CLIP 20

# Force, pressure and surface area - how sharp is a knife?

Key Info
• Force, pressure and surface area - how sharp is a knife?
• Duration: 02:33
• The presenter wants to know if two knives are of identical sharpness, so she sets up an experiment to apply the same force to the knives to cut an apple each. One kilogram masses are gradually added to increase the force on each knife. Eventually one knife cuts the apple with less force than the other. The presenter questions why this might be and talks about force, pressure and surface area of the knife blade.
• Subject:

Topic:

## Phys: Forces - Contact and Non-contact

• Keywords: forces, fair test, surface area, mass, Newton, gravity, pressure
Ideas for use in class
• Use this clip to demonstrate how surface area affects the pressure exerted by an object when the force is constant. An example of a fair test. Could be used as a starting point for a class discussion on what constitutes a fair test (ideally the apples should be the same).
Background details
• Clip language : English
• Aspect ratio : 4x3