Forces and Shape Flashcards
What is a force? List some properties
Something that can: •Start or stop an object moving •Speed up or slow down a moving object •Change the direction I’m which an object is moving •Make an object rotate •Change the shape of an object
Name some examples of forces
frictional force, buoyancy force (upthrust), gravitational force (weight), magnetic force, electrostatic force.
What is the measure of force
Newtons (N)
What is the earths gravitational field strength?
10N (precisely 9.8)
How can you measure the force of an object?
Spring balance or Newton balance.
What is friction?
Friction is a force that opposes motion.
How can you reduce friction?
Keeping moving surfaces apart by using a thin layer of a lubricant - e.g. Oil.
What is the frictional force proportional to?
The force pressing the surfaces together.
What is the elastic limit of a spring?
Where is no longer stretches uniformly and no longer obeys hookes law.
What happens when a spring has reached its elastic limit and you reduce the force on it?
When the force reaches zero again, the string has a permanent stretch.
What happens if you reduce the force on a spring before it reaches it’s limit of elasticity?
It will return to its original length.
What happens at the elastic limit of a wire?
When the extension of the wire becomes much larger for a given increase in force.
What happens at first when you apply force to an elastic band?
The rubber band is easy to stretch. There is a large extension for a small increase in force.
Does Hooke’s law apply to the elastic band?
No, at no stage is the extension of the rubber band proportional to the force applied to it.
What happens as you continue to add force to an elastic band?
It becomes more difficult to stretch. As the force is increased it does not get much longer.
What happens when you reduce the force on an elastic band to zero?
As with a steel spring that has passed its elastic limit the unloading curve does not follow the loading curve but there is mo permanent extension.
What is Hooke’s law?
F=kx where k is called the spring constant. This measures the stiffness of a spring.
What is elastic behaviour?
When a object is able to recover its original shape after the forces have been removed. The loading line should follow the unloading line on the graph.
Is a spring elastic?
Yes, as long as it’s elastic limit is not exceeded as this means it will recover its original length when the force acting on it is removed. Its unloading line will follow its loading line
Is a rubber band elastic?
No as although it will maintain its original shape when the forces are reduced its loading line does not match it’s unloading line.
Describe the force-extension graph for a helical steel spring
Between points A and B (the first straight line), the spring stretches uniformly
At B (when the graph starts to curve) the elastic limit is exceeded and the spring no longer stretches uniformly - as the force is increased further, the extensions suddenly become much larger
If the force on the spring is now gradually reduced (presuming it hasn’t broken) it follows a similiar line, just further right
When the force reaches zero again, the spring has a permanent stretch
Describe the force-extension graph for a copper (metal) wire
Between A and B (straight line) the wire stretches elastically - uniformly - and if the load is removed, the wire will return to its original length
At B (where the graph begins to curve) the wire begins to yield
Between B and C (where the graph line is almost horizontal but curving slightly upwards) the wire yields and exhibits plastic flow - the extension of the wire becomes much larger for a given increase in force (the wire no longer stretches uniformly)
At C (final point) the wire breaks
Describe the force-extension graph for a rubber band
The rubber band is easy to stretch at first - there is a large extension for a small increase in force
But eventually it becomes much more difficult to stretch the rubber band - as the force is increased, the band does not get much longer
At no stage is the extension of the rubber band proportional to the force applied to it
As for a steel spring that has passed its elastic limit, the unloading curve does not follow the loading curve - but there is no permanent extension
Is a metal wire elastic?
Yes, as long as the yield point is not reached, it will return to its original length and the unloading line will match the loading line
