Question

A model car of mass 4kg moves in a straight line along horizontal ground. The resistance to motion of the car has magnitude 4N. The car batteries provide a constant power of 20watts. Calculate:
(i) the acceleration of the car at an instant when it's speed is 2m/s.
(I) when the car passes point A, it is moving with constant spread Vm/s. Show that V=5m/s

Answer 1

Mass of car$=4 Kg$

Power of batteries$=20 W$

Resistive force$=4N$

$(i)$ Velocity of car$=2 m/sec$

Resistive power$=Fv=4\times 2=8 W$

Net power$=20W-8W=12W$

$P_{net}=F_{net}v\implies F_{net}=\frac{12}{2}=6N$

$F_{net}=ma\implies a=\frac{F_{net}}{m}=\frac{6}{4}=1.5 m/sec^2$

$(l)$ When speed is constant ,Resistive power$=$ Battery power

So,$P($ batteries)$=P$ (resistance)$=P=20 N$ ;

$P=Fv\implies 20=4\times v\implies v=\frac{20}{4}=5m/sec$