Physics- a bit confused?

Question

I had to do Physics at college, and i'm struggling with my homework! Can anyone please help me and not just give me the answer- but explain how to do it so in future I can do it. Thankyou!

Large metal bars can be driven into the ground using a heavy falling object. The object (driving the bar into the ground) has a mass 2.0x10^3 kg and the metal bar has a mass of 400kg. The object strikes the bar at 6.0ms/-1. It comes to rest on the bar without bouncing. As a result of the collision, the bar is driven into the ground to a depth of 0.75 m.
(worth 4 marks)

(a) Determine the speed of the bar immediately after the object strikes it.
(3 marks)

(b) Determine the average frictional force exerted by the ground on the floor.

Sorry, (a) is 4 marks, and (b) is 3 marks!

Answer 1

The heavy falling object has velocity and thus momentum. When it strikes the bar, it transfers much of its momentum to the bar. Momentum is ALWAYS conserved in collisions.

Initial momentum: ( 2.0 x 10^3 kg ) ( 6.0 m / s )

Final momentum: ( 2.0 x 10^3 kg + 400 kg ) ( v )

Set these two expressions equal and solve for the velocity of the system after the collision, v.

You now know that the bar is driven .75 m into the ground. We can solve this part using conservation of energy. The initial energy of the system is kinetic plus potential. The final energy is zero, because friction with the ground did work on the bar.

The initial energy is 1/2 m v^2 + m g h
The final energy is zero

E = 1/2 ( 2.0 x 10^3 kg + 400 kg ) ( v^2 ) + ( 2.0 x 10^3 kg + 400 kg ) ( 9.8 m/s^2 ) ( .75 m)

You found v in the first part, so you can solve this for E.

Now work is equal to both the change in energy and to Force multiplied by distance:

W = F d = ΔE

You just calculated the change in energy and you know the distance; now calculate the average frictional force F.

Aside: I installed a fishing dock a while back and the technique for installing dock pilings is just that: a big heavy (and manual) pile driver applied repeatedly to big heavy utility poles. It was a long and hot weekend even though we were standing in water most of the time!

Answer 2

I'll give you a tip about physics: Physics really means a bunch of formulas.

Examine what the problem gives you in this case you know mass and velocity of the force driving the bar.

You need to identify the right equation/s to solve this problem which are probably in the notes in your book prior to the question.