Fred (60 kg) is rollerblading at a velocity of 25 km/h [E] when he sees a broken glass bottle on the path ahead. He brakes and is slowed to a velocity of 8 km/h [E] in 4.2 s. What is the coefficient of friction between Fred's rollerblades and the ground? (Hint: Find FN and Ff first.) Given: m = 60 kg v1 = 25km/h = 6.94m/s [E] v2 = 8km/h = 2.20m/s [E] Δt = 4.2s g = 9.8 N/kg Required: ? Solution: Fg = mg Fg = (60kg) (9.8 N/kg) Fg = 588 N FN = Fg FN = 588 N Ff ≤ µs FN Ff ≤ µs (588 N) ??????? I don't know what to do after. µs is not given, so how do you solve this question?