Riannnne
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a stone is thrown from point A with a speed 30ms-1 at an angle 15degrees below horizontal. the point A is 14m above horizontalground. the stone strikes the ground at B. find the time the stone takes to travel from A to B

my working

resolving downward

s=-14
u= 30sin15
v
a=9.8
t=?

I inserted it into s=ut+0.5at^2

and I got 4.9t^2+30sin15t+14=0

I inserted this into the quadratic formula to solve for t

(-30sin15+-sqroot-214.1114317)/2(4.9)

and I got a math error, assuming this is because of the negative number under the squareroot

I even tried inserting the quadratic into my calculator and got some weird value.

the answer to this 1.1(2sf)

please help I would really appreciate it
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mqb2766
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(Original post by Riannnne)
a stone is thrown from point A with a speed 30ms-1 at an angle 15degrees below horizontal. the point A is 14m above horizontalground. the stone strikes the ground at B. find the time the stone takes to travel from A to B

my working

resolving downward

s=-14
u= 30sin15
v
a=9.8
t=?

I inserted it into s=ut+0.5at^2

and I got 4.9t^2+30sin15t+14=0

I inserted this into the quadratic formula to solve for t

(-30sin15+-sqroot-214.1114317)/2(4.9)

and I got a math error, assuming this is because of the negative number under the squareroot

I even tried inserting the quadratic into my calculator and got some weird value.

the answer to this 1.1(2sf)

please help I would really appreciate it
Think you need to get your signs right.

When positive is upwards
u must be negative (below the horizontal)
a must be negative (accelerating downwards)

Having "a" as positive will mean it will accelerate upwards (for an infinite time?)
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Riannnne
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(Original post by mqb2766)
Think you need to get your signs right.

When positive is upwards
u must be negative (below the horizontal)
a must be negative (accelerating downwards)

Having "a" as positive will mean it will accelerate upwards (for an infinite time?)
but I took downward as the positive direction.

I took a=9.8 because it was accelerating towards gravity(downward)

and I took u = 30sin15 because I was solving with downward as +ve.

I don't get why u must be -ve and a must be -ve.

I'm actually baffled.
Last edited by Riannnne; 1 week ago
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mqb2766
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(Original post by Riannnne)
but I took downward as the positive direction.

I took a=9.8 because it was accelerating towards gravity(downward)

does that mean I need to take the velocity as -ve
s = -14.
Downwards is not positive for the distance. If downwards is positive, this is 14m above the point.

You can take positive as either upwards or downwards, but s, u and a must all be consistent.
Last edited by mqb2766; 1 week ago
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Riannnne
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(Original post by mqb2766)
s = -14.
Downwards is not positive for the distance. If downwards is positive, this is 14m above the point.

You can take positive as either upwards or downwards, but s, u and a must all be consistent.
ahhhhhhh so if I take downward as +ve then s=14 instead of -14 right???
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mqb2766
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Yes, you have to use the +ive direction consistently. speed is dx/dt, acceleration is d^2x/dt^2. The direction of x cannot switch between these related terms.
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