Think the Mass is correct, unsure on Reaction Force

[jaja binks]

Homework Statement

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Homework Equations

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kg/m3 = mass / volume

Downward forces on the column = Weight + Additional force
(F=ma)

The Attempt at a Solution

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kg/m3 = mass / volume
Therefore, 2400 = mass / (0.3 x 0.4)
Mass = 576 kg
Downward forces on the column = Weight + Additional force
(F=ma) = (576 x 9.81) + 10 kN
= 15.7 kN
Every force has an equal and opposite reaction (Newtons 3rd Law). Hence the reaction force at the base of the column = 15.7 kN

 

[SteamKing]

The volume of the concrete is 2 m * 0.3 m * 0.4 m = 0.24 m³.

 

[jaja binks]

The volume of the concrete is 2 m * 0.3 m * 0.4 m = 0.24 m³.

oh ok so does that make the volume 1000?

 

[jaja binks]

oh ok so does that make the volume 1000?

I meant 0.24 sorry

 

[SteamKing]

oh ok so does that make the volume 1000?

1000 what? Always use units.

What I wrote was very plain: the volume of the concrete ... = 0.24 m³. No more. No less.

 

[jaja binks]

sorry yes i did not realize you had worked it out . Thanks.

So would this now make the total mass 1000kg?

Therefore (1000 x 9.81) + 10 KN = overall 19.8 KN ?

 

[Chestermiller]

Where did the 1000 kg come from?

Chet

 

[PhanthomJay]

Math aside for the moment, the so called reaction force at the base has nothing to do with Newtons 3rd law. Rather, it is a consequence of his first law, from equilibrium considerations. This is a very important distinction.

 

[SteamKing]

sorry yes i did not realize you had worked it out . Thanks.

So would this now make the total mass 1000kg?

Therefore (1000 x 9.81) + 10 KN = overall 19.8 KN ?

You seem to be confused by the meaning of density.

Since concrete has a density of 2400 kg/m³, this means that a 1 m³ block of concrete has a mass of 2400 kg.

If you want to calculate the mass of concrete contained in a solid rectangular block, you must calculate the volume of the block, which is what I showed in Post #2.

The formula for density ρ is ρ = M / V, where M is the mass and V is the volume.

For concrete, ρ = 2400 kg/m³ and I showed you how to calculate V. Solve for M.