Explain why the tip of a sewing needle is sharp

Because of its Sharp tip the needle is able to put the force on a very small area of the cloth, producing a large pressure sufficient enough to pierce the cloth being stitched and allow to stitch the thread in and pull it out. 

If the heat engine is a perfect carnot cycle engine, thenefficiency = (t2-t1)/t2   where t1=temperature of cold reservoir ( ° k) = 135            t2=temperature of hot reservoir ( ° k)   = 325thereforeefficiency = (t2-t1)/t2 = (325-135)/325 = 0.5846 = 58%  note: this is the theoretical maximum efficiency of a perfect heat engine.   real-life engines do not achieve this value.