2.5 Energy, Elasticity, Moments, Pressures Flashcards
When is work done on an object?
Work is done on an object when a force causes an object to move through a distance or when a force causes the displacement of the object
What is the formula for work done?
w= fd
Work Done = Force x Distance
W - Joules, F - Newtons, D- Metres
- The distance must be in the line of action of the force
Using a formula, what happens when a force of 1N moves an object by 1m
Work Done = Force x Distance
When a force of 1N (moves) causes a displacement of 1m, then J of work is done
Explain how we know when 1 joule of work is done?
One joule of work is done when a force of 1N causes a displacement of 1m
When it moves an object by 1m
Convert 1J to newton-metres
1J = 1NM
How does the frictional forces of an object cause a rise of temperature in the object?
- The energy is transferred by friction from the kinetic energy store to the thermal energy store
- This causes the temperature to increase and at the same down, slows the object down as it loses kinetic energy
What happens when work is done against the frictional forces acting on an object?
Work done against the frictional forces of an object causes a rise of temperature in the object
How can we can elastically change the shape of an object?
- Stretching the object
- Bending the object
- Compressing the object
Describe the forces needed to stretch an object
To stretch an object, 2 forces should act in opposite directions away from the object
Describe the forces needed to bend an object
To bend an object, one force shoudl act clockwise whilst the other acts anti-clockwise
Describe the forces needed to compress an object
To compress an object, two forces on opposite sides should be pushing towards each other
Why does more than one force have to be applied in order to change the shape of an object?
(when bending, compressing and stretching)
If just one force is applied, the object would just move and not change shape
What is elastic deformation?
Elastic deformation is when the object goes back to its original shape after the force has been removed
What is inelastic deformation?
Inelastic deformation is when the object does not go back to its original shape after the force has been removed
How do we determine if an object is elastic?
An elastic object will always return to their original shape (they are elastically deformed)
What does Hooke’s Law state?
Hooke’s Law states that:
The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality
What is the limit of proportionality?
The point beyond which the extension of an elastic object is no longer directly proportional to the force applied to it
What formulas are used for elasticity?
Force = spring constant x extension
F = ke
F - Newtons, k - N/m, e - metres
________________________________
elastic potential energy = 0.5 × spring constant × extension2
Ee =1/2 ke2
When can the formula f = ke be used?
The formula f = ke can only be used when the object is following Hooke’s law
What does a greater spring constant mean?
The greater the spring constant, the greater the stiffness
What happens when a spring is stretched or compressed in terms of energy?
When a spring is stretched or compressed, Work is done on the spring, and the energy is transferred to its elastic potential energy store
Describe the relationship between the work done on the spring and its elastic potential energy store
The work done on the spring is equal to its elastic potential energy store as long as the spring is not elastically deformed
Describe the difference between a linear and non-linear relationship between force and extension
- In a linear relationship between force and extension, there is elastic deformation
- In a non-linear relationship between force and extension, there is inelastic deformation
What can extension be replaced with in the equation for spring force?
Compression
Define moment of a force?
The moment of a force is the turning effect of a force about a pivot
What is the formula for calculating the moment of a force?
m = f d
moment = force x distance
m - Nm, f - Newtons, d - metres
The distance is the perpendicular distance from the pivot to the line of action of the force
What happens if we increase the distance from the pivot?
By increasing the distance from the pivot, less force is needed to produce the same turning effect
What happens if an object is balanced about a pivot?
If an object is balanced about a pivot, then the total clockwise movement about the pivot is equal to the total anti-clockwise movement about the pivot
How do we calculate the force or distance needed to balance out a pivot?
F1 x d1 = F2 x d2
Give examples of how rotational effects of forces be trasmitted?
A simple lever system and a simple gear system can both be used to transmit the rotational effects of forces.
Add lever and gear flashcards
What can a fluid be?
A fluid can be either a liquid or a gas
Define pressure
Pressure is the force per unit area
What does the pressure in a fluid cause?
The pressure in a fluid exerts a force normal (at right angles) to any surface
How do we calculate the pressure at the surface of a fluid?
pressure = force normal to a surface / area of that surface
p = f/a
p - Pascals (pa), f - Newtons, a - m2
Describe what happens in terms of pressure, if a force is spread over a large area?
If a force is spread over a large area, it will result in a small pressure
Describe what happens in terms of pressure, if a force is spread over a small area?
If a force is spread over a small area, it will result in a high pressure
What happens when an object is immersed in liquid?
When an object is immersed in liquid, the liquid exerts pressure at 90° to the surface which squeezes the object
How does the pressure of a liquid column increase?
In a liquid, the pressure at a point increases with the height of the column above that point and the density of the liquid
What is the formula for calculating pressure due to a column of liquid?
p = h P g
Pressure = height of column x density x gravitational field strength
P - Pascals (Pa), h - metres, P - kg/m3, g - N/kg
Why, in a liquid, does pressure at a point increase with the height of the column of liquid above that point?
- As the depth of the liquid increases, or the height of the liquid above that point increases, so does the mass of the liquid above that point
- This means that the force exerted at that point, due to the mass, also increases
- Since the force increased (because mass above that point increased) whilst the area remained constant, the pressure also increases
Explain why pressure at a point increases with the density of the liquid?
- As the density of the liquid increases, so does the mass
- This is because density = mass/volume, which means there are a more number of particles in a given volume
- As the mass increases, so does the weight of the fluid
- This means that a greater force is exerted due to increase in weight
- As the force increases whilst the area remains constant, the pressure of the fluid also increases
Describe the pressure experienced by a partially or totally submerged object
- An object that is partially or fully immersed in a fluid experiences a greater pressure on the bottom surface than on the top surface, because pressure in a liquid column increases with depth
- This creates a resultant force upwards which is known as upthrust
Describe the pressure experienced along a horizontal line of a column of liquid
The pressure is the same, as the density of the water and height above the horizontal line are the same, so the force experienced is the same across the fixed area, hence the same pressure
What determines if an object floats or sinks?
The weight of the object and upthrust determine if the object floats or sinks
- If the upthrust force is greater than or equal to its weight, then the object will float
- If the weight is more than the upthrust then the object will sink
Density of the fluid also determines if an object floats or sinks
How do we calculate the size of the upthrust acting on an object?
The size of the upthrust acting on an object is equal to the weight of the water it displaces
What does it mean if the weight of displaced water is equal to the weight of the object?
- It means that the force of upthrust is equal to the weight of the object
- …and the object will float
An object will also float if the upthrust is greater than the weight
Describe what happens to an object that has a higher density than liquid
- An object that has a higher density than the liquid sinks
- This is because the volume of liquid that the object displaces will not have the same weight as the object, it will be lower
- Because the weight of the object is greater than the weight of the liquid it displaces, it tells us that the size of upthrust is less than the weight of an object, so it sinks
Tip: Density = mass / volume
Describe what happens to an object that has a lower density than liquid
- An object that has a lower density than the liquid floats
- This is because the liquid has a higher density
- So the volume of liquid that the object displaces is equal to or greater than the weight of the object
- Because the weight of the object is equal to or greater than the weight of the liquid it displaces, it tells us that the size of upthrust is equal to or greater than the weight of an object, so it floats
Tip: Density = mass / volume
What is the atmosphere?
The atmosphere is a thin layer of air around the Earth
Describe the density of the atmosphere with increasing altitude?
The atmosphere gets less dense with increasing altitude
How is atmospheric pressure created?
- Air molecules colliding with a surface creates atmospheric pressure (e.g. an aeroplane’s windows)
- Each impact exerts a tiny force on the surface, but the number of molcules colliding with the surface per second is very large
Why does atmospheric pressure decrease with an increase of height?
- Pressure is created by air molecules colliding with a surface
- The number of air molecules above a surface decreases as the height of the surface above ground level increases
- This means less particles can collide
- So atmospheric pressure decreases with an increase in height
What happens when there is an increase of altitude/height? (in terms of the atmosphere)
- The density of the atmosphere decreases with an increase of height
- Atmospheric pressure decreases with an increase in height
Why does the density of the atmosphere decreases with an increase of height?
COME BACK LATER - WEIGHT OF THE AIR
