Jack and Annie push a cart to the left with forces of 3 N and 4 N, respectively. If the cart is stationary, what is the force of friction?

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Answer 1 · 2018-03-19T13:03:54

I tried this:

The forces together add up to:

#F=3+4=7N# to the left;

but the cart doesn't move so static friction, #f_s# to the right, must compensate these forces to give zero acceleration; into Newton's 2nd Law:

#F-f_s=ma#

with #a=0#:

#F-f_s=0#

so that:

#f_s=F#
#f_s=7N#