another physics question please help?

Question

there is a car going down the street and has a mass of 600 kg. it starts from rest and travels 100 meters in 15 seconds. the car accelerates teh whole time.
a. what net force is acting on the car?

b. if the force of the engine supplies 1000 newtons of force, how much pf the total force is used up in friction?

c. what is it's velocity after accelerationg for 20 seconds?

d. the car slams on the brakes. how long will it take to stop?

please show all your work or explain what formulas to use.
i do not understand this question. thank you

Answer 1

Step 1: Figure out your formulae.
F = ma, but you don't have a. However:
d = vit + 1/2at^2

Step 2: Find a.
d = vit + 1/2at^2
100 = 0 + .5a * 15^2
100 = 112.5a
a = .8889

Step 3: Given a and m, find F.
F = ma
F = 600kg * .8888 m/s^2 = 533.3 N (soluton for a!)

Step 4: Find friction based on the input force and output force.
Fi = Fo + Ff
1000 = 533.3 + Ff
466.7 = Ff (solution for b!)

Step 5: Find velocity after accelerating for 20 sec.
v = at
v = .88888 * 20 sec
v = 17.78 m/s (answer for c!)

Step 6: Find time to stop given car slams on the breaks.
Assumption: You need to know velocity, so assume it's the velocity of c.
Assumption: Since the frictional force while brakes applied is not given, you have to assume it's the frictional force from b. This is of course unrealistic, but it's all we've got. In reality, the frictional force when brakes are applied is greater.

vi = 17.777... and vf needs to be 0.
vf = vi + at

Now we need to find a:
F = ma
F = -466.7 (from b, use negative because you're slowing down)
-466.7 = 600kg * a
a = -.7777... m/s^2

Plug back into our formula for velocity:
vf = vi + at
0 = 17.7777 + -.7777t
.7777t = 17.7777
t = 22.85 sec (solution for d!)