## Simple Constant Force Problem1

A car that weighs 12500.0 N is initially moving at a speed of 58.0 km/hr when the brakes are applied and the car is brought to a stop in 5.0 s.

Find the magnitude of the force that stops the car, assuming it is constant.

What distance does the car move during this time?

Find the magnitude of the force that stops the car, assuming it is constant.

**4.11×103 N**What distance does the car move during this time?

**4.03×101 m****NEW**

im not sure how to solve the second one

What distance does the car move during this time?

At first i though it was a simple problem and i would take

the rate of deceleration in m/s and multiply it by the

number of second it took to stop in my case

-16.11m/s x 4 seconds= 64.44m

however this answer is wrong, there is an example of a hockey puck in the book on pg 65 but the equation uses velocities which we are not given

I would appreciate if someone could help me

thanks guys

**NEW**

I used the equation on pg. 65 of the book. Rearranging to

get x = x(initial) + [v(initial)^2 / (2(μk)(g))]. I got a

little confused with the units but it worked out correctly.

**NEW**

How do we solve the first part. Im using F=G(m/d) but we

have the seconds instead of the distance.

**NEW**

Fahim,

Would you be able to teach me how to do the first

part of this problem?

**NEW**

**Re: Re:**

*Anonymous 6*(Thu Jan 21 09:53:02 pm 2010 (EST))

for the 1st part

get mass from whatever wieght they give you for the car using

weight=mass*(acc. due to gravity)

12500N/9.80m/s^2=1275.5kg

than get acc. from a=(Vf-Vo)/t

acc. is in units of m/s^2 and your answer needs to be in kg*m/s^2 otherwise known a "N"

so than you multiply the acceleration by the mass and that is the force you get.

if you get a negative number try changing it to positive

**NEW**

i am pretty sure the problem gives you initial velocity and final is assumed to be zero because the car has stoped moving

**NEW**

i am doing exactly what you said and i am still getting it

wrong.

13400N/ 9.8= 1367.3 kg

then (53.0 - 0)/ 2.4 = -22.08

22.08(1367.3) = 30195 N

can you help me understand why this is still wrong?

**NEW**

I believe you are getting it wrong because of the units

involved. The problem provides the acceleration in units of

km/hr. When using this for the equation, (Vf-V0)/t, it is

necessary that you convert the acceleration to units of m/s,

rather than km/hr. You simply do this by multiplying your

number in km/hr by 1hr/3600s, and then by 1000m/1km.

Inserting this new number(now in m/s) into your equation

should elicit a correct response.

**NEW**

yep that was it thank you!

**NEW**

how do I find muk for the second part?

**NEW**

*Anonymous 10*(Fri Jan 22 02:48:09 pm 2010 (EST))

To find the second part:

I just used (Vf+Vo)t/2 and got the correct answer. make

sure you have made all the necessary unit conversions first.

**NEW**

For the second part I used (Vi)^2 + 2ax=0 solve for x, you

get the acceleration using 0=Vi + at don't forget to convert

you Vi to m/s by multiplying by 1000 then dividing be 3600

hope that helps