A force that pulls an object to the ground.
Ex: We as humans can stand on Earth because of gravity.
Why?
More Mass= More Gravity we all have strong masses so Earth's gravity attracts us more.
Another ex: Birds can fly, but we can't.
Why?
Less Mass= Less Gravity, birds are VERY light so Earth's gravity does not attract them easily, which is why they fly.
Hope this helps :P
