# Thread: forces and newton's third law

1. ## forces and newton's third law

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.

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?

2. Originally Posted by furor celtica
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.

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?
Since the excavator is moving forward at constant speed the net force on both the sack and the excavator in the horizontal direction is zero. So if there is a horizontal normal force acting on the sack from the excavator then there must be another force of equal size but in the opposite direction acting on the sack .... (it is left for you to think about where that force is coming from).

And from Newton 3, if the excavator exerts a force on the sack, then the sack exerts a force on the excavator of the same size and in the opposite direction .... Again, you are left to thnik about what this means ....

3. thats what i said. the scoop and the excavator arent the same thing. the excavator is not in contact with the sack. the scoop is.
i already noted the normal reaction of the sack on the scoop (horizontally). but does the scoop exert any force on the excavator (which is pushing it)?

4. ok i think i got it, is this correct:
forces on sack: weight, normal reactions from scoop (vertical and horizontal)
forces on scoop: normal reactions from sack (vertical and horizontal), force from excavator

5. Originally Posted by furor celtica
ok i think i got it, is this correct:
forces on sack: weight, normal reactions from scoop (vertical and horizontal)
forces on scoop: normal reactions from sack (vertical and horizontal), force from excavator
You have not understood what I posted. In particular, you have not thought about the implication of the net force on the sack in the horizontal direction being equal to zero. My fault. In my post replace the word "excavator" with "excavator scoop".