Re: What makes it difficult to start rolling a large boulder?

Hi Jennifer

The simple answer to your question is inertia. Inertia is the property of
matter that causes it to resist any change in motion.  Inertia is summed up
by Newton's first law which says that: "a body at rest (like your boulder)
will stay at rest unless acted upon by an external force and a body in
motion will stay in motion in a straight line unless acted upon by an
external force". 

Kilograms are the units of both mass and inertia so an object with a large
mass will also have a large inertia.

Your supposition to the answer of your question is correct.  The mass of
your boulder is large and this means that the boulder has a large inertia,    

In order to start rolling your boulder, you would have to apply a force
that will be greater than the inertia of the boulder.  Once the boulder
starts rolling, you only have to apply enough force to overcome the
friction between the boulder and the ground.

I hope this answer is helpful.  Good luck.

Greg