acceleration applies to changes in direction also; when the direction changes, the acceleration changes Most of the time we will concern ourselves with motion in a straight line, and can look at the change in speed Most of the time we will concern ourselves with motion in a straight line, and can look at the change in speed Gravity – The acceleration that causes objects to move towards the Earth or other large objects Gravity – The acceleration that causes objects to move towards the Earth or other large objects Gravity = 9.8 m/s²

Velocity vs. Acceleration

Observe the animation of the three cars below. Which car or cars (red, green, and/or blue) are undergoing an acceleration? Study each car individually in order to determine the answer. If necessary, review the definition of acceleration.

Free Fall Free Fall – Objects that are only affected by gravity, a lack of air resistance Free Fall – Objects that are only affected by gravity, a lack of air resistance Elapsed Time – The time that has passed since the beginning of the fall Elapsed Time – The time that has passed since the beginning of the fall We can find the instantaneous speed of an object in free fall from the equation: We can find the instantaneous speed of an object in free fall from the equation: Instantaneous speed = acceleration x elapsed time v = gt

The Elephant and the Feather, Free Fall

How Far? You can find the distance fallen in free fall by using the equation: You can find the distance fallen in free fall by using the equation: Distance = ½ acceleration x time² d = ½ at²

Air Resistance Air resistance is responsible for the differences in accelerations that we see between an elephant and a feather Air resistance is responsible for the differences in accelerations that we see between an elephant and a feather With a lack of air, these two items would fall at the same rate With a lack of air, these two items would fall at the same rate Air resistance less noticeably effects more condense objects (i.e. baseballs and stones) Air resistance less noticeably effects more condense objects (i.e. baseballs and stones)

The Elephant and the Feather, Air Drag

Assignment Read Chapter 2 (pg ) Read Chapter 2 (pg ) Do Chapter Assessment #26-50 (pg ) Do Chapter Assessment #26-50 (pg )