Sec. 11.2- Speed and Velocity

Looking out window- you will eventually observe things that are moving Leaf in wind Bird flying Car driving Grass/trees growing

How can we describe the differences between the types of motion we just discussed?

Speed Speed: the ratio of the distance an object moves to the amount of time the object moves When describing the speed of your car, you might say it is traveling at 45 kilometers per hour- what is the ratio?

SI unit for speed: Meters per second: m/s Pick proper units for the motion you are describing: Ex: For a moving worm: meters per second or kilometers per hour? For a car: meters per second of kilometer per hour?

Two ways to express the speed of an object: Average speed Instantaneous speed Average speed is calculated differently than instantaneous speed

Average speed- calculated for entire duration of a trip Instantaneous speed- measured/calculated at a particular instant/ point in time

Average Speed Describing speed of hiker isn’t as easy as describing speed along a straight line Hikers change speed depending on terrain Really rocky vs. really flat, downhill- how does the hiker’s speed differ? For objects that change speed a lot, it is sometimes easier to express speed over the entire trip- so the average speed

Average speed is calculated by taking the total distance traveled divided by the time it takes to travel that distance During the time an object is moving, it may change speed, but this equation tells you the average speed over the entire trip Average speed equation:

Average Speed Calculation While traveling on vaca, you measure and record the times and distances traveled. You travel 35 kilometers in 0.4 hours, and then you travel 53 kilometers in 0.6 hours. What was your average speed?

Instantaneous Speed Sometimes it is more useful to know your instantaneous speed rather than the average speed When driving a car, knowing your instantaneous speed is important- you need to make sure that you are going the speed limit! Instantaneous speed: the rate at which an object is moving at a given moment in time What does a car’s speedometer measure?

According to the speedometer pictured below, what is the vehicle’s instantaneous speed? Average Speed?

Graphing Motion Motion can be described and graphed using a distance vs. time graph Horizontal x-axis: Time Vertical y-axis: Distance Slope=

The slope of a distance vs The slope of a distance vs. time graph is the change in distance divided by the change in time The slope of a distance vs. time graph is SPEED If the slope is a straight line, what do you think is true about the motion of the particular object?

If the slope changes, what do you think is true about the motion of the object? If the slope is zero, what is true about the motion of the object? If the slope is steeper, what is true about the speed of the object?

Distance vs. Time Graph- Constant High Speed

Distance vs. Time Graph- Constant Low Speed

Distance vs. Time Graph- Varying Speed

Velocity Knowing the speed of an object is not always enough information to answer a question Ex: A cheetah runs at a speed of 90 km/hr, and there is an antelope standing still 30 m away from the cheetah Can you tell how long it will take the cheetah to get to the antelope?

The answer is no- you don’t know which DIRECTION the cheetah is running in if you only know it’s speed It is often important to know the speed at which an object moves, as well as the direction it moves (with that particular speed)

Velocity: the speed and direction in which an object is moving Velocity describes both speed and direction of an object- what kind of quantity is velocity?

Vectors- represented by arrows For velocity vectors: Length of arrow would be the relative speed, and arrow head would point in direction of movement Longer arrow= higher speed Shorter arrow= lower speed

Velocity is affected/changed by: Change in direction Change in speed Both change in direction and speed

Sailboat: when moving in a straight line with a constant speed, how would you describe the velocity of the sailboat? How could the velocity of the sail boat change?

Sometimes, the motion of an object involves more than one velocity Combining Velocities Sometimes, the motion of an object involves more than one velocity Ex: Boat traveling with the river current, or against the river current In order to figure out the velocity of the boat relative to the riverbank, what would you have to do?

Ex: Boat traveling with river current A boat is traveling East (downriver) at 12 km/hr The river flows at a velocity of 5 km/hr East What is the velocity of the boat relative to the riverbank?

Ex: Boat traveling against river current A boat is traveling West (upriver) at 12 km/hr The river flows at a velocity of 5 km/hr East What is the velocity of the boat relative to the riverbank?