Stopping Distance and Reaction Time

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Stopping Distance and Reaction Time
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The driver in the car B sees the man A 40 m
away at time t 0.
The velocity of the car changes according to the
graph below.
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Will the car B collide with the man A ?
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From t 0 s to t 0.5 s , the driver is
thinking what to do. He does not apply the brakes
during this period.
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From t 0.5 s to t 3 s ,the brakes are
applied. The car slows down until it stops at t
3 s.
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Stopping Distance Thinking Distance Braking
Distance 10 m 25 m 35 m
Stopping Distance lt 40 m Therefore the car will
not collide with the man A.
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During braking, the slope of the v-t graph -8 m
s-2. This is the acceleration of the car. It
points in a direction opposite to the motion of
the car.
Given the mass of the car is 500 kg.
Braking Force mass x acceleration 500 kg x
(-8 m s-2) -4000 N
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The braking force is in fact the friction between
the tyres and the road surface. When the brakes
are applied, the wheels are locked and the tyres
rubs against the road surface. Skid mark is left
on the road surface.

• The braking force decreases if
• the road surface is wet,
• the surfaces of the tyres are worn out.

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Suppose the initial velocity of the car is 40 m
s-1. The reaction time of the driver remains as
0.5 s and the braking force remains as 4000
N. Will the car hit the man A with an initial
separation of 40 m ?
For the same braking force and same mass, the
acceleration is the same. The slope of the graph
remains as -8 m s-2
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Suppose the initial velocity of the car is 40 m
s-1. The reaction time of the driver remains as
0.5 s and the braking force remains as 4000
N. Will the car hit the man A with an initial
separation of 40 m ?
Stopping distance gt 40 m . Therefore the car will
hit the man A.
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Suppose the reaction time of the driver is 1
s. The initial velocity remains as 20 m s-1 and
the braking force remains as 4000 N. Will the car
hit the man A with an initial separation of 40 m
?
For the same braking force and same mass, the
acceleration is the same. The slope of the graph
remains as -8 m s-2
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Suppose the reaction time of the driver is 1
s. The initial velocity remains as 20 m s-1 and
the braking force remains as 4000 N. Will the car
hit the man A with an initial separation of 40 m
?
Stopping distance gt 40 m . Therefore the car will
hit the man A.
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Suppose the braking force is reduced to 2000 N
(e.g. due to wet surface on a rainy day). The
initial velocity remains as 20 m s-1 and the
reaction time of the driver remains as 0.5 s Will
the car hit the man A with an initial separation
of 40 m ?
Given the mass of the car is 500 kg.
Braking force -2000 N Acceleration -2000
500 -4 m s-2
When the brakes are applied the slope of the
velocity-time graph is -4 m s-2.
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Suppose the braking force is reduced to 2000 N
(e.g. due to wet surface on a rainy day). The
initial velocity remains as 20 m s-1 and the
reaction time of the driver remains as 0.5 s Will
the car hit the man A with an initial separation
of 40 m ?
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Suppose the braking force is reduced to 2000 N
(e.g. due to wet surface on a rainy day). The
initial velocity remains as 20 m s-1 and the
reaction time of the driver remains as 0.5 s Will
the car hit the man A with an initial separation
of 40 m ?
Stopping distance gt 40 m . Therefore the car will
hit the man A.
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• Stopping distance depends on
• the initial speed of the car,
• the braking force available,
• the mass of the car,
• the reaction time of the driver.

On a rainy day, when the road is wet (smaller
braking force available), cars should travel with
lower speed and the separation of the cars should
be increased.
With the same braking force and initial speed, a
car with larger mass requires a longer braking
distance than one with smaller mass.