The biggest problem with T.rex is what it did with it's short forelimbs. The answer could give an insight into it's feeding habits. Some believe that the arms were used for balance or helping to position it's mate during mating. One theory is that T.rex used it's arms to grapple onto prey while it ate with it's powerful jaws but to do this it would have to have brought it's immense head back into a position that's unlikely. If it couldn't grasp it's prey what did it use to stop the huge moving targets. It could have used it's powerful hind legs to jump on the dinosaur, but we know that creatures like today's elephants that are the only comparison large enough, can't jump.