When the driver applies the brakes of a light truck traveling at [itex]40\,\frac{km}{hr}[/itex], it skids 3 m before stopping. How far will the truck skid if it is traveling [itex]80\,\frac{km}{hr}[/itex] when the brakes are applied?(adsbygoogle = window.adsbygoogle || []).push({});

Here is what I have done, we are supposed to use the work energy formula, but I can't figure out how to relate the small amount of information. There is no mass specified, no kinetic friction coeeficient specified? Please help!

[tex]V_{i_1}\,=\,40\,\frac{km}{hr}\,=\,11.11\,\frac{m}{s}[/tex]

[tex]V_{f_1}\,=\,0[/tex]

I figured [itex]t_1[/itex] using kinematics:

[tex]v_f\,=\,v_i\,+a\,t[/tex]

[tex]0\,=\,11.1\,+\,a\,t[/tex]

[tex]s\,=\,s_0\,+\,v_0\,t\,+\,\frac{1}{2}\,a\,t^2[/tex]

[tex]6\,=\,0\,+\,22.2\,+\,a\,t^2[/tex]

[tex]t_1\,=\,0.54\,s[/tex]

[tex]a\,=\,-20.6\,\frac{m}{s^2}[/tex]

Then I use these numbers in another kinematic equation for the 80 km/hr instance:

[tex]V_f\,=\,V_0\,+\,2\,a\,(s\,-\,s_0)[/tex]

[tex]0\,=\,22.2\,\frac{m}{s}\,+2\,\left(-20.6\,\frac{m}{s^2}\right)\,(s\,-\,0)[/tex]

[tex]s\,=\,0.538\,m[/tex]

The answer is actually 12m though.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Driver going 40km/hr slams on the brakes & goes 3m before stopping - how far for 80?

**Physics Forums | Science Articles, Homework Help, Discussion**