# Pretension of spring based on sideway shock

1. Mar 31, 2014

### YeeHaa

Hi all,

First to clarify: this is no homework question. I am working on a construction and am really puzzled about the following problem.

I need a spring to hold an object in place during sideway shocks.

I am really puzzled about the fact if friction helps one or two times. In other words: do I count friction only between the block and the floor, or also between the pushing part of the spring and the block (meaning that friction counts two times and they both "help" in keeping the block in its place).

This would mean that:
Frictionforce-spring_block = staticfriction_coeff(spring-block) * (pretension_force + weight)
Frictionforce-block_floor = staticfriction_coeff(block-floor) * (pretension_force + weight)

2. Apr 2, 2014

### Baluncore

Static friction is a force that must be overcome before movement can occur.
When you have a stack of layers, the static friction between the top surface and the bottom surface will be the minimum static friction of any layer interface present. The separation will occur first at the interface with the minimum static friction.

You can change the friction by inserting a sheet of different material at an interface. Paper or cloth may increase friction, PTFE or stainless steel sheet tend to reduce it. To hold a steel block in place on a steel bench, put a sheet of paper between the two. That replaces one low friction interface with two high friction interfaces.

Where a sheet is held between two “friction pads”, the force needed to start movement will be the sum of the combined static friction forces from either side.

In your example, if the ball is held in position so it cannot move sideways then the force needed to overcome static friction will be the sum of the ball and floor static friction forces. But if the ball is able to move sideways very slightly then the static friction at the floor will break, followed by the ball contact. The ball contact will break when the sum of the moving floor dynamic friction plus the ball static friction is exceeded.

Energy will not be expended until static friction is overcome and movement occurs against the dynamic friction force.