Chapter 4 Making Sense of the Universe: Understanding Motion, Energy, and Gravity

4.1 Describing Motion Our goals for learning: How do we describe motion? What is a frame of reference? How is velocity different from speed? What is acceleration?

We measure movement relative to a frame of reference, usually an object that appears stationary Earth is the most common frame of reference

What is Motion? Motion: A change in position of an object compared to a reference point

The rate of change in position Speed The rate of change in position this is the distance traveled in a given amount of time

s = d t

Velocity Speed plus direction Example: 50 km/hour north

If either the speed or the direction(or both) change then velocity is changing, so the object is accelerating! What are the three controls on your car that can be used to change your velocity?

The acceleration due to gravity: g = 10 m/s2 Galileo showed that acceleration caused by gravity is the same for all falling objects on earth. The acceleration due to gravity: g = 10 m/s2 The speed of a falling object increases 10 m/s with each second of falling. When you show this video clip, be sure to point out what is going on since it is not easy to see… Apollo 15 demonstration

velocity = acceleration due to gravity x (time) To find the velocity of a falling object after a period of time we use: velocity = acceleration due to gravity x (time) v = gt To find the distance the object fell we use: distance = ½ (accelertion due to gravity ) x (time)2 d = ½ g t2

What have we learned? What is a frame of reference? Stationary point to measure motion against How do we describe motion? Speed = distance / time Speed & direction => velocity If the velocity is changing => acceleration We can use the acceleration due to gravity to find the velocity of a falling object and the distance that it fell