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A car moving at 40 km/h skids 16 m with locked brakes...

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  • Q : (A) A car moving at 40 km/h skids 16 m with locked brakes. How far will the car skid with locked brakes at 120 km/h? (B) A 74 kg skydiver moving at terminal speed falls 50 m in 1 s. What power is the skydiver expending on the air?
  • A :  (1) For the first problem, you can use a ratio. This ratio is valid if you're assuming a linear relationship (40 km/h / 16 m) = (120 km/h / x m) then (40/16) = (120/x) then  2.5 = (120/x) then x = 120/2.5 and x = 48 m. The car will skid 48 m with locked brakes at 120 km/h. (2) At terminal velocity, the downward force due to gravity equals the upward force due to drag. Therefore the skydiver is not accelerating and is traveling at constant velocity: Power = force x velocity  then  Power = m g x velocity  then  Power = (74 kg) x (9.81 m/s2) x (50 m/s) then  (The skydiver falls 50 m in 1 s, or 50 m/s) so Power = 36297 Watt.

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