Equation of uniformly accelerated motion

[fcb]

Homework Statement

A ball is thrown vertically upwards at 16ms^-1. What is the velocity after 2 seconds
(acceleration is 9.8 m/s/s)

Homework Equations

v = u+at
V=velocity
u= Initial speed
a=acceleration down to earth
t=Time

The Attempt at a Solution

I don't quite understand it.

 

[dink87522]

v = u + at

v = final velocity, u = initial velocity, a = acceleration (i.e. gravity of 9.8 m/s^-1 downwards), t = time

v = 16 + (-9.8)(2)
= -3.6 m/s^-1, i.e. the ball is now traveling downwards.

 

[fcb]

v = u + at

v = final velocity, u = initial velocity, a = acceleration (i.e. gravity of 9.8 m/s^-1 downwards), t = time

v = 16 + (-9.8)(2)
= -3.6 m/s^-1, i.e. the ball is now traveling downwards.

Thanks so much

You're a life saver.

 

[fcb]

Ohh and is the displacement 51.6m?

 

[dink87522]

I could be wrong on this, but wouldn't the displacement be 0? There is no motion in the horizontal plane, there is only motion in the vertical plane and the ball returns to its inital starting position, hence displacement is 0.

 

[fcb]

I could be wrong on this, but wouldn't the displacement be 0? There is no motion in the horizontal plane, there is only motion in the vertical plane and the ball returns to its inital starting position, hence displacement is 0.

Well the answer book said 12.4m

Using the equation: r=ut + 1/2at^2
r = displacement
u = Initial
t = time
a = acceleration

 

[tiny-tim]

Ohh and is the displacement 51.6m?

Hi fcb!

You've got a sign wrong … this u and a have opposite signs.

 

[fcb]

Hi fcb!

You've got a sign wrong … this u and a have opposite signs.

I thought to myself that can't be right because gravity is 9.8m/s/s relative. So maximum number in 2 seconds can only be 19.6m/s

thanks heaps lad.

 

[karkas]

@tiny-tim : Well his equation is kind of correct, as in fact it's when you substitute acceleration that the - sign comes up. Though you can easily forget to substitute the sign too.

@dink87522: You kinda confused acceleration units there, it's ms^-2 instead of ms^-1. I know it's a mistake though, don't worry.

 

[fcb]

Is m/s/s the same as ms^-2?

 

[tiny-tim]

Is m/s/s the same as ms^-2?

(try using the X² tag just above the Reply box )

Yes.

 

[fcb]

I am on my phone so I can't use it. Well not I currently know of

 

[tiny-tim]

ah! you can type [noparse]"^{" and "}" instead.[/noparse]