Describing and Measuring Motion When is an object

Describing and Measuring Motion When is an object in motion? How do you know an object’s speed and velocity? How can you graph motion?

When an object moves, its position changes as a function of time. A reference point is used to determine if an object has moved. It should be a stationary object. A change in position is called a displacement.

It is not always enough to just specify the distance from the reference point. We also have to specify the direction for most objects. Think of an airplane pilot making a landing. Why does she have to know the position, speed and direction of flight of other pilots?

Relative motion- Depends on the reference point. Objects which are moving together do not appear to have any motion. Think of your motion in a car that is moving. You are not moving relative to the car.

Graphing Motion Slope is the steepness of a line. It can help you determine how fast an object is moving. The steeper the slope the faster an object is moving. A horizontal line on a distance vs time graph shows motion stopping.

Measuring Speed and Distance Scientists use the metric system to measure distance. The meter is the unit of length. Prefixes help you to determine the unit of measurement. Measure accurately and use a meter stick. Instantaneous speed is not the same as average speed. Instantaneous is at a specific time Average is over a period of time Velocity is speed and direction.

• • • DISTANCE-TIME GRAPHS Time is always plotted on the X-axis Distance is plotted on the Y-axis (side of the graph). If an object is moving at a constant speed, it means it has the same increase in distance in a given time:

Calculating Speed • To find the speed of an object you need to know the distance it traveled and how long it took to go that distance. • SPEED and VELOCITY EQUATION: • speed = distance OR v= d • time • Always give a direction for velocity t

Constant Velocity Changing Velocity (acceleration) Negative (-) Velocity Slow to Fast Velocity Fast to Slow

ACCELERATION • Acceleration is the rate at which an object changes its motion. – Increasing speed – Decreasing speed – Changing direction a car accelerating around a curve, it does not have to change its speed ANYTHING MOVING IN A CIRCLE IS ACCELERATING EVEN IF IT MAINTAINS THE SAME SPEED. IT IS STILL CHANGING DIRECTION