Think the Mass is correct, unsure on Reaction Force

1. Apr 28, 2015

jaja binks

1. The problem statement, all variables and given/known data

2. Relevant equations

kg/m3 = mass / volume

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

3. The attempt at a solution

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

2. Apr 28, 2015

SteamKing

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

3. Apr 28, 2015

jaja binks

oh ok so does that make the volume 1000?

4. Apr 28, 2015

jaja binks

I meant 0.24 sorry

5. Apr 28, 2015

SteamKing

Staff Emeritus
1000 what? Always use units.

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

6. Apr 28, 2015

jaja binks

sorry yes i did not realise 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 ???

7. Apr 28, 2015

Staff: Mentor

Where did the 1000 kg come from?

Chet

8. Apr 28, 2015

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.

9. Apr 28, 2015

SteamKing

Staff Emeritus
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.