Level 1: When You Walk, Which Way Does Friction Push?
A box riding on a truck.
A box sits on the flat bed of a truck. The coefficient of static friction between them is 0.4 (g = 10 m/s²). The largest acceleration the truck can have without the box slipping is:
The only horizontal force on the box is friction from the truck bed, and it points forward, along the acceleration. It can be at most μs times the normal force mg.
The mass cancels, so any box on this truck bed starts to slip when the truck accelerates faster than 4 m/s². As in walking, the friction on the box points forward, because it stops the box sliding backward relative to the truck.
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