Kinematic Question -- Car braking to avoid hitting a pedestrian

In summary: And what does that tell you about the distance you need to use in your calculation?In summary, you are trying to calculate the maximum reaction time a driver has to avoid hitting a child who is 150m away. You can do this by calculating the distance it takes for the car to come to a stop after hitting the brakes, which is equal to the distance traveled at a constant speed of 26 m/s until the brakes are applied. This distance can then be compared to the 150m separation distance to determine the maximum reaction time the driver has to avoid hitting the child.
  • #1
Erin Pashanov
5
0

Homework Statement


Driving along a steady speed of 26m/s and suddenly see a child 150m from you. Breaks can produce acceleration of -2.5m/s² but it takes time to get the foot from the gas to the brake pedal. How much time do you have, if to avoid hitting the child?
Known:
d=150m
a=-2.5m/s²
Vi=26m/s
t=?

Homework Equations


d=Vit+½at²

The Attempt at a Solution


150=26(t)+½(-2.5)t²
150=26t+(-1.25)t²
150=26t-1.25t²
1.25t²-26t+150=0
Quadratic formula:
qform01.gif

x=[26±√(-26)²-4(1.25)(150)]/2(1.25)
x=[26±√676-750]/2.5
x=[26±√-74]/2.5
x=[26±8.6i]/2.5
Here I got stuck. How can I solve it if I have an imagery number? I guess my way is wrong?
 
Physics news on Phys.org
  • #2
Erin Pashanov said:
Driving along a steady speed of 26m/s and suddenly see a child 150m from you. Breaks can produce acceleration of -2.5m/s² but it takes time to get the foot from the gas to the brake pedal. How much time do you have, if to avoid hitting the child?
Known:
d=150m
a=-2.5m/s²
Vi=26m/s
t=?

Homework Equations


d=Vit+½at²
i think you are trying to solve without realising the physical situation.
try to ask - what should be car;s deceleration such that it comes to velocity zero after traversing the separation distance ? using other kinematical equation relating initial velocity , final velocity , distance and acceleration you can get info of required acceleration -compare this value with the maximum one can have -then relate the time taken ...
 
  • #3
The problem is asking about your "reaction" time, that is , the time it takes for you to hit the brakes after first seeing the child. You are traveling at 26 m/s constant speed during this reaction time. You should first calculate how far you will travel to a stop after hitting the brakes , by using another kinematic equation. Then you can calc your max reaction time based on what distance you have left.
 
  • #4
Kinda confused.. what should I solve for then? Vf?
 
  • #5
Erin Pashanov said:
Kinda confused.. what should I solve for then? Vf?
No, you should solve for the distance it takes to go from Vo (26 m/s ) to Vf (0 m/s ) when you hit the brakes.
 
  • #6
PhanthomJay said:
No, you should solve for the distance it takes to go from Vo (26 m/s ) to Vf (0 m/s ) when you hit the brakes.
Got 135.2, now I sub this as my distance?
 
  • #7
Erin Pashanov said:
Got 135.2, now I sub this as my distance?
135.2m is the distance for which your motion will be ... what? What will your motion be prior to that?
 

Related to Kinematic Question -- Car braking to avoid hitting a pedestrian

1. How does the speed of the car affect the stopping distance when braking to avoid hitting a pedestrian?

The speed of the car has a direct impact on the stopping distance when braking to avoid hitting a pedestrian. The higher the speed, the longer the stopping distance will be. This is because the car has more momentum and will take longer to come to a complete stop.

2. What role does the mass of the car play in braking to avoid hitting a pedestrian?

The mass of the car also plays a significant role in braking to avoid hitting a pedestrian. A heavier car will have more momentum and will take longer to stop compared to a lighter car. This means that a heavier car will require a longer braking distance to avoid hitting a pedestrian.

3. Is there a difference in stopping distance when braking on different road surfaces?

Yes, the stopping distance can vary depending on the road surface. A car will take longer to stop on a wet or icy road compared to a dry road. This is because the tires have less traction on a slippery surface, making it harder for the car to come to a complete stop.

4. How does the reaction time of the driver affect the likelihood of hitting a pedestrian when braking?

The reaction time of the driver is crucial in determining the likelihood of hitting a pedestrian when braking. The faster the reaction time, the quicker the driver can apply the brakes and potentially avoid hitting the pedestrian. This is why it is important for drivers to stay alert and attentive while driving.

5. Can the braking distance be affected by the condition of the car's brakes?

Yes, the condition of the car's brakes can have a significant impact on the braking distance. Worn out or faulty brakes will take longer to stop the car, increasing the risk of hitting a pedestrian. It is important to regularly maintain and replace the brakes to ensure they are functioning properly.

Similar threads

"How far did the car move while braking?"
    • Introductory Physics Homework Help
Replies
14
Views
2K
Kinematic Question -- Car braking to avoid hitting a fox
    • Introductory Physics Homework Help
Replies
2
Views
2K
Pushing a stalled car out of an intersection
    • Introductory Physics Homework Help
Replies
1
Views
1K
1D Kinematics: Distance between 2 cars
    • Introductory Physics Homework Help
Replies
3
Views
1K
Kinematics question - An automobile accelerating and braking
    • Introductory Physics Homework Help
Replies
1
Views
5K
B How long to travel 150m at 26m/s with an acceleration of -2.5m/s^2?
    • Other Physics Topics
Replies
2
Views
996
What is the maximum speed you could have and still avoid hitting the deer?
    • Introductory Physics Homework Help
Replies
5
Views
4K
Kinematics - Can the car avoid the moose
    • Introductory Physics Homework Help
Replies
1
Views
4K
Minimum Deceleration for Car to Avoid Truck: Physics Homework
    • Introductory Physics Homework Help
Replies
5
Views
2K
Will the Car Stop in Time to Avoid Hitting a Child in a School Zone?
    • Introductory Physics Homework Help
Replies
3
Views
8K

Hot Threads

Recent Insights

Back
Top