I am wondering what type of biological material are you working on?
If you are working on plant cells, your task will be more complicated..
Anyway If I were you, I would try freeze-thaw as physical stress because the involvement of apoptotic cell death in response to very low temperature has been well reported in wide range of animal cells such as renal cells, fibroblasts, blood cells, cornea, stem cells, lymphocytes, sperm, and ovarian tissue. So try to kill the cells through immediate freezing in liquid nitrogen for 5 min, followed by quick thawing at 60 °C for 5 min. then you must see internucleosomal DNA fragmentation as the main hallmark of apoptotic death upon running DNA extracts on gel. In addition you can irradiate your cells by UV (254 nm light) at 60 J/m2 dose for 24 h ; this kind of stress is well used as positive control for apoptosis.
Concerning necrosis cell death induction, you should apply severe oxidative stress by treating your cells with H2O2 at more than 100 µM as it is well known that hydrogen peroxide triggers different type of cell death by dose-response manner; at high concentration (you can even use 500 µM) it will induce necrosis. In such condition you will not detect DNA laddering.
Mechanical detachment of mammalian cells, such as scraping, can lead to both apoptosis and necrosis. The severity of cell death is dependent on the cell type and mechanical technique. Extensive physical damage to cells will most likely result in necrosis. Necrotic cells can release chemicals (including cytokines) that can induce apoptosis in the surrounding cells.