This is Giancoli answers with Mr. Dychko. When this child on a sled reaches the bottom of the hill, they have an initial speed of 10 meters per second. And the retarding force is due to friction and so, that's what I've drawn here on this free body diagram, and there's also normal force up and gravity down although it turns out those won't be relevant for this calculation here. Final velocity is zero and the child travels a distance of 25 meters, they have a mass of 60 kilograms. So, the friction force is the only force horizontally and so, that makes it the net force. And net force always is mass times acceleration. So, we'll have to find the acceleration first using kinematics and then multiply by the mass to get our answer for the friction force. So, final velocity squared equals initial velocity squared plus 2ad and V initial, or sorry, V final is zero, so, get rid of that term. And then we'll subtract Vi squared from both sides and then divide both sides by 2d to solve for a. And we have acceleration is negative initial velocity squared divided by 2d. So, it's negative 10 meters per second squared divided by 2 times 25. And it gives negative 2.00 meters per second squared. And the retarding force then is mass times acceleration, 60 kilograms times negative 2 which gives negative of 1.20 times 10 to the 2 kilograms. And I wrote it in scientific notation instead of writing 120 because writing it this way is ambiguous as to how many significant figures are suggested, strictly speaking there's only two. And so, since we should have three I wrote it this way. And the negative sign means that this force is in the opposite direction to the initial velocity. And with the initial velocity being positive, that means this has to be negative.