12.1 Acceleration

Acceleration Acceleration A vector that changes speed or direction or both. When a object changes velocity Measured in m/s2 Acceleration may be positive, negative or zero. Positive acceleration causes an object to speed up Negative acceleration causes an object to slow down Zero acceleration is an object is at rest or moving with constant speed or velocity

Acceleration Acceleration Acceleration may be positive, negative or zero Positive acceleration causes an object to speed up Negative acceleration causes an object to slow down Zero acceleration is an object is at rest or moving with constant speed or velocity

How is acceleration occurring??

Calculating Acceleration Acceleration = Change in velocity Total Time Acceleration = (vf – vi) t Vf = final velocity Vi = initial velocity t = time

Acceleration Example #1 A ball at rest, on the top of a ramp, starts rolling down the ramp. After 2 seconds, its velocity is 6 m/s. What is the acceleration of the ball?

Acceleration Example #2 While traveling along a highway a driver slows from 24 m/s to 15 m/s in 12 seconds. What is the automobile’s acceleration?

Acceleration Example #3 A parachute on a race car opens and changes the speed of the car from 50 m/s to 15 m/s in a period of 7.3 seconds. What is the acceleration of the car?

Free Fall 1600’s Studied how things fell Didn’t have a good clock Galilei Galileo 1600’s Studied how things fell Didn’t have a good clock Rolled balls down an inclined plane Found that the speed increased as it rolled down the ramp 1st person to explain acceleration of moving objects and falling bodies.

Free Fall Acceleration Due to the Earth’s Gravity Galileo When dropped, these two different masses will fall with the same acceleration!

Free Fall - Falling Objects When gravity is the only force acting on an object (free fall acceleration) is directed toward the center of the earth. For free fall, it is customary to use the letter g to represent the acceleration because the acceleration is due to gravity. Although g varies slightly in different parts of the world, its average value is nearly 9.8 m/s2 (This number is VERY important, remember it!!)

Free Fall – All objects fall at the same rate If you drop a coin and a feather at the same time you will notice that the coin reaches the ground way before the feather. Why??? However, if you were to take the air out of the container (vacuum) you would find that the coin and feather fall together and hit the bottom at the same time!

Free Fall – Air Resistance In Air: A stone falls faster than a feather Air resistance affects stone less In a Vacuum: A stone and a feather will fall at the same speed. Thinker: What does it means if you were on the moon and the acceleration due to gravity is 1.62 m/s2?

Videos http://www.youtube.com/watch?v=zXDZWKmRxI0&feature=related http://www.youtube.com/watch?v=KDp1tiUsZw8 http://www.youtube.com/watch?v=_XJcZ-KoL9o

Free Fall The constant acceleration of an object moving only under the force of gravity is "g". The acceleration caused by gravity is 9.8 m/s2 If there was no air (Vacuum), all objects would fall at the same speed Doesn’t depend on mass

Speed-Time Graph The slope of a speed-time graph equals the acceleration of the object

Speed-Time Graph What is the acceleration of A? What is the acceleration of B? What is the acceleration of C?

11.6 Assessment Question #1 Describe three types of changes in velocity which creates acceleration.

11.6 Assessment Question #2 What is the equation for acceleration?

11.6 Assessment Question #3 What shows acceleration on a speed-time graph?

11.6 Assessment Question #4 What is the acceleration of an object moving at a constant speed?