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

4.1 Describing Motion How do we describe motion? How is velocity different from speed? What is acceleration?

Motion Is Relative An object is moving if its position relative to a fixed point is changing.

We measure movement relative to a frame of reference, usually an object that appears stationary Earth is the most common frame of reference Even things that appear to be at rest move.

Although you may be at rest relative to Earth’s surface, you’re moving about 100,000 km/h relative to the sun.

Speed You can calculate the speed of an object by dividing the distance covered by time.

Galileo is credited as being the first to measure speed by considering the distance covered and the time it takes. Speed is how fast an object is moving.

Any combination of units for distance and time that are useful can be used: miles per hour (mi/h) kilometers per hour (km/h) centimeters per day light-years per century In physics we mostly use meters/second (m/s)

If we know average speed and travel time, the distance traveled is easy to find. total distance covered = average speed × travel time For example, if your average speed is 80 kilometers per hour on a 4-hour trip, then you cover a total distance of 320 kilometers.

Velocity is speed in a given direction. Example: 60 km/h = speed. 60 km/h to the north= velocity.

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?

Acceleration A car is accelerating whenever there is a change in its state of motion.

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 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

Free Fall: How Fast During each second of a fall the instantaneous speed of an object increases by an additional 10 meters per second. This gain in speed per second is the acceleration.

Free Fall: How Fast The change in speed each second is the same whether the object is going upward or downward.

Free Fall: How Fast To find the velocity of a falling object after a period of time we use:

Free Fall: How Far At the end of time t, the object starting from rest falls a distance d.

You can describe the motion of an object by its position, speed, direction, and acceleration.