Chapter 11 - Motion

Physics – The science that studies the relationship between matter and energy. 5 major areas of study in Physics: Mechanics Electricity and Magnetism Waves and Optics Thermodynamics Modern Physics 11.1 Distance and Displacement

How can I describe the motion of this dog? How fast is it moving? Is it running toward the owner or away from the owner?

To describe motion accurately and completely, a frame of reference is needed Distance and Displacement

To describe motion accurately and completely, a frame of reference is needed Distance and Displacement Frame of reference – a system of objects that are not moving with respect to one another.

To describe motion accurately and completely, a frame of reference is needed Distance and Displacement Frame of reference – a system of objects that are not moving with respect to one another. clipclip The Earth is the most commonly used frame of reference I can consider the road and trees stationary and measure the dog’s motion relative to them

Relative motion – movement in relation to a frame of reference How fast are the passengers on this high speed train moving? How fast are you moving? 11.1 Distance and Displacement Are they moving? The choice of a frame of reference is arbitrary Should allow motion to be described in a clear and relevant manner

11.1 Distance and Displacement

Measuring Distance Distance – the length of a path between two points SI Unit for measuring distance? meter (m) Small distances with the centimeter (cm) 11.1 Distance and Displacement A B Of course, large distances are measured with the kilometer (km)

Displacement – the direction from the starting point and the length of the straight line path between the starting and ending point 11.1 Distance and Displacement Measuring Displacement A B

“Walk 5 blocks.”NOT A DISPLACEMENT!! “Walk 5 blocks North from the bus stop.” Direction Distance Displacement is a vector quantity – a quantity that has magnitude (size) and direction 11.1 Distance and Displacement Measuring Displacement THIS IS A DISPLACEMENT!!

11.1 Distance and Displacement Procedure Take a piece of graph paper and label the top North, the bottom South, the right side East, and the left side West. Draw a dot at the intersection of two lines near the bottom edge of a sheet of graph paper. Label the dot “Start”. Draw a second, similar dot near the top of the paper. Label this dot “End”. Draw a path from the “Start” dot to the “End” dot. Choose any path that stays on the grid lines. Your path cannot be a straight line from one dot to the other. You must have a least 5 turns and a maximum of 10. Use a ruler to determine the distance of your path. Record it on your paper. Use a ruler to determine the displacement from start to end. Record it. Anaylze and Conclude Which is shorter the distance or the displacement? How could you have made the distance shorter? What was the distance between your 2 nd and 3 rd turn? The displacement? If you keep the “Start” and “End” points the same, is it possible to make the displacement shorter? Explain your answer.

Combining Displacements Represented on paper by arrows Length of the arrow represents the magnitude Direction of the arrow represents the direction 11.1 Distance and Displacement

When two displacements have the same direction you can add their magnitudes If two displacements are in opposite directions, the magnitudes are subtracted from each other Displacement Along a Straight Line

Displacement NOT Along A Straight Line Combine vectors with different directions by graphing Resultant Vector – the vector sum of two or more vectors 11.1 Distance and Displacement

Each day Galileo walks 5 blocks to school and then returns home for lunch. After lunch he goes back to school. On one particular day after lunch, when he was half way back to school, he realized he forgot his Physical Science textbook on the kitchen table. He ran home to get it and then sprinted back to school so as not to be late. What was his displacement once he arrived back at school?

11.1 Distance and Displacement A girl who is watching a plane fly tells her friend that the plane isn’t moving at all. Describe a frame of reference in which the girl’s description would be true.

11.1 Distance and Displacement Should your directions to a friend for traveling from one city to another include displacements or distances?

11.1 Distance and Displacement The resultant vector of two particular displacement vectors does not equal the sum of the magnitudes of the individual vectors. Describe the directions of the two vectors.

You might describe a car as going 45 kilometers per hour Speed – the ratio of the distance an object moves to the amount of time the object moves 11.2 Speed and Velocity Some things move fast (i.e. – a car) Some things move slowly (i.e. – a tree growing) Speed

The SI unit of speed is (m/s) There are two ways to express speed: 11.2 Speed and Velocity Your units should make sense for the object you are describing: - A car might be described by km/h - A tree might be described by cm/year Average speed – computed for the entire duration of a trip Instantaneous speed – measured at a particular instant

11.2 Speed and Velocity Average speed – computed for the entire duration of a trip Average speed is the total distance traveled, d, divided by the time, t, it takes to travel that distance A bicyclist travels for 1.5 hours at an average speed of 32 km/hr. How far does the bicyclist travel during that time? Example 1 A car travels 85 km from Town A to Town B, then 45 km from Town B to Town C. The total trip took 1.5 hours. What was the average speed of the car? Example 2

Math Skills and Math Practice on p Speed and Velocity Graphing Motion The slope of the line on a distance-time graph gives the speed of the object

11.2 Speed and Velocity

10 250

11.2 Speed and Velocity = __ ΔyΔy ΔxΔx _________ = 10 ____ 250 =25 m/s

11.2 Speed and Velocity 25 m/s

11.2 Speed and Velocity m/s

11.2 Speed and Velocity = __ ΔyΔy ΔxΔx22-16 _________ = 6 ____ 300 =50 m/s 25 m/s 50 m/s

11.2 Speed and Velocity 25 m/s 50 m/s

11.2 Speed and Velocity 25 m/s 50 m/s 8 40

11.2 Speed and Velocity 25 m/s 50 m/s 8 40 = __ ΔyΔy ΔxΔx36-28 _________ = 8 ____ 40 =5 m/s

11.2 Speed and Velocity 25 m/s 50 m/s 5 m/s

11.2 Speed and Velocity 25 m/s 50 m/s 5 m/s 4 60

11.2 Speed and Velocity 25 m/s 50 m/s 5 m/s 4 60 = __ ΔyΔy ΔxΔx42-38 _________ = 4 ____ 60 =15 m/s

11.2 Speed and Velocity 25 m/s 50 m/s 5 m/s 15 m/s

Velocity – a description of both speed and direction of motion. Velocity is a vector. Velocity 11.2 Speed and Velocity This cheetah can run 25 m/s. The zebra are 75 m away. How long will it take the cheetah to reach the zebra? You also need to know the direction the cheetah runs.

11.2 Speed and Velocity

Combining Velocities Just like displacements, velocities add using vector addition

11.3 Acceleration

Acceleration - The rate at which velocity changes Acceleration is: A change in speed A change in direction A change in both speed and direction Acceleration is a vector quantity 11.3 Acceleration

Changes in Speed as Acceleration An increase or a decrease in speed is an acceleration Free fall is a type of acceleration (p. 343) Free fall – the movement of an object toward Earth solely because of gravity The unit of acceleration is m/s 2 Objects near Earth free fall at 9.80 m/s Acceleration A force is needed for an acceleration

11.3 Acceleration

Riding a merry-go-round is an example of a change in direction as an acceleration 11.3 Acceleration Changes in Direction as Acceleration Your speed may remain the same, but your direction constantly changes

11.3 Acceleration

Riding a roller coaster is an example of a change in speed and direction as an acceleration 11.3 Acceleration Changes in Speed and Direction as Acceleration

Constant acceleration - a steady change in velocity 11.3 Acceleration Acceleration, a, is the final velocity, v f, minus the initial velocity, v i, divided by the total time, t, of the velocity change A sprinter accelerates from the starting block to a speed of 8.0 m/s in 4.0s. What is the magnitude of the sprinter’s acceleration? Example 1 A car is traveling at 14 m/s. Stepping on the gas causes the car to accelerate at 2.0 m/s 2. How long does the driver have to step on the pedal to reach a speed of 18 m/s? Example 2

11.3 Acceleration Math Skills and Math Practice on p. 346 Just as we can have instantaneous velocity, we can have instantaneous acceleration - how fast a velocity is changing at a specific instant

11.3 Acceleration 4 43

11.3 Acceleration 20 2

11.3 Acceleration