## johnweldon1993 one year ago Question with Dynamics?

1. johnweldon1993

So I'm going to use a circle for my FBD First thing is...without the brakes being applied...would I not have a friction force? |dw:1436399040735:dw| Meaning for impulse $\large mv_1 + \Sigma \int_{0}^{0.06} F_{wall} = mv_2$ $\large (\frac{2700lb}{32.2ft/s^2})(4ft/s) + F_{wall}(0.06s) = 0$ $\large F_{wall} = \frac{337.5lb/s}{0.06s} = 5625lb$

2. johnweldon1993

I mean, that doesnt make sense to me at all, there is always friction, otherwise there would be no traction...regardless since the coefficient of kinetic friction wasn't introduced until the second part of the problem...it's the only thing I can think of

3. johnweldon1993

So now with the friction force added in because of the applied breaks we would have $\large F_y = ma_y$ $\large W = N$ $\large N = 2700lb$ $\large F_f = \mu_k N = 0.3(2700lb) = 810lb$ Impulse again $\large mv_1 + \Sigma \int_{0}^{0.06}Fdt = 0$ $\large (\frac{2700lb}{32.2 ft/s^2})(4ft/s) + F_{wall}(0.06s) - 810(0.06s) = 0$ $\large F_{wall} = 4780.06lb$

4. johnweldon1993

Just review so reply with whatever you can correct me on because again I'm not even 75% sure on this :)

5. anonymous

The friction force, if any, would be negligible compared to the force by the wall.

6. anonymous

The correct units in line 3 is -337.5 lb.seconds / .06s = -5625 lb. negative total Force -5625lb (total) = -810 lb (brakes) + -4815 lb (wall) You _should_ apply the brakes _before_ hitting the wall, so their Force lasts longer.