What Is the Role of Inertia in Newton's First Law of Motion?

[Girix]

Let's say a shovel carrying a pile of dirt is moving right and then make a sudden stop. The dirt then falls off the shovel moving in the same direction as the shovel was moving. This is an example of Newton's first law of motion. For this to happen, the net force must equal 0 on the object, so when does the net force on the object (dirt) equal zero? Is the net force of the dirt pile zero when it is on the shovel? If so, then how would it fall off the shovel if the shovel makes a sudden stop? The net force would equal zero because the applied force is canceled out by the force of friction on the shovel. Or is the net force unequal to zero when it is flung off the shovel, which means the applied force overcomes the frictional force?

 

[Drakkith]

The net force is zero when the shovel (and dirt along with it) isn't accelerating. As soon as you go to stop the shovel, you have to apply a net force, which accelerates it in the opposite direction as it is moving, thus slowing it down. If the net force, which is transferred through the shovel to the dirt, is too great, then the dirt falls off of the shovel (either by sliding off or by the pile "toppling" over).

 

[Girix]

The net force is zero when the shovel (and dirt along with it) isn't accelerating. As soon as you go to stop the shovel, you have to apply a net force, which accelerates it in the opposite direction as it is moving, thus slowing it down. If the net force, which is transferred through the shovel to the dirt, is too great, then the dirt falls off of the shovel (either by sliding off or by the pile "toppling" over).

So, the net force being applied to the shovel (and the dirt) turns zero once the shovel stops accelerating, therefore the dirt has a net force of zero, which means the inertia causes it to continue moving in a constant velocity to the right until it falls down due to gravity, correct?

 

[jbriggs444]

So, the net force being applied to the shovel (and the dirt) turns zero once the shovel stops accelerating, therefore the dirt has a net force of zero, which means the inertia causes it to continue moving in a constant velocity to the right until it falls down due to gravity, correct?

If you've ever shoveled dirt, you should know that's not how it works.

For sticky earth, you give the shovel a large acceleration. A backwards jerk at the release point. This exploits Newton's second law, F=ma. A large acceleration requires a large force. More force than the adhesion between Earth and shovel can sustain. The dirt breaks loose.

Having broken loose, there is little force left acting on the dirt. It continues in a free fall trajectory toward the pile where you've aimed it. Except for gravity, that's pretty much Newton's first law in action.

 

[Drakkith]

So, the net force being applied to the shovel (and the dirt) turns zero once the shovel stops accelerating, therefore the dirt has a net force of zero, which means the inertia causes it to continue moving in a constant velocity to the right until it falls down due to gravity, correct?

Let's say that, starting from rest, you slowly accelerate the shovel and dirt up to some velocity and then cease applying any net force. The shovel and dirt both continue their movement with no change in velocity. Then, you suddenly pull backwards (or sideways) hard, such that you quickly decelerate the shovel. The dirt, being held onto the shovel only by friction and together with relatively weak forces, cannot stay together under the net force the shovel is trying to apply to it (this force being transferred from your hands through the shovel to the dirt). So it falls apart and slides off of the shovel, as Newton's laws tell us it should.