# I need help with my Physics Homework

I need help with my Physics Homework!!!

A Car of mass 1000kg decelerates from 20m/s2 to rest in a time of 5 seconds. Calculate the braking force of the car, assuming that there is no friction.

Ans:
20 m/s2 / 5 seconds = 4m/s2
Force= ma= 1000 * 4 = 4000N

Can anyone explain this to me?

Welcome to PF. :)

First make sure you got all the relevant data written down right.
Was the initial velocity of the car 20 m/s? (NOT m/s², those are the units of acceleration!)

Data:
Mcar = 1000 kg
Vi = 20 m/s
thalt = 5 s
Vf = 0 (The car breaks to a halt)

What formula do you know linking initial velocity, final velocity, and acceleration?
What formula do you know linking acceleration, mass and force?

4ms is acceleration .. the formula is vt=vo-at (if decelerates) after that you can multyplie with car mass 1000 kg (f=m.a)

Yes, I know the answer is 4000N, and the solution is there, I got the answer wrong in my test paper, I nned to know why the correct answer is 4000N.

The given solution was:
20 m/s divided by 5 seconds = 4m/s2
Force= ma= 1000 X 4 = 4000N

I need someone to explain step-by-step...

Pengwuino
Gold Member

Starting with Newton's 2nd Law: F = ma = mass * acceleration. We know the mass obviously. The acceleration, however, for a constant force, is the change in velocity divided by the change in time, that is:

$$\frac{{\Delta v}}{{\Delta t}} = \frac{{20m/s}}{{5s}} = 4m/s^2$$

Now to find the force, it's simply:

$$F = ma = 1000kg*4m/s^2 = 4000N$$