Projectile Problem: Marble falls from Desk

[aatari]

Hey Guys, I have attempted the question below and need someone to check my work, please.

Thanks!

1. Homework Statement

A marble rolls off a desk that is 0.86 m above the floor with a horizontal speed of 1.2 m/s.

a) What is the marble's horizontal velocity when it is half way to the floor?
b) What is the marble's vertical velocity at that point?
c) Describe the value of the vertical component of velocity as the marble falls to the floor.​

Homework Equations

The Attempt at a Solution

 

[PeroK]

For c), I don't think that $0$ is the answer they are looking for!

 

[PeroK]

Also, if you are using $g$ to one decimal place, you cannot have an answer for velocity to four decimal places.

 

[aatari]

For c), I don't think that $0$ is the answer they are looking for!

Will it be 2.9008 m/s?

 

[Caio Graco]

It is not correct to write $\vec g = - 9.8 m/s^2$ , because a vector is never equal to a number.

After the ball leaves the table, the vertical component of its speed increases from $9.8 m/s$ to every second until it collides with the ground.

 

[PeroK]

Will it be 2.9008 m/s?

No, you just do what you did to find its velocity half way to the floor.

You can safely ignore post #5.

 

[aatari]

No, you just do what you did to find its velocity half way to the floor.

You can safely ignore post #5.

I get it, thank you.
It will be 4.1 m/s.

 

[PeroK]

I get it, thank you.
It will be 4.1 m/s.

That looks about right.

 

[Caio Graco]

I get it, thank you.
It will be 4.1 m/s.

In letter c he does not ask for speed by hitting the ground. He asks what happens to the speed over the fall.

 

[haruspex]

In letter c he does not ask for speed by hitting the ground. He asks what happens to the speed over the fall.

Good point.

 

[Caio Graco]

In letter c he does not ask for speed by hitting the ground. He asks what happens to the speed over the fall.

For this, see post # 5 "ignored".

 

[PeroK]

I get it, thank you.
It will be 4.1 m/s.

It looks like they weren't asking for the speed as it hits the floor, but a "description of the value" during the fall.

 

[Caio Graco]

It looks like they weren't asking for the speed as it hits the floor, but a "description of the value" during the fall.

Exactly

 

[PeroK]

For this, see post # 5 "ignored".

The thing to ignore was about the vector. The OP's diagram is good and clear in its vector representation, in my opinion.

 

[Caio Graco]

The thing to ignore was about the vector. The OP's diagram is good and clear in its vector representation, in my opinion.

The problem is that he wrote $\vec g= - 9.8 m/s^2$
The correct is $\vec g= -9.8 \vec j m/s^2$ (vector equation) or else I could write $g=-9.8 m/s^2$ (scalar equation)

 

[PeroK]

The problem is that he wrote $\vec g= - 9.8 m/s^2$
The correct is $\vec g= -9.8 \vec j m/s^2$ (vector equation) or else I could write $g=-9.8 m/s^2$ (scalar equation)

The OP's diagram is clear and that's just pointless pedantry.

 

[Caio Graco]

g=−9.8m/s2

When writing this it is being said that a vector is equal to a number. A vector can only be a vector and a number can only equal a number.

 

[Caio Graco]

The OP's diagram is clear and that's just pointless pedantry.

Ok.

 

[haruspex]

When writing this it is being said that a vector is equal to a number. A vector can only be a vector and a number can only equal a number.

Yes, you've made your point, but it really is a bit excessive to complain about that in an explanatory note in a diagram.

 

[Caio Graco]

Yes, you've made your point, but it really is a bit excessive to complain about that in an explanatory note in a diagram.

I agree. It's just a matter of formality.