How Do You Solve Projectile Motion for a Car Driving Off a Cliff?

Alvl tay
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Homework Statement



A car is drove off a cliff at 8 m/s. it lands in the sea 3 seconds later.

Homework Equations



a) how far does it land?
b)how high is the cliff? (g = 10m/s^2)
c) at what angle relative to the horizontal does it land?

The Attempt at a Solution


convention chosen : upwards is positive
initial velocity= 0m/s
a= -g
Vx=8m/s

a) FROM HORIZONTAL COMPONENT

Vx= x(displacement)/ t( time)
8m/s = x/3s
x=24m

b) FROM VERTICAL COMPONENT
using the eq'n s=ut + 0.5at^2
ie. y = 0.5 gt^2

y = 0.5 (-10)(9)
y= - 45 m

c) ** my problem is that I am not sure which eq'n to use

since its relative to the horizontal there is only one... i think.

Vx = Vt cosθ
i have t= 3s
i have x=24m
i don't have V ,or at least i don't know what V exactly is.

Help please?:confused:
 
Last edited:
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It will be clear if you make a drawing indicating the initial and final positions of the car. You need the angle of the displacement vector, not the angle of the initial velocity which is horizontal.

ehild
 
The x-component of the velocity is just the initial 8m/s.The final velocity of a uniformly accelerated object is
Vf=Vi+gt=0+(-9.8m/s^2)*3s=-29.4m/s
which is the y-component.

Then the angle should be the angle made up from those vector components: tan(8/24), or is it tan(24/8), can't think straight about it at the moment.
 
i solved it already :/ a long time ago.
am i supposed to delete the post then?
 
Alvl tay said:
i solved it already :/ a long time ago.
am i supposed to delete the post then?

You should have written that the problem was solved.

ehild
 
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To solve this, I first used the units to work out that a= m* a/m, i.e. t=z/λ. This would allow you to determine the time duration within an interval section by section and then add this to the previous ones to obtain the age of the respective layer. However, this would require a constant thickness per year for each interval. However, since this is most likely not the case, my next consideration was that the age must be the integral of a 1/λ(z) function, which I cannot model.
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