There are many factors to consider when determining what makes a safe heavy. The most important factor is the weight of the item. If an item is too heavy, it can pose a serious safety hazard. Other factors to consider include the size and shape of the item, as well as the material it is made from.
The weight of an object is determined by its mass. The heavier an object is, the more force it takes to move it. This can be a problem if the object is not properly supported. An overloaded shelf could collapse, for example, injuring anyone nearby.
The size and shape of an object also play a role in its safety. A large, bulky object may be difficult to control and could cause injury if it falls on someone. Likewise, objects with sharp edges or pointy corners can be dangerous if they are not handled carefully.
Finally, the material an object is made from can affect its safety. Some materials are much stronger than others and can better support heavy weights without breaking or collapsing. Others might be more likely to shatter or break if they are dropped or hit hard enough.