Pile Driver Energy Homework: Calculate Change in PE

[AutumnBeds]

Homework Statement

A pile driver of mass 220 kg is allowed to fall from rest through a distance of 1.8m on to a 380 kg concrete pile to drive it into the ground. Each impact drives the pile into the ground by 200 mm. Assuming that the pile driver remains in contact with the pile after impact and that momentum is conserved, use the principle of

conservation of energy to calculate

b. The change in potential energy of the driver and pile combination on moving into the ground.

Homework Equations

b. Conversion of potential energy into kinetic energy

The Attempt at a Solution

No attempt made as of yet as I am struggling to understand the question.

I am assuming that tp work this out two parts are required;

Fist calculate potential energy for driver = 220*9.81*1.8 = 3884.76 joules
Then calculate potential energy of the pile - which is either= 380*9.81*-20 or 380*9.81*20 (I am unsure of the 20mm if it easier positive or negative with being driven into the ground, but the direction of force is the same)

Then calculate the difference and convert to kinetic energy.

Any help would be really appreciated. I am not looking for the exact workings out, but a point in the right direction.

Thanks in advance!

 

[gneill]

The question only asks about the change in potential energy for the driver and pile combination. Presumably that means when they are together and moving as one. So, what potential energy do you think they have in mind?

 

[AutumnBeds]

If they are moving as one, they would have a total mass of 500kg and moved a total of 2m.

so, MGH = 500x2x9.81 = 9810 joules

 

[gneill]

If they are moving as one, they would have a total mass of 500kg and moved a total of 2m.

so, MGH = 500x2x9.81 = 9810 joules

2 m is not right: that would include the distance that the driver falls before it makes contact with the pile. How far do they move while they are together?

 

[AutumnBeds]

Ah ok, if so would the total distance only be 0.2m?

Giving = MGH = 500*0.2*9.81 = 981 joules

 

[gneill]

Ah ok, if so would the total distance only be 0.2m?

Yes.

Giving = MGH = 500*0.2*9.81 = 981 joules

Check your mass sum. 220 + 380 = ?

 

[AutumnBeds]

Thanks!

1177.2 joules