3. An 18.0 N force pulls a cart against a 15.0 N frictional force. The speed of the cart increases 1.0 m/s every 5.0 s. What is the cart's mass?

The total force on the cart is:  

F = F(applied) - F(friction) = (18 N) - (15 N) = 3.0 N  

The acceleration of the cart is:  

a = (change in velocity)/(time) = (1.0 m/s)/(5.0 s) = 0.20 m/s^2  

Using F = ma, the mass of the cart is:  

m = F/a = (3.0 N)/(0.20 m/s^2) = 15 kg

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