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[atar90plus]

I wondering if anyone could help me with these homework questions because my answers were wrong. Could you guys please show the working out and solutions.

11. An object is travelling forwards at 20m/s is subjected to an acceleration of 4m/s^2 in reverse direction.
Calculate
a) displacement after 2 seconds
b) velocity after 3 seconds

12. A stone is dropped from a cliff 50m high. Calculate
a) the time taken to reach the bottom.
b) the velocity just before impact

 

[Nooblet94]

11-a)s=ut+.5at^2=20*2-.5*4*(2)^2=40-8=32metres

b)v=u+at=20-4*3=20-12=8m\s in the direction it was originally travelling



12- a) s=ut+.5at^2
50=0t+.5*9.8t^2
t^2=50/4.9
t=3.19...seconds

b) v=u+at=0+9.8*root(50/4.9)=31.3...m/s

Note: in Q12 I've used down as the positive direction to simplify everything.