CHAPTER 2—MOTION & SPEED 2.1—Describing Motion

Why should we learn about motion? You will be able to figure out how fast to go to get somewhere in a certain length of time, how long it will take you to get somewhere, how fast you can speed up or slow down, how hard you have to push or pull something to move it, etc. You will be able to predict how an object will move if you hit, throw, push, or pull it

Position = an object’s location   Motion = a change in an object’s location

Reference point = a location against which the location and motion of another object are compared From the reference point of the yellow girl or the Earth, the red girl and the vehicle are moving.

From the reference point of the red girl, she and the vehicle are not moving, but the yellow girl is moving!

Distance = total length an object moves (different definition than in ch. 1); SI base unit is meter, m   Displacement = the distance and direction of an object’s change in position; will be the same as distance if, and only if, the motion was in only 1 direction dis = not, none, away

Vector quantity = a measurement which includes direction (v for vector and velocity) Scalar quantity = a measurement which does not include direction (s for scalar and speed)   Displacement is a vector quantity. Distance is a scalar quantity.

Which one includes distance? DISTANCE & DISPLACEMENT Distance is greater than displacement because the mouse took a longer-than-necessary route. Which one includes distance?

Distance is 6 km (3 km going to the store + 3 km returning home). Displacement is 0 because the starting and ending positions are the same.

Distance and displacement are both 4 m because the entire motion was in 1 straight direction. However, displacement also has direction. So, distance = 4 m displacement = 4 m to the left

Speed (s) = (how fast an object is moving); the distance an object travels per unit time; the rate of change of position speed = distance s = d = d/t time t (all lowercase letters) SI base unit is m/s   Direction doesn’t matter, so speed is a scalar quantity.

Average speed = the average of all the speeds on a trip; average speed is the total distance traveled divided by the total time of the trip. (This is s = d/t)

Instantaneous speed = the speed at a given point in time

Velocity (v) = the speed and direction of motion of an object Velocity (v) = the speed and direction of motion of an object. Velocity is a vector quantity. The SI base unit is m/s. v = 25 m/s North s = 25 m/s

Speed-Distance-Time problems Example 1: What is the average speed of a car that travels 90 miles in 2 hours? s = d = 90 mi = 45 mi (or 45 mi/h) t 2 h h

Example 2: What distance does a racer run if he travels at a speed of 100 m/s in 5 s? This asks for d, not s, so rearrange the equation to say “d =” Use your speed-distance-time triangle. Use d = s x t: d = s x t = 100 m x 5 s s = 100 m x 5 s = 500 m

Example 3:. How long does it take a snail moving 0 Example 3: How long does it take a snail moving 0.5 cm/s to travel 12 cm? t = d = 12cm = 12cmŸs = 12cms s 0.5 cm/s 0.5 cm 0.5 cm = 24 s

Example 4: How long does it take you to walk home from school if you walk at 3 mi/h and you live 1 mi from school?

Example 5: If an airplane travels the 4,000 km from New York City to Los Angeles in 4 hours, how fast does it fly?

Example 6: If a wolf travels for 4 days at an average of 10 km/day, how far does it travel?

Graph these data: A jogger is taking a break, sitting on a bench in a park.   TABLE 1: DISTANCE TRAVELED WHILE RESTING Time (min) Distance (m) 0 0 1 0 2 0 3 0 4 0 5 0

Graph these data: TABLE 2: DISTANCE TRAVELED WHILE JOGGING ON A TRACK AT A CONSTANT SPEED Time (min) Distance (m) 0 0 1 400 2 800 3 1,200 4 1,600 5 2,000

Graph these data: (If the jogger has to slow on hills, speed up going downhill, slow to move around obstacles, stop at a red light) TABLE 3: DISTANCE TRAVELED WHILE JOGGING ON A SIDEWALK Time (min) Distance (m) 0 0 1 400 2 600 3 900 4 1,500 5 1,500

Distance-time graph = a line graph with time on the x-axis (independent variable) and distance on the y-axis (dependent variable). The slope is the speed; a steeper slope means a faster speed.