Work done using time, velocity and mass?

[TDizzl]

Homework Statement

The law of conservation of energy applies to the motion of vehicles. Find the work done on a 1200kg vehicle when it slows from 90kmh-1 to 50kmh-1 in 8.0 seconds.

Homework Equations

W=1/2mv^2 - 1/2mu^2

The Attempt at a Solution

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W=0.5*1200*50^2 - 0.5*1200*50^2

 

[cnstntcnfsn]

If you assume a constant deceleration, you can use constant acceleration equations such as x = 1/2(vo + v)*t and v = vo + a*t. Work = Fd, where F is the force and d is the displacement. This can be expanded to W = m*a*d. Using the acceleration equations, you should be able to find all of your unknowns and plug them into the work equation.

 

[haruspex]

W=0.5*1200*50^2 - 0.5*1200*50^2

Is that what you meant to post? What happened to the 90kmph?

If you assume a constant deceleration...

Why would TDizzl care what the rate of deceleration is?

 

[TDizzl]

Yes, my bad
Its W=0.5*1200*50^2 - 0.5*1200*90^2

 

[TDizzl]

Using the acceleration equations

The question is from a paper which does not require the use of constant acceleration equations. It would seem bizarre to use a formula that hasn't been introduced to us yet.
Also, the solution is written as -2.59*10^5 Joules in the paper.

 

[PeroK]

Yes, my bad
Its W=0.5*1200*50^2 - 0.5*1200*90^2

You're not proposing to leave that as your answer, are you?

 

[TDizzl]

You're not proposing to leave that as your answer, are you?

No, that was how I substituted the values in as an attempt to answer the question.

 

[haruspex]

Yes, my bad
Its W=0.5*1200*50^2 - 0.5*1200*90^2

In what units?

 

[TDizzl]

In what units?

m=1200kg
v=50kmh-1
u=90kmh-1

 

[haruspex]

m=1200kg
v=50kmh-1
u=90kmh-1

So what is it in Joules?

 

[TDizzl]

So what is it in Joules?

Yea its like -3360000J, that's what I got

 

[haruspex]

Yea its like -3360000J, that's what I got

No, the speeds you are given are in kmh^-1, not ms^-1.

 

[TDizzl]

So how do I get the answer as -2.59×10⁵?

 

[haruspex]

So how do I get the answer as -2.59×10⁵?

How do you convert km/h to m/s?

 

[TDizzl]

Alright -2.59×10⁵ is the answer I got, the problem was that I was doing 50²/3.6 instead of (50/3.6)².
Thanks for the help, feeling extremely stupid.