Q.A passenger standing in a bus that suddenly stops lurches forward, while a passenger in a bus that suddenly starts moving is thrown backward. Explain both observations using Newton's first law and the idea of inertia.

By Newton's first law, a body in motion (or at rest) tends to continue in that same state unless an external force acts on it to change it. A standing passenger's feet are in direct contact with the floor of the bus, so friction between feet and floor allows the floor to drag the passenger's feet along with whatever the bus itself does; the passenger's upper body, however, is not rigidly attached to the bus at all, and so continues, by its own inertia, in whatever state of motion it was already in.\n\nWhen the bus is moving and then suddenly stops, the passenger's feet (in contact with the floor) are brought to rest along with the bus almost immediately, but the passenger's upper body, by inertia, tends to keep moving forward at the speed the bus (and the passenger) had before -- so the passenger lurches forward, relative to the now-stopped bus.\n\nWhen the bus is at rest and then suddenly starts moving, the opposite happens: the passenger's feet are dragged forward along with the accelerating floor, but the upper body, by inertia, tends to stay behind, at rest -- so the passenger is thrown backward, relative to the now-moving bus.\n\nIn both cases, it is not some new force pushing the passenger's body around; it is simply the passenger's own inertia -- their body's resistance to a change in its state of motion -- combined with the fact that only their feet, not their whole body, are directly forced to follow the bus's own sudden change in speed.\n\n> [!ANSWER]\n> Both effects are the passenger's own inertia: their body tends to keep its previous state of motion even as the bus (in contact with their feet) suddenly changes speed underneath them.

Apply Newton's first law separately to the passenger's feet (dragged along by friction with the floor) and to the rest of the body (not rigidly attached), and compare what each part does when the bus's own speed changes suddenly.

• Describing the lurch as being caused by some new force pushing the passenger, rather than by the absence of a force acting on the upper body while the bus itself changes speed.
• Mixing up the two cases -- lurching forward on a sudden stop, backward on a sudden start.