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# Friction

## The resistance that one surface or object encounters when moving over another.

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Practice Friction
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Race to The Finish

### Race to The Finish

Credit: Jen Durfey
Source: http://www.flickr.com/photos/dottiemae/5798161768/

In most drag racing competitions, competitors race down the track to compete either against each other or for the fastest time. After racing down the pavement for 14\begin{align*}\frac{1}{4}\end{align*} mile, a parachute is deployed to help slow down the speeding car. When the parachute is deployed the velocity of the car is quickly reduced due to the increased drag forces on the car that is provided by the open parachute.

#### Why It Matters

Credit: RVWithTito
Source: http://www.flickr.com/photos/28781820@N08/4811561148

Drag racing consits of cars competing for the fastest time and speed [Figure2]

• As drag race competitors leave the start line, their velocity and acceleration vectors point in the same direction. As they continue down the track their velocity vector increases while their acceleration vector stays constant, or decreases depending on how far they are from the start line. Once they have crossed the finish line, each competitor must quickly reduce the speed of their vehicle without losing control. By deploying a parachute they are able to change the direction of the acceleration vector and stabilize the vehicle at the same time.
• The same thing can be seen when you throw a ball vertically in the air. When you throw a ball vertically upward, you give it an initial velocity vo\begin{align*}v_o\end{align*}. As the ball leaves your hand, the acceleration due to gravity takes effect because your hand is no longer supporting the ball. Because of this, the acceleration vector points vertically downward, causing the ball to slow down.

#### Can You Apply It?

Using the information provided above, answer the following questions.

1. Some of the fastest drag cars have use jet engines. When the car is leaving the start line, are the velocity vectors and the acceleration vectors in the same direction?
2. What direction must the frictional force be between the tires and the ground on the race car as it accelerates from the start line?
3. What is the name of the force that actually causes the car to accelerate?

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