Sketches and Forces in an Excavator System

[furor celtica]

Homework Statement

a sack is in contact with both the base and the vertical back of an excavator scoop. the excavator is moving forward at constant speed in a straight line. make separate sketches showing the forces acting on the sack and the scoop.

Homework Equations

The Attempt at a Solution

ok the sketches aren't important, i just want to make sure i have all the forces correct. so for he sack I'm pretty sure that the vertical forces are its weight and the normal reaction from the scoop, while the horizontal forces would be the force exerted by the scoop and the normal reaction from the sack, is this correct?
however for the scoop i am less sure. there is the normal reaction exerted by the sack, the force from the excavator, but then what?

 

[tiny-tim]

hi furor celtica !

… for he sack I'm pretty sure that the vertical forces are its weight and the normal reaction from the scoop, while the horizontal forces would be the force exerted by the scoop and the normal reaction from the sack, is this correct?

the normal reaction from the sack cannot be a force on the sack

otherwise, that's ok

(except I'm not sure whether to include a vertical friction force from the back of the scoop … sacks are floppy, so they tend to fall down a wall much like a ladder would, so is there a friction force as there would be on a ladder? i don't know )

however for the scoop i am less sure. there is the normal reaction exerted by the sack, the force from the excavator, but then what?

every reaction force on the sack has an equal and opposite reaction force from the sack

also, you've missed out something really obvious!

 

[furor celtica]

ok so forces on the sack: normal reactions (vertical and horizontal) from scoop, weight
forces on scoop: normal reactions from sack (vertical and horizontal), force from excavator (does this count?)

 

[tiny-tim]

ok so forces on the sack: normal reactions (vertical and horizontal) from scoop, weight
forces on scoop: normal reactions from sack (vertical and horizontal),

yes

force from excavator (does this count?)

yes of course! all the forces on the scoop should be shown …

however, which direction does the force from the excavator point in?

and why doesn't the scoop shoot off in that direction (in other words, what are you missing? )

 

[gomunkul51]

"force from excavator" is a part of the normal reaction from the scoop of the excavator.
don't make unneeded equations.

 

[tiny-tim]

"force from excavator" is a part of the normal reaction from the scoop of the excavator.

i don't understand

 

[furor celtica]

does this have to include air friction?

 

[tiny-tim]

in exam questions, no

(in reality, yes, but it will make very little difference)

 

[furor celtica]

so what did i forget with the scoop?
normal reactions from sack, force from excavator, and?

 

[tiny-tim]

weight?

 

[furor celtica]

weight of the scoop? isn't that taken care of by the excavator itself?

 

[tiny-tim]

no …

assuming that the excavator is supporting the scoop only from underneath, the weight of the scoop is a force downward on the scoop, and the force from the excavator is a force upward on the scoop

 

[furor celtica]

ok thanks
so forces on scoop are
normal reactions vertical and horizontal from sack, weight and force exerted by excavator
correct?

 

[tiny-tim]

correct!

 

[furor celtica]

thanks