contestada

25. (a) How high a hill can a car coast up (engine disengaged) if work done by friction is negligible and its initial speed is 110 km/h? (b) If, in actuality, a 750-kg car with an initial speed of 110 km/h is observed to coast up a hill to a height 22.0 m above its starting point, how much thermal energy was generated by friction? (c) What is the average force of friction if the hill has a slope 2.5º above the horizontal?

Respuesta :

Answer:

a) 47.63 m

b) 188572.725 joule

c) 373.88 N

Explanation:

Given data:

initial speed  = 110 km/hr = 110,000 m/3600 s = 30.6 m/s

[tex]E_{inital} = E_{final}[/tex]

[tex]mgy_i + \frac{1}{2} mv_i^2 = mgy_f + \frac{1}{2} mv_f^2[/tex]

solving y_f we have

[tex]y_f = \frac{m(gy_i +\frac{1}{2} v_i^2 - \frac{1}{2} v_f^2}{mg}[/tex]

[tex]y_f = \frac{(0 + \frac{1}{2}(110 \times \frac{1000}{60\times 60})^2 - 0}{9.8})[/tex]

[tex]y_f = h = 47.63 m[/tex]

b) loss of energy = m g (47.63 - 22)

= 750(9.81)(25.63) = 188572.725 Joules lost

c)  work done = force acted in motion direction × distance moved

[tex]188572.725 = F (\frac{22}{sin2.5})[/tex]

F = 373.88 N