The second difference is the equal sign instead of less-than-or-equal. This means the frictional force is constant as long as the object is sliding—it doesn’t equal the applied force anymore. That means the net force isn’t zero. Push harder on the chair by running and the chair will speed up.
She added: "You have to look at the pennies and see how far they stretch and if they don't stretch, then what do you do?"
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