A rifle with a barrel length of 50.86 cm fires a 7.06 g bulletwith a horizontal

Question

A rifle with a barrel length of 50.86 cm fires a 7.06 g bulletwith a horizontal speed of 449 m/s. The bullet strikes a block ofwood and penetrates to a depth of 12.9 cm.

What resistive force (assumed to be constant) does thewood exert on the bullet?
5516.6785 N How long does it take the bullet to come torest? Shouldn't it just be t =v/a         t = 449/5516.6785 = 8.14*10-2 Thats what I'm getting but its saying im wrong... Am I missingsomething?                         A rifle with a barrel length of 50.86 cm fires a 7.06 g bulletwith a horizontal speed of 449 m/s. The bullet strikes a block ofwood and penetrates to a depth of 12.9 cm.

What resistive force (assumed to be constant) does thewood exert on the bullet?
5516.6785 N How long does it take the bullet to come torest? Shouldn't it just be t =v/a         t = 449/5516.6785 = 8.14*10-2 Thats what I'm getting but its saying im wrong... Am I missingsomething? Thats what I'm getting but its saying im wrong... Am I missingsomething?                        

Explanation / Answer

Energy before Ke is equal to work done We 0.5mV2 = F d F= 0.5mV2 / d F = 0.5 x 0.00706x (449)2 / 0.129 F= 5,520 N (confirmed?) since d=0.5at2 and a= V/t now d=0.5(V/t)t2 t= 2d/V t= 2 x 0.129/ (449) t=5.76 x10-4 s ( this only if theblock remains stationary) or using your method t=V/a= V/(F/m)= Vm/F t= 449 x 0.00709/ 5520 = t= 5.76 x10-4 s Do you see the difference? Do you know why? 'cause you we rigt ( alsmost right;-)   )

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