Motion and Energy Chapter 9 Section 1 and 2 By Mrs Nazarali

Vector & Scalar Quantity A vector quantity is a quantity that has both magnitude and direction. A scalar quantity is a quantity that has magnitude, but no direction.

Which is a vector and which one is scalar ? Velocity – 55 miles/hr driving east Speed -55 miles/hr Weight (gravitation pull) Mass Time Volume Force

Extracting Key Points Questions: 1. When do we say that a body is in motion? 2. When do we say that an object moves faster than another object?

So, where are you? Where you will meet is the position as described by a reference point. A reference point is an object that does not move. Click the object for a history of clocks You and your friend agreed to meet for a movie tonight. You decided to get together at the corner of Orange and Denni. in 30 minutes.

It’s about time! The time you will meet is measured with a clock. Time is defined as the interval between two events (when you make the decision to meet and when you meet).time Over the years, time has been measured with sundials, hour glasses, watches, and the most accurate clock, the atomic clock.

As you walk to meet your friend, you change your position with every step you take. Distance is how far you travel along a path while you change your position. Distance is measured inunits. SI stands for the International System of Units called the metric system, a system based on the number ten. The basic unit of length is the meter (m). Do you want to know more about units of length and the metric system? Visit these links Metric SystemSIMetric NumbersHow Many?

While you were walking to meet your friend, you were in motion. Changing position creates motion. You have gone a distance, and time has passed while you were changing your position.

Let’s talk displacement- Displacement is the change in position of an object. If you end up where you started, your displacement is zero. Displacement is described by both a length and a direction. Distance is described only by length. To say that you live 6 km from the amusement park isn’t very helpful unless you state in which direction (6 km south).

Distance vs. Displacement Distance Total length of a path b/w two points. Displacement Length and direction of a straight line b/w starting and ending point. Vector – msmt with a size and direction

Is Displacement a vector or scalar quantity Is Distance a vector or scalar quantity?

The better way to describe the difference between displacement and distance is here: 1. Displacement is the directed line segment between initial and final position of a moving body. 2. Distance is the total length of path traversed by the moving body irrespective of direction.

O.K. Just how fast are you moving? Speed is the distance traveled by an object during a given time interval (distance divided by time).Speed Speed = Time = Distance Speed = distance travelled per unit of time SI unit: m s

and speeds up again to 60 km h –1. Average speed Its average speed over the whole journey overall distance travelled total time of travel slows down to 0 km h –1, A car travels at 50 km h –1, =

Velocity When you know the speed and direction of an object’s motion, you know the velocity of the object. For example, the meteorologist (one who studies the weather) gives the velocity of the wind. The wind is out of the southwest at 10 km/hr. Velocity is the displacement of an object divided by time.

The formula for velocity is- Velocity = Displacement Time = In the formula, v stands for average velocity and d for displacement. Time is t. v = d t

These are your equations Speed = Distance Time Velocity = Displacement Time

Is Velocity a Vector or Scalar? How about Speed? Why?????????

speed = 300 km h –1 direction = west Speed with direction Velocity Pilots are concerned about velocity (direction & speed). speed = 90 km h –1

Graphing Motion Graph distance on the y-axis and time on the x- axis Slope = rise = distance = speed run time run time

Learning Checkpoint This graph shows several stages of motion: Stage 1: 100 m in 10 s Stage 2: 50 m in 10 s Stage 3: 150 m in 20 s Calculate the speed as indicated by each of the colors. Calculate the average speed. What is the total distance?

Solution Stage 1: S= d/ t 100 m/ 10 s= 10 m/s Stage 2: S= d/t 50 m/ 10 s= 5 m/s Stage 3: S= d/t 150 m/ 20 s= 7.5 m/s Ave Speed= Tot d/ Tot t 300 m/ 40 s= 7.5 m/s Distance = 300 meters

Consider the following illustrations:

average speed = total distance time interval A cheetah can maintain a constant speed for short periods of time. Its speed is the same at all times during its motion. In 4 minutes, the cheetah traveled 8 km. speed = 8 km 4 min = 2 km/min Most objects do not move at constant speeds for very long. Dividing the total distance by the total time does not tell you the speed for every point of the journey. Instead it gives you the average speed. The rabbit had a burst of speed during the second minute. What is going on between the 3 rd and 4 th minutes? Time Answer The rabbit is not moving. Time passes but no distance is covered. DistanceDistance DistanceDistance

v = 2 km/min t = 4 min d = 8 km Let’s manipulate that formula- Suppose you were given the speed and the time. Could you determine the distance? This time you know the distance and the speed. Can you find out how long it took for the cheetah to go the distance? Yes. d = v t Yes. t = d / v

Instantaneous speed- Instantaneous speed is the rate of motion at any given instant. What does that mean? The picture at the right gives you the speed of the runner at the instant the picture was taken. 8.5 meters/second

The speedometer in your car gives you a reading of instantaneous speed. While the speedometer is showing the speed at any given moment meters/second

Let’s work on the following problems:

500 km Distance = speed x time The speed of a ship is 50 km/hr. How far will it travel in 10 hours? (Something different here!) Check for your answer  A friend lives 4 km away. Walking at 2 km/hr, how long will it take you to go this distance? (Be careful---it’s another version of the formula!) Check for your answer  2 hrs time = distance/speed

Let’s work a problem or two- An airplane is flying from Bridgeport, WV, to Washington, DC., a distance of 330 km. The trip takes 1.5 hours. What is the airplane’s velocity? The total displacement divided by total time is a velocity of 220 km/hr east. Click here for the answer. A girl walks 2 km west, she turns and walks 3 km east. What distance does she walk? What is her displacement? The distance she walked 5 km. Her displacement is 1 km east. Click here for the answer.

Problems Car trip: 100 km first 2hr 150 km next 3hr Avg Speed = total distance total time = = = 50km/hr Car trip: 80km/hr for first hr 40km/hr next 2hr Avg Speed = total distance total time First hr = 80km Next 2hr = 80km = = = 53km/hr