Forces and Shape

Question 1

What is a force? List some properties

Answer 1

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

Question 2

Name some examples of forces

Answer 2

frictional force, buoyancy force (upthrust), gravitational force (weight), magnetic force, electrostatic force.

Question 3

What is the measure of force

Answer 3

Newtons (N)

Question 4

What is the earths gravitational field strength?

Answer 4

10N (precisely 9.8)

Question 5

How can you measure the force of an object?

Answer 5

Spring balance or Newton balance.

Question 6

What is friction?

Answer 6

Friction is a force that opposes motion.

Question 7

How can you reduce friction?

Answer 7

Keeping moving surfaces apart by using a thin layer of a lubricant - e.g. Oil.

Question 8

What is the frictional force proportional to?

Answer 8

The force pressing the surfaces together.

Question 9

What is the elastic limit of a spring?

Answer 9

Where is no longer stretches uniformly and no longer obeys hookes law.

Question 10

What happens when a spring has reached its elastic limit and you reduce the force on it?

Answer 10

When the force reaches zero again, the string has a permanent stretch.

Question 11

What happens if you reduce the force on a spring before it reaches it’s limit of elasticity?

Answer 11

It will return to its original length.

Question 12

What happens at the elastic limit of a wire?

Answer 12

When the extension of the wire becomes much larger for a given increase in force.

Question 13

What happens at first when you apply force to an elastic band?

Answer 13

The rubber band is easy to stretch. There is a large extension for a small increase in force.

Question 14

Does Hooke’s law apply to the elastic band?

Answer 14

No, at no stage is the extension of the rubber band proportional to the force applied to it.

Question 15

What happens as you continue to add force to an elastic band?

Answer 15

It becomes more difficult to stretch. As the force is increased it does not get much longer.

Question 16

What happens when you reduce the force on an elastic band to zero?

Answer 16

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.

Question 17

What is Hooke’s law?

Answer 17

F=kx where k is called the spring constant. This measures the stiffness of a spring.

Question 18

What is elastic behaviour?

Answer 18

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.

Question 19

Is a spring elastic?

Answer 19

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

Question 20

Is a rubber band elastic?

Answer 20

No as although it will maintain its original shape when the forces are reduced its loading line does not match it’s unloading line.

Question 21

Describe the force-extension graph for a helical steel spring

Answer 21

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

Question 22

Describe the force-extension graph for a copper (metal) wire

Answer 22

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

Question 23

Describe the force-extension graph for a rubber band

Answer 23

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

Question 24

Is a metal wire elastic?

Answer 24

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