Question

Would the friction force increase, decrease, or stay the same if Marco added two more blocks to the sled or wagon? Explain your answer.

Answer 1

yes the friction will increase but co.effecient of friction remains constant ..
explanation :
    it is so because more the normal reaction of the surface increases more will be the friction .. 
here normal reaction increases as more force ( weight )  is applied on it ,, which in turn increases friction ..
doubling applied weight means doubling the normal force that leads to double the friction as well ..
weight will be equal to the normal reaction when the surface is balanced or levelled .. but opposite in direction ,, it will be upward as weight acts directly downward ..