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When a bus moves suddenly, the person standing in it tilts backwards. This concept is explained using inertia (tendency of body to resist change in its state of motion) but when the bus moves suddenly, can't we also say that the torque applied due to friction force on our feet causes our upper body to move backwards?

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  • $\begingroup$ The upper body is not moving - interpreted by me as accelerating - backward. It does experience torque. If the person falls it will be accelerated. $\endgroup$ – my2cts Aug 17 at 12:06
  • $\begingroup$ By moving backwards I meant the tilt caused in upper part of the body $\endgroup$ – Physics freak Aug 19 at 18:20
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It should also be noted that the person tilting backward on the bus is not experiencing a real force but a pseudo force on the upper body, that is, a force that appears to be acting on the person because the persons motion is described in a non-inertial reference frame.

can't we also say that the torque applied due to friction force on our feet causes our upper body to move backwards?

The static friction force prevents the bottom part of the body from moving backwards as opposed to causing the upper part of the body to move backwards.

Hope this helps.

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  • $\begingroup$ What if we observe the person from non accelerating frame. Is the cause for tilt only explained by inertia? What basically I wanted to ask was that is the possible cause for tilt in non inertial frame -inertia(or torque)? $\endgroup$ – Physics freak Aug 19 at 18:22
  • $\begingroup$ Yes, if we observe someone from a non accelerating frame (e.g., from the road on which the bus is traveling) the tilt is due to inertia. This is because a pseudo, or fictional, force is one that appears to act on a mass whose motion is described using a non-inertial frame of reference, in this case the frame of the bus, although there is no contact force. Although one in an inertial frame can observe the tilting back, one would not experience such a force. For the tilting (rotation) with respect to the floor to be caused by torque, an actual contact force would need to be applied. $\endgroup$ – Bob D Aug 19 at 20:15

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