Sec. 11.3- Acceleration

When playing basketball, how would you describe the velocity of the ball? What about the velocity of a player who is staring from a jog and speeding up?

Knowing the velocity of an object is important, but knowing the rate at which the velocity of an object changes is also important Velocity changes frequently for objects in our physical world- it is very difficult to move with a constant velocity Describing how quickly something changes velocity, as well as how much it changes, is a necessary part of describing motion

What is Acceleration? Acceleration- The rate at which velocity changes Acceleration- a vector quantity- describes changes in speed, and changes in direction of motion What do you think I mean by “changes in speed”?

Acceleration changes when the following things change: Direction Speed Both direction and change in speed Units for Acceleration: meters per second per second- how much velocity is changing per second… written as m/s2

Changes in Speed We often think of acceleration when something is speeding up Ex: Car commercials often brag about how fast their car goes from 0 to 60 mph- what is this a measure of? However, acceleration applies to any change in velocity

This change in velocity may be either an increase or a decrease in speed When the velocity of an object is increasing, this results in a positive acceleration When the velocity of an object is decreasing, this results in a negative acceleration

Ex: Cheetah starting a 0 mi/hr then reaching a speed of 65 mi/hr- while this cheetah was ramping up to 60 mi/hr, was its acceleration positive, or negative? Why? Ex: Imagine you are driving a car on a country road at a constant velocity and suddenly spot a deer walking out into the road. You slam on your breaks to come to a stop to avoid hitting the cute little deer. Was your acceleration positive or negative? Why?

Whenever you feel “pulled back” or “pushed forward” while traveling in a car/bus, you are experiencing acceleration Bus sits a stop light, then light turns green. Bus begins moving- what do you feel? Would this be positive or negative acceleration?

Bus is traveling down the road at a constant velocity- what do you feel? What kind of acceleration would this be? Bus spots the next bus stop and begins slowing down- what do you feel? Would this be positive or negative acceleration?

Same as when you are riding in an elevator to a higher floor When elevator starts- what do you feel? Positive or negative acceleration?

When elevator gets ready to stop at a floor- what do you feel? After elevator has started, and you are listening to the beeps as it passes floor after floor- what do you feel? Acceleration? When elevator gets ready to stop at a floor- what do you feel? Positive or negative acceleration?

Another example of acceleration due to change in speed- skydiving! When you are skydiving, you experience free fall Free fall: when an object is falling and the only force acting upon it is gravity

Believe it or not, gravity’s force causes acceleration! When you are skydiving, you are accelerating toward Earth’s surface due to gravity Acceleration due to gravity is a constant value: 9.8m/s2… so every second an object free falls (on Earth), it is traveling 9.8 m/s faster than it was 1 second before that!

Changes in Direction Acceleration- not always due to change in speed When riding a bike and you turn a corner- even if you are keeping constant speed, what do you feel? Even though you are keeping a constant speed, your change in direction means you are accelerating

When riding on a carousel (after it has started, before it is close to stopping- at constant speed), what is the acceleration you experience due to?

Think of a roller coaster and the motion you feel while riding one Think of a roller coaster and the motion you feel while riding one. Everyone experiences acceleration on the coaster- what is this acceleration due to?

Constant Acceleration Constant acceleration- steady change in velocity Constant acceleration is felt when an object speeds up at a steady rate- meaning it increases speed the same amount each second Any ideas for examples of objects that display constant acceleration?

Calculating Acceleration You can calculate acceleration for straight-line motion by dividing the change in velocity by the total time Equation: SIGN IS IMPORTANT!

According to the equation- if you were coasting on a bike downhill, would you experience positive or negative acceleration? WHY? Again, according to the equation- if you continued coasting on your bike after you reached the bottom of the hill, would you experience positive or negative acceleration? WHY?

Example Acceleration Calculation A ball rolls down a ramp starting from rest. After 2 seconds of rolling, its velocity is 6m/s. What is the acceleration of the ball?

Graphs of Accelerated Motion Two different graphs that you can see acceleration on: Speed vs. Time Position vs. Time

Speed vs. Time Graphs Constant acceleration on a speed time graph= straight line Slope of speed vs. time graph = acceleration

How would slope of speed vs. time graph look for negative acceleration? How would slope of speed vs. time graph look for positive acceleration?

Speed vs. Time graph: Bus moving at a constant speed What is acceleration when speed is constant?

Speed vs. Time Graph: Bus slowing down- Negative Acceleration Speed vs. Time Graph: Bus speeding up- Positive Acceleration

Position vs. Time Graphs If the line of a distance vs. time graph is not straight, this means the speed of the object is not constant

A CURVED line on a position vs. time graph= acceleration Given the following Position vs. Time graphs, you should be able to tell if acceleration is present and if it is positive or negative

Position vs. Time Acceleration positive or negative?

Position vs. Time Acceleration positive or negative?