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Homework Statement

A mobile (M) is animated by a uniformly varied rectilinear motion.
At time=0, it is on abscissa x= -1 meter
At time = 1s, it is on abscissa X= +2 meters
At time = 3 s , it is on abscissa x= -4 meters

Relevant Equations

Find the time equation of the movement

This is where I'm stucked i don't even know if my reasoning is correct.

 

[PeroK]

When it says at time $t = 0$, $x = -1$ that means you can substitute $t=0$ into your equation and get $x = -1$. That may help!

 

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I

When it says at time $t = 0$, $x = -1$ that means you can substitute $t=0$ into your equation and get $x = -1$. That may help!

I fond that the accelaration is -4
The initial velocity is 5
And the initial distance is -1

The time equation is : -4(t^2) + 5t -1

Is it correct?

But i don't understand something the accelaration is -4 and how is it possible that a time=1, The abscissa is positive and after time=3, it becomes negative. Shouldn't the mobile stop?

 

[PeroK]

I fond that the accelaration is -4
The initial velocity is 5
And the initial distance is -1

The time equation is : -4(t^2) + 5t -1

Is it correct?

Yes, although you should include the relevant units in metres and seconds.

But i don't understand something the accelaration is -4 and how is it possible that a time=1, The abscissa is positive and after time=3, it becomes negative. Shouldn't the mobile stop?

This motion is similar to a ball being thrown up and falling back down: it starts $1m$ below some reference point (perhaps $1m$ below the garage roof); it has an initial velocity of $5m/s$ upwards; and, the acceleration due to gravity is $-4m/s^2$. That might be nearer the figure for Mars than Earth, I guess.

At time $t = 1$ it has moved above the reference point and is still moving up. By time $t = 3$ it has reached its maxium height and fallen all the way back to $4m$ below the reference point ($3m$ below where it started.

It might be worth plotting a velocity against time and a position against time graph for your equation.

 

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Yes, although you should include the relevant units in metres and seconds.

This motion is similar to a ball being thrown up and falling back down: it starts $1m$ below some reference point (perhaps $1m$ below the garage roof); it has an initial velocity of $5m/s$ upwards; and, the acceleration due to gravity is $-4m/s^2$. That might be nearer the figure for Mars than Earth, I guess.

At time $t = 1$ it has moved above the reference point and is still moving up. By time $t = 3$ it has reached its maxium height and fallen all the way back to $4m$ below the reference point ($3m$ below where it started.

It might be worth plotting a velocity against time and a position against time graph for your equation.

Should i do two different graphs? One for the velocity and the other for the position?

 

[PeroK]

Should i do two different graphs? One for the velocity and the other for the position?

Yes.

 

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Yes.

Oh, thank you Perok

 

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Oh, thank you Perok

I'll show you the graphs