### E82

```Do Now for 5/14/13
Take out write up for E82
HW: Complete analysis
E82 Braking Distance
• Today’s Target: I will be able to observe and
state the effects of increased friction on the
braking distance of a car.
• Introduce Activity
• Lab set up
• Gather data
• Analysis
• Key Points
E82 Braking Distance
• We are going to look at some of the features
designed to slow a car down and prevent
collisions from happening.
• Braking Distance – The distance a vehicle
travels in the time between the moment the
brakes are applied and the moment the car
comes to a stop.
• A vehicle’s brakes cause the car to stop by
increasing friction.
E82 Braking Distance
• Set up
– When you cover the track with cloth, will the cart’s
braking distance become longer or shorter?
• We will investigate what happens to a cart’s
breaking distance when it hits the cloth “brakes”
at different speeds.
• The higher the cart is released, the faster it is
travelling at the bottom of the ramp.
• What variables are the same? What variable(s)
are different?
E82 Braking Distance
E82 Braking Distance
• 1. In this model of a stopping car:
• a. What does the cloth represent?
– The cloth models the brakes on a real vehicle.
• b. List some strengths and weaknesses of this
• model.
– This is a good model because it adequately slows
down the cart. But it is not representative of the real
braking mechanism in a car.
– For example, the driver of a car can apply the brakes
with a varied force when pressing the pedal, while the
model provides a more constant frictional force.
E82 Braking Distance
• 2. What effect does speed have on the braking
distance? Explain, using evidence from this
activity.
– As the initial speed was increased, the braking
distance also increased. For example, from Notch
C the cart traveled 29 cm before stopping. From
the Notch A the cart traveled 69 cm before
stopping. Small changes in initial speed resulted
in very large changes in distance.
E82 Braking Distance
• 3. (RE ASSESSMENT) Does the evidence you
gathered in this investigation support the
statement below? Use evidence from you
investigation to explain why or why not.
• A car moving at 55 mph needs less distance to
stop than an identical car moving at 45 mph.
E82 Braking Distance
• 4. Reflection: Why do you think speeding is a factor in
a high percentage of car accidents?
– There may be many reasons speed is involved in a high
number of fatalities. This activity showed that the faster a
car is traveling, the more distance it takes it to come to a
stop. This makes it more likely that the car will hit
something.
– Also, if after braking the car does hit something, a
speeding car will be traveling faster than it would have
been had it been traveling more slowly initially.
– Evidence from Activity 76, “Speed and Collisions,” showed
that collisions at higher speeds result in greater forces
during impact that are more likely to cause serious damage
and fatalities.
E82 Braking Distance – Key Points
• 1. Friction is a force that will cause changes in the
speed of an object’s motion.
• 2. The motion of an object can be described by its
position, direction of motion, and speed.
• 3. Students should be able to design and conduct
appropriate scientific investigations. This may
include the use of multiple trials.
• 4. Good experimental design requires keeping as
many variables as possible the same—except for
the one being tested—so that any results can be
ascribed to the variable changed.
```