Homework help: Work and Energy, Free fall

[Syeda]

Homework Statement

A 200 kg object moves at 10 m/s at 20m (vertically). It falls to a height of 5 m. Find the new speed.

Homework Equations

vf²=vi²+2ad

The Attempt at a Solution

vf²=(10m/s)²+2(9.8m/s²)(15m)
vf=19.8m/s

To find the distance for the new speed I did 20-5, but I'm not sure it's correct.
Also I'm not sure if I'm supposed to use 9.8 for the acceleration

 

[kuruman]

Use energy conservation.

On edit: I noticed that you have posted two different threads with the same title. Please try to avoid this because it can be confusing.

 

[CWatters]

"moves at 10m/s" in which direction? Vertically? Horizontally?

 

[Syeda]

"moves at 10m/s" in which direction? Vertically? Horizontally?

Vertically, from 20 m to 5 m

 

[CWatters]

Then I believe your working in #1 is correct.

You could use conservation of energy method to check you get same answer if you want.

Edit: I just noticed the title says work and energy so perhaps your tutor intended you to use conservation of energy method rather than the equations of motion.

 

[kuruman]

I just noticed the title says work and energy so perhaps your tutor intended you to use conservation of energy method rather than the equations of motion.

Arguably, OP's relevant equation, vf²=vi²+2ad is the energy conservation equation with a factor of (½)m multiplying both sides canceled out.

 

[GroundState]

Everything you have done in step #3 is correct.
Yes, it moves 15 metres.
Yes, it is freefall, so you can use 9.8 for the acceleration.
Remember that if the solution has to be in proper significant digits, you will need to round it to 1 digit because the lowest number of significant digits in your original given values is 1 digit. In fact, ALL of the given numbers are 1 significant digit long.