1. why does a hiker have to lean slightly forward when her backpack is heavy?
2. why is it that car tires moving at 30 m/s experience four times greater forces than when moving at 15 m/s or half the speed?

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To compensate for the shift in her center of gravity.
So she doesn't fall

Since the backpack is heavy and is carried on the shoulders the centre of mass of the "body+backpack"system is raised. As a higher CM means greater instability,hence the hiker bens forward to lower her own CM so that the CM of the system remains at its original level.
The centripetal force on the centre of wheel = mv^/r if we consider rotation about point of contact.

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wel i heard the less time an object takes in an impact the greater the force it will experience, something to do with newtons second law F = (mu-mv)/t ie change in momentum per second. the faster its going the greater the objects momentum if it collides and comes to rest the resultant force acting upon it is greater than that of a slower object. (completly ignore what im saying if it makes no sense im only in year 12)

Think acceleration