Where we've got to with contact forces

A summary of contact forces

There are three kinds of contact forces that can support an object.

Warp forces can be found wherever a solid is distorted by an object:

• Add a compression force exerted by a neighbouring solid acting on the object if that solid is compressed by the object.
• Add a tension force exerted by a neighbouring solid acting on the object if that solid is compressed by the object.

You might, for teaching purposes, combine these two and call them warp forces – with the forensic clue that if an solid in contact with the object is stretched or squeezed then you can add an arrow labelled warp force.

• Add a buoyancy force if the object is partially or wholly immersed in a fluid.

Frictional forces of three kinds can be found at the surfaces of the object when it moves, or makes to move, past other particles its environment.

• If the environmental particles are a solid and no movement occurs, add an arrow at the contacting surface and label it grip force.
• If the environmental particles are a solid and movement occurs, add an arrow at the contacting surface and label it slip force.
• If the environmental particles are a liquid and movement occurs, add an arrow at the most significant surface and label it drag force.

You might, for teaching purposes, combine these three and call them frictional forces – but we'd not recommend that as it obscures the very different reasons for adding the arrows.