Physics Help Pls!! Rolling Friction, Lift, Aerodynamic drag!?
In the original question, it was assumed that rolling friction was constant. With that assumption, we did not need the weight of the vehicle. This new problem changes that and says that we are now going to assume that rolling friction is proportional to the force pushing down on the wheels. Now we need the weight. At low speeds, the normal force is the same as the weight of the car because there is essentially no lift. The weight is 1500 kg * 9.81 m/s^2 14,715 N. We were told that the rolling friction was 220 N at low speed. At 65 km/h, we are told the lift is 300 N. So we subtract this from the weight of the vehicle to get the downward force on the wheels: Normal force 14,715 N - 300 N 14,415 N. We are told that the rolling friction is proportional to the normal force. So that means the normal force is 220N * (14,415 / 14,715) 215.5 N. That's not a whole lot less than before! But now you just use that instead of the 220 N in figuring out the total force opposing the motion of the vehicle (aerodynamic drag plus rolling friction). At 65 km/h, that's 220 N + 215.5 N 435.5 N. The speed is 65 km/h * 1000 m/km * 1 h/3600 sec 18.06 m/s The power will now be P F * V 435.5 N * 18.06 m/s 7863 W 7.86 kW. At 120 km/h, we are told the lift is 900 N. So now the downward force is 14715 - 900 13815 N. The rolling friction will now be 220 N 8 (13815/14715) 206.5 N As before, the drag goes up in proportion to the speed. The speed is 120 km/h * 1000/3600 33.33 m/s So drag 220 N * (33.33/18.06)^2 749.8 N The total force opposing the motion is 749.8 + 206.5 N 956.4 N Power 956.4 N * 33.33 m/s 31,879 W 31.9 kW
