If a child is injured and the physician is uncertain of whether the injury is physical child abuse, should social service be called?

Yes. The risk of missing a serious, potentially life threatening case of child abuse outweighs, in my opinion, the embarressment and aggravation of an innocent family being investigated.
Yes. Anytime a physician, other health professional, teacher, or anyone working with children observes an injury they suspect may be the result of child abuse, they are legally obligated to report to social services.