Topic 15 - Forces and Matter Flashcards
Paper 2
What does stretching, compressing or bending do?
It transfers energy
What happens when you appply a force to an object?
+You may cause it to stretch, compress or bend.
+This requires more than one force acting on the object [otherwise the object would simply move in the direction of the applied force, instead of changing shape].
When has an object been elastically distorted?
+It has been elastically distorted if it can go back to its original shape and length after the force has been removed.
What are objects that have been elastically distorted called?
Elastic objects [eg. a spring]
When has an object been inelastically distorted?
+It has been inelastically distorted if it doesn’t return to its original shape and length after the force has been removed.
What is the elastic limit?
+The point where an object stops distorting elastically and begins to distort inelastically.
When is work done?
+When a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object.
+If it is elastically distorted, ALL this energy is transferred to the object’s elastic potential energy store.
What is extention directly proportional to?
Force
When does a spring stretch?
+If a spring is supported at the top and then a weight is attatched to the bottom.
What is the extention?
+The extention of a stretched spring [or other elastic object] is directly proportional to the load or force applied
+So F ∝ x
+This means that there is a linear relationship between and extention graph for the spring - [If you plotted a force-extension graph for the spring, it would be a straight line]
What is the force-extension equation?
What is the spring constant?
+The spring constant depends on the material you are stretching - a stiffer spring has a greater spring constant.
What does the force extention equation also work for?
+Compression - [where x is just the difference between the natural and compressed lengths - the compression]
For a linear relationship, what is the gradient of an object’s force-extention graph?
This is equal to its spring constant
When does extension being directly proportional to force stop working?
When the force is great enough.
What stops force and extension being directly proportional?
+There’s a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally
What does the graph show?
+The graph shows force against extension for an elastic object.
+There is a maximum force above which the graph curves, showing that extension is no longer proportional to force - the relationship is now non-linear as the object stretches more for each unit increase in force [this point is known as the limit of proportionality - shown on graph marked point P]
+The elastic limit is marked as E - past this point the object is permanently stretched.
How can you investigate the link between force and extension?
+Do an experiment with a spring and weights - make sure you have plenty of extra masses, then measure the mass of each [with a mass balance] and calculate its weight [the force applied] using W=mg
Describe the process of the experiment investigating force and extension.
- Measure the natural length of the spring [when no load is applied] with a millimetre ruler clamped to the stand - make sure you take the reading at eye level and add markers [eg. thin strips of tape] to the top and bottom of the spring to make the reading more accurate.
- Add a mass to the spring and allow the spring to come to rest - record the mass and measure the new length of the spring [the extension is the change in length]
- Repeat this process until you have enough measurements [no fewer than 6]
- Plot a force-extension graph of your results - it will only start to curve if you exceed the limit of proportionality but don’t worry if yours doesn’t [as long as you’ve got the straight line bit]
Describe the quick pilot experiment to find out what size masses to use.
- Use an identical spring to the one you will be testing, load it with masses one at a time and record the force [weight] and extension each time.
- Plot a force-extension graph and check that you get a nice straight line for at least the first 6 points. If it curves to early you need to use smaller masses.
What would you find that a larger force causes?
+A bigger extension - you can also think of this as more work needing to be done to cause a larger extension
+The force doing work is the gravitational force and for elastic distortions, this force is equal to F=kx
How can you also find the work done for particular forces [or energy stored]?
+By calculating the area under the linear section of your force-extension graph up to that value of force.
What is the elastic limit always at or beyond?
+The limit of proportionality - this means for a linear relationship, the distortion is always elastic - all the energy being transferred is stored in the spring’s elastic potential energy store.
As long as the spring is not stretched past its limit of proportionalty, what is work done to the spring equal to?
+It is equal to the energy stored in its elastic potential energy store
For a linear relationship, what is the equation for energy in the elastic potential energy store [work done]?
What is pressure?
The force per unit area
What is the equation for pressure?
The following equation can be used for solids, liquids and gases:
What are both gases and liquids?
+They are both fluids [their particles are free to move, or ‘flow’]
What is fluid pressure?
+The pressure caused by the collisions of gas or liquid particles on a given surface.
What does fluid pressure always exert a force at?
+Right angles [normal] to any surface in contact with the fluid - as the area of this surface increases, the pressure exerted decreases.
What does pressure exerted by a fluid depend on?
+The area the force is being exerted on, the properties of the fluid and the surrounding atmospheric pressure.
What does fluid pressure depend on?
+Depth and Density
What can density be thought of as?
+A measure of the compactness of a substance [ie. how close together the particles in a substance are]
+For a given liquid, the density is uniform [the same everywhere] and it doesn’t vary with shape or size.
+The density of a gas can vary though
Assuming their particles have the same mass, what does a denser fluid have?
+More particles in a certain space than a less dense one
+This means there are more particles that are able to collide so the pressure is higher at a given depth in the denser fluid.
What happens as the depth of a fluid increases?
+As the depth of a fluid increases, the number of particles above that point increases - the weight of these particles adds to the pressure felt at that point, so fluid pressure increases with depth
How can you calculate pressure?
+You can calculate pressure due to the column of liquid above a certain depth using:
What do objects in fluids experience?
Upthrust
What happens when an object is submerged in a fluid?
+When an object is submerged in a fluid [either partially or completely], the pressure of the fluid exerts a force on it from every direction
What does pressure increase with?
Depth - so the force exerted on the bottom of the object is larger than the force acting on top of the object.
+This causes a resultant force upwards known as upthrust.
What is upthrust equal to?
+The weight of the fluid that has been displaced by the object [eg. the upthrust on an old boot in water is equal to the weight of a boot-shaped volume of water.
What happens when weight = upthrust?
An object floats
What happens if the upthrust on an object is equal to the object’s weight?
+Then the forces balance and the object floats
What happens if an object’s weight is more than upthrust?
The object sinks
What will an objects density cause?
Whether or not an object will float.
What does an object that is less dense than the fluid it is placed in do?
+It displaces [pushes out of the way] a volume of a fluid that is equal to its weight before it is completely submerged.
+At this point, the objects weight is equal to the upthrust, so the object floats.
What does an object that is denser than the fluid it is placed in do?
+It is unable to displace enough fluid to equal its weight -this means the weight is always larger than the upthrust, so it sinks.
How do submarines make use of upthrust?
+To sink, large tanks are filled with water to increase the weight of the submarine so that it is more than upthrust.
+To rise to the surface, the tanks are filled with compressed air to reduce the weight so that it’s less than the upthrust.
What does atmospheric pressure decrease with?
Height
What is the atmosphere?
+A layer of air that surrounds Earth - it is thin compared to the size of the Earth.
How is atmospheric pressure created on a surface?
+It is created on a surface by air molecules colliding with the surface.
What happens as altitude increases?
+As the altitude [height above Earth] increases, atmospheric pressure decreases - as shown on graph.
+The graph is curved because atmospheric pressure is affected by the density of the atmosphere, which also varies with height.
+As altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface.
+There are also fewer air molecules above a surface as the height increases - this means that the weight of the air above it [which contributes to atmospheric pressure], decreases with altitude.
