Chapter 1
Describing and Measuring Motion
What is a force
?
Force
Is a push or a pull
Can cause things to start or stop
Can cause things to
Can cause things to change direction
Write down as many examples of forces as you can think of in 1 minute.
Ready?
Key Terms
Motion
Distance
Velocity
Reference point
Meter
Slope
International System of Units
Speed
Reference point
When is an Object in Motion?
An object is in motion if its position changes relative to another object.
To decide if you are moving, you use a
reference point, a place or object used for comparison to determine if something is in motion.
An object is in motion if it changes position relative to a reference point.
Relative Motion
• If you use your chair as a reference point as you sit and read, you are not moving.
• Suppose you use the sun as a reference point instead of your chair.
• If you compare your position to the sun, you are moving because you & your chair are on the Earth which revolves around the sun.
Measuring Distance
To describe motion, you need to use units of measurement.
Scientists use a system of measurement called the International System of Units (SI)
Distance is the length of the path between 2 points, the SI unit for length is the meter (m).
Speed and Velocity
How Do You Calculate Speed?
How Do You Describe Velocity?
How Do You Graph Motion?
How do you calculate speed?
• Speed is the distance an object moves per unit of time.
• Formula for speed: speed = distance time
Example: If an athlete bikes a distance of 50 kilometers in 2 hours, what is the athlete’s speed?
50 km = 25 km/hr.
2 hrs.
Average Speed
Average Speed = total distance divided by total time.
Example: A triathlete swims a total distance of 3 km in 1 hour, bikes a distance of 50 km in 3 hours and runs a distance of 12km in 1 hour. What is the athlete’s average speed?
Total distance = 3km + 50km + 12km = 65km
Total time = 1h + 3h + 1h = 5 h
Average speed = 65 km = 13km/h
5h
Instantaneous Speed
• Instantaneous speed is the speed at which an object is moving at a given instant in time.
What is the instantaneous speed of the speedometer on the left?
Velocity
Velocity is the speed and direction of an object’s motion.
For example if a train is traveling west at 100 mph, its velocity is 100mph west.
How do you graph motion?
• You can show the motion of an object on a line graph in which you plot distance versus time.
• The steepness of a line on a graph is called
slope.
You can calculate the slope of a line by dividing the rise by the run. Rise is the vertical difference between any 2 points on the line. Run is the horizontal difference between the same 2 points.
