# How to Calculate if a Car Will Stop in Time?

• Eddard
In summary: ENDMENT: In summary, the conversation discusses a problem involving a 920 kg car traveling at 125 km/h and attempting to stop before reaching a concrete barrier. The frictional force acting on the car is 8600N and the driver is unsure if they can stop in time. The suggested solution involves using energy and calculating the stopping distance to determine if it is less than 65m.
Eddard
Hey... I need help on a question i know how to calculate the decceleration of the truck in the problem below but I am not completely sure about how you would be able to find the conclusion to this question? Help would be appreiated

A 920 kg car is 65m from a concrete barrier traveling at 125 km/h, when the driver notices and slams on the breaks. The fritional force acting on the car is 8600N. Is the driver able to stop the car in time?

so this is what i got:M= 920kg
D=65m
V1=125 km/h
F(friction)=8600N
A=?
v1=0

F(net)=ma
-F(friction)=ma
A=-F(fric..)
=-0.935 m/s ^2V2^2=V1^2 + 2a*∆d

Rearranged the equation:
∆d= V2^2/2a∆d - square of V1

Then when i sub in the numbers and solve the equation I conclude that the car stops 2.11 meters away from the concrete barrier...but I am not sure if i did this right and if any part of this equation doesn't make sense ill try to explain...

Last edited:
Eddard said:
Hey... I need help on a question i know how to calculate the decceleration of the truck in the problem below but I am not completely sure about how you would be able to find the conclusion to this question? Help would be appreiated

A 920 kg car is 65m from a concrete barrier traveling at 125 km/h, when the driver notices and slams on the breaks. The fritional force acting on the car is 8600N. Is the driver able to stop the car in time?
I am not sure what you are doing here. The easiest way to do this is to use energy. The force x stopping distance must equal the initial kinetic energy of the truck. If the stopping distance is greater than 65 m then it hits the wall.

Since Fd = KE, the stopping distance is: d = KE/F

Work that out and see if d is less than 65 m.

AM

Hi there,

It looks like you have correctly calculated the acceleration of the car and used the kinematic equation to find the distance it will travel before stopping. However, it is always a good idea to double check your work and make sure you have used the correct units and conversions. Also, keep in mind that there may be other factors at play in a real-life scenario, such as reaction time and road conditions, that could affect the outcome. Overall, your approach seems sound and with some minor adjustments, you should be able to confidently conclude whether or not the driver is able to stop the car in time. Keep up the good work!

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