# Force and speed issues

The braking distance for a car at 30mph is 13.5m and 6.0m at 20mph.
Thinking distance is directly proportional to speed. Show that the thinking distance at 20mph is 6.0 m. This question has baffled me, pls help

haruspex
Homework Helper
Gold Member
The braking distance for a car [is] 6.0m at 20mph.
..... Show that the thinking distance at 20mph is 6.0 m.
I think you must have misstated the question.
When you've sorted that out, try to write a general equation for braking distance in terms of whatever variables you deem relevant.

I've check
I think you must have misstated the question.
When you've sorted that out, try to write a general equation for braking distance in terms of whatever variables you deem relevant.
ed

haruspex
Homework Helper
Gold Member
I've checked
Ok, but it can't be right. I suggest we just do it algebraically, so put variables for the given data and ignore the actual values for now.
Create another unknown variable for each of:
- think time
- deceleration while braking
Using those, write equations corresponding to the given information.

Delphi51
Homework Helper
The thinking distance at 20 mph could be 6 m if the thinking time happens to be 0.671 s. The thinking distance depends on the thinking time, which isn't given. It could be deduced from some other information such as the total stopping distance including the thinking distance for some speed. Might there be something like that given in another part of the question?

mfb
Mentor
"Thinking distance" means the distance the car travels within the reaction time of the driver?
As the two given braking distances do not include that, I don't see how we could make any statement about that distance without additional assumptions.