Changing Speeds

1. Changing Speeds In order to understand how your car will be able to change speed you need to know your car’s acceleration, its positive acceleration and its brakes, its negative acceleration. Your cars acceleration is the change in velocity with respect to a change in time. Your cars positive acceleration is how it can increase its velocity with respect to time. A car’s negative acceleration is its decrease in velocity with respect to time.

2. Changing Speeds Through this we can understand that you can have constant acceleration and not a constant velocity. Also you can have a velocity with no acceleration.

3. Stopping A Car

4. Negative Acceleration • When a driver breaks to stop their car, the car goes through negative acceleration, which in this case means a change in velocity where the object slows down. • This can be expressed through the equation : 0- vi / t • Since the car will come to a complete stop, the final velocity (vf ) is zero • If a car is going -90 m/s^2 , that does not mean that the car is accelerating at a negative speed; it is impossible to drive at a speed less than 0m/s. The negative sign shows that the object is slowing down, not speeding up.

5. Braking Distance • Many people assume that if they are driving at 3 m/s it will take them 3 m/s to stop. This is wrong. These people forget that braking distance is calculated by negative acceleration, not velocity. • In fact, it will take the velocity squared to get the braking distance . Acceleration is velocity squared. • The relationship between velocity (the car’s speed) and it’s negative acceleration can be expresses through the equation: • v2 = -2ad • v= the velocity of the car a= acceleration d= distance • Because the relationship between speed and breaking distance is exponential, a driver must be very aware of how fast they are driving, They should know that if they are driving 3 m/s it will take them around 9m/s to stop. This is also the reason why speed limits are so slow in heavily populated areas.

6. Braking Distance • Many people assume that if they are driving at 3 m/s it will take them 3 m/s to stop. This is wrong. These people forget that braking distance is calculated by negative acceleration, not velocity. • In fact, it will take the velocity squared to get the braking distance . Acceleration is velocity squared. • The relationship between velocity (the car’s speed) and it’s negative acceleration can be expresses through the equation: • v2 = -2ad • v= the velocity of the car a= acceleration d= distance • Because the relationship between speed and breaking distance is exponential, a driver must be very aware of how fast they are driving, They should know that if they are driving 3 m/s it will take them around 9m/s to stop. This is also the reason why speed limits are so slow in heavily populated areas.

7. Section 6 Using Models: Intersections with a Yellow Light • able to make it through the intersection before the light turns red. It is in the GO Zone. • able to come to a safe stop when the light turns red because it is in the STOP Zone. In this zone, automobiles can stop safely before they reach the intersection. • When approaching a signalized intersection at high speeds and the light turns yellow, the dilemma zone is the space from the intersection to the point on the road where it may be difficult for the driver to discern whether they should run the yellow light or brake to be safe. Yellow Light Dilemma

8. The diagram illustrates what happens when an automobile approaching an intersection sees the yellow light. Drivers who are in the "Can't Go" zone as the light turns yellow know they are too far back and won't be able to reach the intersection before the light turns red -- they must stop. Drivers who are in the "Can't Stop" zone know they're too close to the intersection to stop safely -- they must proceed. But when the yellow time is inadequate, there is place in between both zones where the driver can neither proceed safely, nor stop safely. Engineers call this the "Dilemma Zone." http://www.google.com/imgres?imgurl=http://www.thenewspaper.com/rlc/reports/images/dilemma2.gif&imgrefurl=http://www.thenewspaper.com/rlc/reports/rlcreport3.asp&h=188&w=260&sz=4&tbnid=OR5AjA1NlD7h4M:&tbnh=81&tbnw=112&prev=/images%3Fq%3Ddilemma%2Bzone&usg=__zf-nLn4ZnUSbaFMqKAwnnP6QPyI=&sa=X&ei=Ah0zTKGdNcG88gbm5JiwAw&ved=0CCcQ9QEwBA http://www.denverpost.com/nationworld/ci_15405725

9. Forces of the Road • Forces are a push or a pull that affect the motion of an object. • There are four forces: friction, gravity, normal, and tension. • Equation for Force: F=m*a (force= mass *acceleration) • Force is Expressed in Newtons= N • Friction is the force that works between the tires of a car and the road. • Coefficient of Friction: qualifies the interaction between two surfaces , and is represented by the sign μ • Equation for Friction: friction = μ* mg (mg is weight, mass times acceleration due to gravity)

10. Curves and Centripetal Force • The force of friction is the reason why a car can turn. Friction exerts a centripetal force on the car which pulls it inwards. • Due to inertia, the property of matter that makes it want to resist change, like getting my lazy butt off the couch, the car wants to go straight. • Because the car wanting to go straight , and the friction of the road, the car turns. • Equation of centripetal force: Fc= m *v^2/ r (centripetal force equals mass time velocity squared divided by the radius of the turn) • The faster a car goes in a turn the less friction the car has on it, which makes it harder to turn. This is the reason why drivers must slow into curves. • Also, the tighter the turn (smaller radius) the more friction is needed to turn the car. • Weather conditions like rain, snow, and flooding decrease the friction on a road, making it harder to turn and stop. Therefore, driver must slow down when driving though bad weather.