Chapter 5 Matter in Motion Section 1 Measuring

Chapter 5: Matter in Motion Section 1: Measuring Motion

Observing Motion • Motion is observed when an object appears to change position relative to a reference point (frame of reference). • The earth is the most common reference point. • When an object changes position over time relative to a reference point, as you see in figure 1 page 118, the object is in motion.

Speed: Distance and Time • Speed is the distance traveled by an object divided by the time taken to travel that distance. • The SI unit for speed is meters per second (m/s). Other common units for speed are kilometers per second (km/s), feet per second (ft/s), and miles per hour (mi/h).

Average Speed • Most of the time, objects do not move at a constant speed. • Average speed is calculated using the following formula: Average speed = total distance total time • The formula for distance is speed • time. • The formula for time is distance/speed.

Speed on a Graph • Speed can be represented on a distance-time graph. Time is on the x-axis, distance is on the yaxis. • The slope of the line on a distance-time graph for speed represents the rate of speed. • The steeper the slope, the greater the rate of speed. • If the line is straight, the speed is zero. • Draw a simple distance-time graph for the following: high rate of speed, low rate of speed, and a speed of zero.

Figure 2, page 119: This figure shows a graph of the speed of a car over a four hour period. • How far did the car actually travel in 2 hours? • 200 km • How far did the car travel in 4 hours? • 360 km • What was the average speed of the car? • 90 km/h • Try the practice problems in the Math Focus sections of page 120.

Velocity • Velocity is the speed of an object in a given direction. • An example of velocity would be an airplane traveling 600 km/h south. • An object’s velocity is constant only if its speed and direction are constant. • Constant velocity is always motion along a straight line.

Representing Velocity & Finding Resultant Velocity • Velocity is represented using an arrow. The point of the arrow shows the direction. The length of the arrow shows how much. • Combining velocities of objects will determine resultant velocity. • When combining velocities in the same direction, add them together. • When combining velocities in opposite directions, subtract the smaller velocity from the larger velocity. The direction is the same as the larger.

Figure 4 page 121 • Figure 4 shows a person walking along the aisle of a bus while the bus is in motion. • The resultant velocity of the person in the first illustration is 16 m/s east. • The resultant velocity of the person in the second illustration is 14 m/s east. • Why are these two velocities different?

Acceleration • Acceleration is the rate at which velocity changes over time. • Remember: velocity involves both speed and direction. • An object accelerates if either speed or direction change. • The faster the velocity changes the greater the accleration.

Types of Acceleration • Acceleration is commonly associated with an increase in speed, but an object accelerates if it slows down as well. (refer to the definition of acceleration) • An increase in velocity is called positive acceleration. (This is the same as speeding up in the positive direction or slowing down in the negative direction. ) • A decrease in velocity is called negative acceleration. (This is the same as slowing down in the positive direction or speeding up in the negative direction. )

Velocity-Time Graph

Velocity-Time Graph

Calculating Acceleration • Acceleration is found by using the following formula: A = final velocity – initial velocity time • Try the Math Practice question on page 122.

Graphing Acceleration • On a distance-time graph, acceleration is represented using a curved line.

Other Information Re. Acceleration • Some common units for acceleration are meters per second (m/s/s or m/s 2) or kilometers per hour per second (km/h/s). • Using algebra, rewrite the formula for acceleration to solve for final velocity. – vf = at + vi

Circular Motion • With circular motion, only the direction is changing, not the speed. • Centripetal acceleration is the acceleration that occurs in circular motion and is directed toward the center of the circular path.