Help?! First day in physics and already lost!?

Question

I need help understanding a homework question. I don't need the exact answer, although that would be fine, too. I haven't gotten a text book yet and need help!

An object moving at an initial velocity of 80 ft/sec north accelerates at a rate of -16 ft/sec^2.

How long does it take to stop?

How far has it moved?

What I don't get is the negative acceleration rate.

What equations do I use?

Answer 1

negative acceleration is slowing down. usually called deceleration.

everything needs to be in meters... so i will convert these

80ft/sec=24.38400m/s and -16ft/s^2=-4.8768m/s^2

use equations vf=vi+at and vf^2=vi^2+2as

0=24.384+(-4.8768)t

t=5 seconds

0^2=24.384^2+2(-4.8768)s

s=60.96m which is 200ft

make it a good day

Answer 2

<< How long does it take to stop? >>

Working formula is

Vf - Vo = aT

where

Vf = final velocity = 0 (when the train stops)

Vo = initial velocity = 80 ft/sec (give

a = -16 ft/sec^2 (given)

T = time for object to stop

Substituting values,

0 - 80 = (-16)T

Solving for T,

T = 80/16

T = 5 seconds

<< How far has it moved? >>

Formula is S = VoT + (1/2)aT^2

where

S = distance travelled by object within the given time interval

and all the rest of the terms have been previously defined.

Substituting values,

S = 80(5) + (1/2)(-16)(5)^2

S = 400 - 200

S = 200 feet

<< What I don't get is the negative acceleration rate. >>

The negative acceleration indicates that the train was slowing down. If you compare this to a car --- note that when you apply the brakes, the car will slow down till it comes to a full stop. Now, during this time interval, the car is experiencing a deceleration OR a NEGATIVE acceleration.

Hope this helps.

<< What equations do I use? >>

All the equations that you need to solve this problem are all up there.

Answer 3

negatice acceleration means you are gradually slowing down.

you'll want to use the equations:

v = v(i) + a*t

d = v(i)*t + .5*a*t^2

where

v = velocity in question

v(i) = initial velocity = 80 ft/sec

a = acceleration = -16 ft/sec^2

t = time

d = distance traveled assuming you're starting at any random point

the tricky part of this question is finding everything that's given to you, even though it might not be spelled out. for example, it's obvious you're given initial velocity and acceleration. however, you also know your velocity = 0 because at the end of the problem, the object is stopped.

so in the first equation, you have all the information you need to solve for the time. once you know the time, then you can use the second equation to solve for the distance.

Answer 4

-16   negative means you are stopping.. slowing down

Answer 5

The negative sign only means the acceleration is in the opposite direction of the motion of the object. The object is simply slowing down and coming to a stop. The following formulas ought to solve the problem (I think) :

Use the velocity formula to find the time:

Vf = Vo + A(t)

t = time, Vo = initial velocity, Vf = final velocity and A is the acceleration.

0 = 80 - 16(t)

80 = 16(t)

t = 80/16 = 5 seconds

Now, use the displacement formula to find the distance:

X is the displacement (the distance)

X = Vo(t) + 1/2A(t)^2

X = 80(5) - 1/2(16)(5)^2

X = 400 - 200 = 200 feet

Therefore, the object ought to come to a stop after traveling a distance of 200 feet in 5 seconds.

Hopefully these are the right equations. I'm not a physicist.