Classical mechanics
We sometimes write Newton's second law of motion as an equation: .  is the amount of force you need,  is the mass of the object you want to move, and  is the amount of acceleration (change of velocity) that the object gets when you try to move it. We can also write the equation as , where  is momentum and  is the way we write "change in momentum over time" using calculus. Momentum is the mass of an object times how fast it is moving (its speed): , where  is the speed. A bumblebee has a small mass, but it moves very fast so it has lots of momentum. The Earth moves very slow, but it has a large mass so it also has lots of momentum.