Physics Clarification

**Caturdayz** · December 17th 2009, 05:48 PM

A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
_

I'm getting a negative mass, so I must be doing something wrong.

$v = V_0 t + 1/2 a t^2$
$ 0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

$104.1 + 125a$

Which simplifies to $-.93 m/s$

I plug that into $F= ma$

so...

$100= (m)(-.93)$

Which comes out to -107.53

**mathaddict** · December 17th 2009, 08:04 PM

Originally Posted by Caturdayz

A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr in 15 seconds. Solve for mass.
_

I'm getting a negative mass, so I must be doing something wrong.

$v = V0t + 1/2at^2$
$ 0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

$104.1 + 125a$

Which simplifies to $-.93 m/s$

I plug that into $F= ma$

so...

$100= (m)(-.93)$

Which comes out to -107.53

HI

using $v=u+at$

$0=6.94+a(15)$

$a=-0.463 ms^{-2}$

Negative here means deceleration .

$F=ma$

$100=m(0.463)\Rightarrow m=216 kg$

Force here is used to produce a deceleration .

**mr fantastic** · December 18th 2009, 02:19 AM

Originally Posted by Caturdayz

A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
_

I'm getting a negative mass, so I must be doing something wrong.

$v = V_0 t + 1/2 a t^2$
$ 0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

$104.1 + 125a$

Which simplifies to $-.93 m/s$

I plug that into $F= ma$

so...

$100= (m)(-.93)$

Which comes out to -107.53

Do you realise that $a = {\color{red}-} \frac{100}{m}$ ....?

**CaptainBlack** · December 18th 2009, 02:36 AM

Originally Posted by Caturdayz

A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
_

I'm getting a negative mass, so I must be doing something wrong.

$v = V_0 t + 1/2 a t^2$
$ 0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

$104.1 + 125a$

Which simplifies to $-.93 m/s$

I plug that into $F= ma$

so...

$100= (m)(-.93)$

Which comes out to -107.53

Post the entire question before proceeding with you work. That way we don't have to work out what you have omitted ourselves.

CB

**Caturdayz** · December 18th 2009, 02:41 AM

Sorry I thought I had come back and put the time back into the question.

Regardless, Mr. Fantastic answered my question, thanks.

For some reason the fact that this was deceleration completely slipped my mind.

**BabyMilo** · December 18th 2009, 03:14 AM

Originally Posted by Caturdayz

A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.
_

I'm getting a negative mass, so I must be doing something wrong.

$v = V_0 t + 1/2 a t^2$
$ 0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

$104.1 + 125a$

Which simplifies to $-.93 m/s$

I plug that into $F= ma$

so...

$100= (m)(-.93)$

Which comes out to -107.53

$v = V_0 t + 1/2 a t^2$
$0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

this is wrong.

it should be

$x = u t + 1/2 a t^2$

where
x= displacement
u= inital velocity.

and you cant have x=0 because the car was initally moving.

**mr fantastic** · December 18th 2009, 03:20 PM

Originally Posted by BabyMilo

$v = V_0 t + 1/2 a t^2$
$0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

this is wrong.

it should be

$x = u t + 1/2 a t^2$

where
x= displacement
u= inital velocity.

and you cant have x=0 because the car was initally moving.

The required equation is most likely meant to be $v = u + at$ .

**1LISTEN** · December 19th 2009, 12:56 AM

Wow! Mr.Fantastic your my inspiration!

It's time to play the music
It's time to light the lights
t's time to meet the Muppets on the Muppet Show tonight.

It's time to put on makeup
It's time to dress up right
It's time to raise the curtain on the Muppet Show tonight.

Why do we always come here
I guess we'll never know
It's like a kind of torture
To have to watch the show

And now let's get things started
Why don't you get things started
It's time to get things started
On the most sensational inspirational celebrational Muppetational
This is what we call the Muppet Show!

(Gonzo blows his trumpet)

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