Force
Forces and Motion

Forces exerted by distorted solids

Physics Narrative for 5-11 11-14 Supporting Physics Teaching

Forces and acrobats

The Tombolas are a circus act. The bottom row support the rest. They are supporting the three extra bodies. They must push upwards more than normal.

Pile up three large cushions. The shape of the cushions changes. What is happening to the one at the bottom?

Both situations involve things in equilibrium. The forces acting down on the cushions and on the Tombola brothers are each being balanced by a force acting upwards. We might call this upward force a support force.

But what about the ground under Papa Tombola's feet? It is also being squeezed and is supporting him (and his sons). The floor under the cushions is doing the same job. How can the ground exert a force upwards?

Compression forces

If you look closely at the ground it will be slightly distorted, dipping a little due to the force acting on it. We can explain these surface support forces by thinking of the particles which make up the surface. This takes some imagination.

These sketches show how we might imagine the particles in the floor. A simple model is to consider the atoms in the floor being connected by spring-like bonds.

It is rather like a tiny mattress structure. When a force acts downwards on the floor, the floor distorts slightly and the floor structure becomes compressed. The reaction to being compressed provides an upward force to support the gravity force and this upward force is often referred to as the reaction force from the surface (the floor in this case). It is an example of a compression force: something is compressed and exerts a force as a result of this compression.

This is how we can think of a solid inanimate object providing an upward supporting force. This upward supporting force is our first example of a contact force, where one object exerts a force on another object through being in direct contact with it (the floor pushes up directly on the Tombolas' feet).

Since the distortion does not have to be made by pushing downwards (you could push sideways by learning on a wall) a better general name for such forces is compression forces. Expect such forces wherever one object is in contact with its surrounding in such a way as to compresses the solids in the surroundings. Compression forces are drawn from the surface of the object, to remind you of the warp or distortion that caused the push.

Tension forces

When you lie on a bed the gravity force acting on you squashes the springs and they react, providing an upward force to support you. The mattress is a person-size model of what is happening at a microscopic level under the bottom row Tombola's feet. Forces that tend to squash objects are called compression forces. Forces that tend to extend objects are called tension forces.

If you extend a metal wire, perhaps a guitar string, you can feel the effect of the tension forces between the particles in the wire. As the material extends, the particles separate and the attraction between them pulls them back together. We say the wire is in tension.

Expect tension forces wherever one object is in contact with its surrounding in such a way as to stretch the solids in the surroundings (for example, a climber hanging on a rope). Tension forces are drawn from the surface of the object, to remind you of the warp or distortion that caused the pull.

Both tension and compression forces arise when solids in the surroundings are distorted or warped. We might therefore group tension and compression forces as warp forces.

But what about objects surrounded by fluids (liquids and gases)? Read on.

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