An engine with a net power of 15 kW, installed on a car, can tell it a speed of 90 km / h when driving on a horizontal

| September 14, 2021 | education

An engine with a net power of 15 kW, installed on a car, can tell it a speed of 90 km / h when driving on a horizontal section of the road. The same engine installed on a motor boat provides it with a speed not exceeding 15 km / h. Determine the resistance force Fc to the movement of the car and the motor boat at the given speeds.

Np = 15 kW = 15000 W.

Vа = 90 km / h = 25 m / s.

Vl = 15 km / h = 4.2 m / s.

Fа -?

Fl -?

We will assume that the car and the boat are moving uniformly in a straight line, therefore, the net engine power Nп is used to overcome the resistance forces of the movement of the car and the boat.

The useful power of the car engine Nп will be expressed by the formula: Nп = Аа / t = Fа * Sa / t = Fа * Vа, where Aa is the useful work of the car, t is the time of movement of the car, Fа is the force of resistance to the movement of the car, Sa is the path traveled by the car, Vа – vehicle speed.

Fа = Nп / Vа.

Fa = 15000 W / 25 m / s = 600 N.

The useful power of the engine of the motor Np is expressed by the formula: Np = Al / t = Fl * Sl / t = Fl * Vl, where Al is the useful work during the movement of the boat, t is the time of movement of the boat, Fl is the force of resistance to the movement of the boat, Sl is the traversed by the boat path, Vl – boat speed.

Fl = Np / Vl.

Fl = 15000 W / 4.2 m / s = 3571 N.

Answer: at given speeds, the resistance force to the movement of the car is Fа = 600 N, of the motor boat Fl = 3571 N.



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