Changing the shape of an object

1. An object undergoes deformation when the forces acting on it change its shape or size.
2. The three basic types of deformation are stretching, compressing and bending.
   1. Stretching pulls an object so that it becomes longer, as when you pull a spring or a rubber band.
   2. Compressing squashes an object so that it becomes shorter or thinner, as when you squeeze a sponge or sit on a cushion.
   3. Bending makes an object curve, as when a diving board sags under a diver or a ruler bows when you press its middle.
3. When an object is stretched, the increase in its length is called its extension.
4. The extension is found by subtracting the original length from the stretched length.
   1. A spring with an original length of $8.0cm$ that is stretched to $11.0cm$ has an extension of $3.0cm$.
5. When an object is compressed, the decrease in its length is called its compression.

Why one force is not enough

1. If only one force acts on an object, the resultant force on it cannot be zero.
2. An unbalanced resultant force makes the whole object accelerate in the direction of that force, so the object moves off instead of changing shape.
   1. If you pull one end of a spring lying loose on a smooth bench, the whole spring slides towards you and does not stretch.
3. To change the shape of an object, more than one force must act on it, and the forces must act on different parts of the object.
4. The forces together stop the object as a whole from moving off, while its different parts are pulled apart, pushed together or made to curve.
   1. To stretch the spring on the bench, you must hold its other end still, so that a second force pulls on it in the opposite direction.
5. The forces that deform an object are often balanced, so the object stays still while its shape changes.

Stretching forces

1. To stretch an object, forces must pull on it in opposite directions, away from each other.
2. A spring hanging from a clamp stand with a mass on its lower end is stretched by two forces.
   1. The weight of the mass pulls the bottom of the spring downwards.
   2. The clamp pulls the top of the spring upwards.
3. The pulling force that is set up inside a stretched spring, rope or cable is called tension.
4. In a tug of war, the two teams pull the rope in opposite directions, so the rope is stretched tight and is under tension.
5. A bungee cord is stretched between the jump platform that holds its top end and the weight of the jumper hanging from its lower end.

Compressing forces

1. To compress an object, forces must push on it in opposite directions, towards each other.
2. When you press down on a sponge resting on a table, your hand pushes down on the top of the sponge while the table pushes up on its bottom, so the sponge is squashed between them.
3. A car's suspension springs are compressed because the car body pushes down on the top of each spring while the axle pushes up on its bottom.
4. The legs of a chair are compressed between the weight of the person sitting on it and the upward push of the floor.
5. The sole of a trainer is compressed between your foot and the ground every time you take a step.

Bending forces

1. To bend an object, forces must act at different places along its length, usually with forces in one direction at the ends and a force in the opposite direction in between, or the other way round.
2. A ruler resting on two books bends when you press down on its middle, because the books push up at each end while your finger pushes down at the centre.
3. A shelf sags under a row of heavy books because the brackets push up near its ends while the weight of the books pushes down along its length.
4. A diving board curves downwards because the clamp and the support hold one end while the weight of the diver pushes down on the free end.
5. When an object bends, the material on the outside of the curve is stretched and the material on the inside of the curve is compressed.
6. A fishing rod bends when a fish pulls on the line at the tip while the angler holds the handle firmly at the other end.

Deformation in everyday objects

1. A trampoline works because the springs around its edge are stretched between the frame and the mat each time you land on it.
2. A climbing rope is stretched between the anchor point above the climber and the weight of the climber below.
3. A tennis ball is squashed against the racket strings during a shot, because the strings push on one side of the ball while the ball presses back on the strings.
4. In every case you can name two objects in contact with the deformed object, and each of them supplies one of the forces that changes its shape.