# Two force problems

kevinlikesphysics
3. [SFHS99 4.P.22.] An 1680 kg car is moving to the right at a constant velocity of 2.94 m/s.

(a) What is the net force on the car?
wrong check mark N

(b) What would be the net force on the car if it were moving to the left?
in Newtons = ??

4. [SFHS99 4.P.23.] A freight train has a mass of 5.5 107 kg. If the locomotive can exert a constant pull of 15 105 N, how long would it take to increase the speed of the train from rest to 85 km/h?
in seconds =??

thanks alot

bayan

The net force will be zero if there is no acceleration! Newtons first law of motion.

same would apply to part b if it moves at constant velocity towards left.

For second question.

we know the mass and we know the force it can apply.
$$F=ma$$

$$\frac{F}{m}=a$$

$$\frac{15105}{5.5*10^7}=a$$

$$a=2.75*10^-^4$$

$$v=u+at$$

velocity is 85KM/h which is 23.61 m/s

$$23.61=0+2.75*10^-^4t$$

$$t=\frac{23.61}{2.75*10^-^4}$$

$$t=85972 s$$

23 hours 52 mins 52 seconds!

if the force applied by engine is 15105N.

if it is $$15*10^5$$ then the answer is 865.7 seconds

kevinlikesphysics
thanks a lot they are correct, your explanations helped alot

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