So my Friend Timmy C. is not the best driver. See he has just stated and doesn't quite get that you cant hit the brakes really hard. So as you can imagine it is a very jerky car ride. And thinking more about this it is because of out mass that we will continue to move forward even though the car has slowed. This is because of inertia and mass is a property of inertia. Inertia is the resistance to acceleration based on mass. So therefore a heavier mass will be harder to accelerate and a lighter mass will be easy to accelerate. Applying this to the car situation above it is easy to understand why we continue to move forward the driver applies the brakes hard because you do not want to decelerate, having mass, so you continue to move forward. Here is an extreme example, caution this clip may be violent in some eyes.