# Homework Help: What actually happens when slicing occurs

1. Jan 15, 2013

### sgstudent

1. The problem statement, all variables and given/known data
I have only learned that when i apply a net force on an object, it will have an acceleration. However, when i use a knife to cut an object from what I've learned the object should have no net force as the contact force from the table to the object will be equal to the force from the knife onto the object. So what allows the object to be sliced?

2. Relevant equations

F=ma

3. The attempt at a solution
I can't think of an answer. The board below shouldn't allow the knife to pass through it. Thanks for the help!

2. Jan 15, 2013

### ehild

The board does not act on the knife. The knife experiences your force and some force from the object: There is a net vertical force on the knife, so it will accelerate and move through the object. Also, the knife is like a wedge, so it exerts side-way forces on the parts of the object,the parts will move horizontally away.

ehild

3. Jan 15, 2013

### sgstudent

But won't the board act on the object? So won't it counter the downwards vertical force acted on the object by the knife? I understand the force being applied to the sides but not vertically downwards. Thanks for the help!

4. Jan 15, 2013

### ehild

The board acts on the object, but not on the knife. The net vertical force on the object is zero, so its CM will not move.
On the other hand, the object -if it can be sliced- is not completely rigid. The part under the knife is compressed, and the parts at the sides of the object move sideways. That relaxes the stress under the knife edge and it can go deeper.

ehild

5. Jan 15, 2013

### sgstudent

Oh that makes sense but could you elaborate on the "The part under the knife is compressed, and the parts at the sides of the object move sideways. That relaxes the stress under the knife edge and it can go deeper"? or is there a topic about the cutting so that i can read more about it?

Are we required to know about non-rigid bodies and how cutting and such stuff affect them in the UK based O levels? Cos' i kinda just thought about this and felt like it might be part of my syllabus haha :)

Thank ehild

Last edited: Jan 15, 2013
6. Jan 15, 2013

### ehild

That is connected to the elasticity of the solids, it is a bit more advanced topics. You can make experiments. If you press a balloon, for example, it becomes shorter and wider... When you step on wet sand, you can see your footprint, which is deeper than the ground, but the mud raised a bit around the footprint.

ehild

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7. Jan 15, 2013

### sgstudent

Oh! So it's something like when the knife sinks down the other parts gets risen. Haha I'll have to leave this for now, until i have a better concept about this :)

8. Jan 15, 2013

### CompuChip

Try cutting a squishy cake - the effect is clearer there!