Distance and acceleration problem

  • #1
david_duhig
3
0

Homework Statement


A police radar has an effective range of 1.0 km, and a motorist's radar detector has a range of 1.9 km. The motorist is going 120 km/h in a 80 km/h zone when the radar detector beeps. At what rate must the motorist decelerate to avoid a speeding ticket?

Homework Equations


v^2=u^2+2as
v=288m/s
u=432m/s
a=
t=
s=900m

The Attempt at a Solution


82944=186624+(2*a*900)

(82944-186624)/900/2

=57.6m/s^2
 

Answers and Replies

  • #2
Charles Link
Homework Helper
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60 m.p.h.=88 ft/sec. Your velocities of 288 m/sec and 432 m/sec are much too high. 1 meter is slightly greater than 3 ft.
 
  • #3
david_duhig
3
0
60 m.p.h.=88 ft/sec. Your velocities of 288 m/sec and 432 m/sec are much too high. 1 meter is slightly greater than 3 ft.
Ahh of course, thank you for your help.
 
  • #4
drvrm
963
214

Homework Equations


v^2=u^2+2as
v=288m/s
u=432m/s
a=
t=
s=900m

The Attempt at a Solution


82944=186624+(2*a*900)

(82944-186624)/900/2

=57.6m/s^2
check the calculation;
for example 80 km/h = 80 x 10^3 m/ (60x60) sec.= 800/36 m/s=200/9 m/s and you are writing v=288m/s so correct it and other velocity also.
 
  • #5
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
12,798
1,670

Homework Statement


A police radar has an effective range of 1.0 km, and a motorist's radar detector has a range of 1.9 km. The motorist is going 120 km/h in a 80 km/h zone when the radar detector beeps. At what rate must the motorist decelerate to avoid a speeding ticket?

Homework Equations


v^2=u^2+2as
v=288m/s
u=432m/s
Interesting fact: the speed of sound at sea level is about 340 m/s, also known as Mach 1.

If you're going Mach 1 in an 80 kph zone, no crummy Fuzz is going to catch you. o_O
 
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  • #6
david_duhig
3
0
check the calculation;
for example 80 km/h = 80 x 10^3 m/ (60x60) sec.= 800/36 m/s=200/9 m/s and you are writing v=288m/s so correct it and other velocity also.
Thank you.
 

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