Acceleration

Changing velocity: Let's suppose that a car is dripping oil at the rate of one drop per second. Now let's suppose that the car is moving with a velocity that is __increasing__ as the car moves from left to right. Since the car is moving with an increasing velocity, it travels a longer distance during every second of motion. The pattern of oil drops you would expect to see on the road after the car passed would look something like this:

Note that the distance between the drops is increasing. This tells you that the velocity of the car is increasing. Since the velocity is increasing the car travels further during every second of motion.

How would these oil drops look if the car were __slowing down__? Like this!

If the velocity of the car slows while the car travels from left to right, the oil drops will become closer and closer together.

When the velocity of a moving object changes the object is **accelerating**. How quickly the velocity changes is called acceleration. Acceleration is the rate of change of position or __rate of change in velocity__. Stopped at a stoplight your velocity is 0km/h. However, when the light turns green and you step on the gas pedal ( often called the accelerator), Your velocity starts to increase. This increase is a positive change in velocity or a positive acceleration. Slowing down for the next light you find your velocity decreasing. This is a negative acceleration. So, speeding up is a positive acceleration, and slowing down is a negative acceleration. Now look what happens when you turn a corner. Since acceleration is a change in velocity and velocity is a vector quantity, velocity changes when direction changes. One way to think about this is to remember that when you are traveling in a straight line at an unchanging speed you have a constant velocity. As you turn a corner, you start to increase your speed in a new direction. This is acceleration. So, there are three ways your velocity can change, and therefore, three ways to accelerate: speed up, slow down, or change direction. The chart below summarizes the ways that an object can accelerate.

SPEED UP

POSITIVE ACCELERATION

VELOCITY INCREASES

SLOW DOWN

NEGATIVE ACCELERATION

VELOCITY DECREASES

CHANGE DIRECTION

POSITIVE ACCELERATION

VELOCITY INCREASES IN A NEW DIRECTION