Mechanics/ biomechanics - unit 1 deck 3 Flashcards
Define what is meany by the term statics in mechanics
This is the branch of mechanics which deals with forces and moments acting on bodies which are at rest or moving with constant velocity
==> by static we mean that the bodies are not accelerating; they may be moving with a constant linear &/ or angular velocity
Define what is meant by the term body in mechanics
This means the object of interest, it does not refer necessarily to the human body
What type of quantity is force ?
It is a vector quantity ==> has a magnitude and direction in which it acts
What are the two effects a froce will have on a body?
- Firstly it will tend to change the bodys position in space
- Secondly it will tend to deform the bodys shape
Which effect of a force will we only consider in this module and what are we therefore assuming ?
We will only consider the first effect - a force will tend to change a bodys position in space
==> assuming that the bodies under consideration are rigid i.e. they cannot be deformed - known as rigid body mechanics
In reality do all bodies deform under the action of an applied force?
Yes - but we dont need to worry about that
State one way of measuring a force and explain how it works
By using a calibrated spring balance
This works as the spring stretches in proportion to the applied force. The spring balance’s scale shows the magnitude of force acting parallel to the spring
What is the SI unit of force ?
Newtons (N)
Define both mass and weight
- Mass is the quantity of matter of which a body is composed
- Weight is the force of gravity acting on a body
e.g. On the moon the astronauts weigh less than they do on Earth. (Their mass however remains the same as it did on Earth.) The reason for the astronauts’ light weight on the moon is that the moon’s gravitational field is approximately 1/6th the magnitude of that on Earth. In outer
space an astronaut can be described as being weightless as the gravitational field is
negligible. However in space, on the moon and on earth an astronaut is still made up of
the same quantity of matter; his or her mass
State the SI unit of mass and the SI unit of weight
- SI unit of mass = kilogram (kg)
- SI unit of weight = Newton (N)
What are the symbols used to represent both mass and weight
- Mass - m
- Weight = W
Define what density of a material is
This is the mass per unit volume
State the equation for calculating density
ρ = m / V
- ρ = density of the materal
- m = mass of the body made from the material
- V = volume of the body made from the material
What is the SI unit of density?
kilograms per meter cubed (kg m-3)
Describe the relationship of density, mass and volume of a material
The density of a material always remains constant
Mass and volume are therefore proportional to each other i.e. if the mass increases then the volume must also increase as the density always remains constant
Do SAQ 1 pg. 3 of force in mechanics binder
Define what gravity is
This is the acceleration due to the gravitational attraction between two bodies
e.g. The gravitational attraction between the moon and the Earth keeps the moon in orbit about the Earth. On Earth, the gravitational attraction between two bodies is negligible in comparison to their attraction to the Earth. (Two cups of coffee sitting on a glass table top don’t move towards each other however smooth the table’s surface, yet, if they were placed too near the edge, they would both “fall” off the table due to the gravitational attraction between them and the Earth.) Therefore, gravity accelerates bodies if they are allowed to freefall
What causes the gravitational attraction between two bodies to increase?
It increases as the mass of the bodies increase
Gravity accelerates a body if it is allowed to do what?
Free-fall
What does gravity cause a body to have ?
Weight
What is the equation for calculating weight ?
W = mg
- W = the bodies weight
- m = the bodies mass
- g = the acceleration due to gravity
What is the earths gravity ?
9.81 m s-2
Define what the centre of mass of a body is
This is the point where all the mass of a body can be assumed to act
What does the symbol in the pic represent ?
This is the symbol used to represent the centre of mass of a body
What does the position of the centre of mass of a body depend on ?
It depends on how the mass is distributed in a body
How does the centre of mass in a rigid, symmetrical body of uniform density (i.e. a body with an even mass distribution) compare to the centre of mass in a complex system (e.g. the human body)
In a rigid, symmetrical body of uniform density (i.e. a body with an even mass distribution) it will have a centre of mass lying at its geometric centre (e.g. a cylinder and a sphere)
However in a complex system (e.g. the human body) it will not have its centre of mass coinciding with its geometric centre, since the mass is not distributed evenly. This uneven distribution may be due to varying densities within the body (e.g. muscle has a higher density than fat) or due to the structure of the body (e.g. a golf club or boomerang)
Does the centre of mass of a body need to lie within the body itself?
No
e.g. When someone stands upright, his or her centre of mass will be positioned in the lower abdomen. However, when bending over it will move to lie outside the body in front of the pelvis
What can the human body be considered constructed of to make up the whole body?
A number of parts or segments
What does each individual body segment (e.g. the right thigh) have ?
Its own centre of mass
What will happen if one body segment is moved to the centre of mass of the whole body?
As one segment is moved relative to the others the centre of mass of the whole body will shift accordingly. (human body centre of mass usually lies in the lower abdomen)
An example of the shift in centre of mass is illustrated in Figure 19 Pg. 5 in forces section of mechanics binder, which shows how Biomick’s centre of mass position varies as he performs a long jump
Along with a centre of mass what will a body in a gravitational field also have ?
A centre of gravity
Define what is meant by the centre of gravity of a body?
This is the point where the weight of the body can be assumed to act
Does the centre of mass and centre of gravity occur at the same point ?
Coincidentally they do, but its important to remember that centre of gravity only occurs when a body is in a gravitational field e.g. an astronaut in outer space will have a centre of mass but not a centre of gravity
Do SAQ 2 pg. 5 of force unit in mechanics binder
Define what friction is
This is the force arising between two surfaces when they rub against each other
What direction does friction act
It acts as a tangent (straight line) to the surfaces rubbing against eachother and opposes motion (i.e. acts in the opposite direction to motion)
What is the SI unit of friction?
Netwton (N) since it is a force
What is the maximum magnitude of friction force dependent on ?
It is dependent on the textures of the two surfaces and the magnitude of the force pressing them down
What is the difference in friction between wet or lubricated surfaces and dry surfaces ?
The wet or lubricated surfaces will have a smaller friction force than the dry surfaces
As the force pressing two surfaces together increases what happens to the friction force?
The friction force increases
Define what the coefficient of friction is
This is the measure of the maximum friction force between two surfaces
It is the ratio of the friction force to the force acting normally (perpendicular) to press the two surfaces together
Define what is meant by the term perpendicular
This means a straight line at an angle of 90 degrees to a given line, plane or surface
State the equation for calculating the coefficient of friction between two surfaces
μ = F / N
- μ = the coefficient of friction between two surfaces
- F = the friction force
- N = the force acting normally to the surfaces
What are the 3 different circumstances in which friction can occur ?
- Static
- Sliding
- and Rolling friction
Does the co-efficient of friction vary between the 3 different circumstances in which friction occurs ?
Yes it does - there is three different values for the coefficient of friction for the interaction between the same materials
When does static friction occur ?
It occurs only when motion is about to occur between two surfaces
When static friction is present just before motion is about to occur, what is the static friction force present just sufficient enough to do ?
To oppose the applied force that is trying to more the surfaces forces over one another
What needs to be exceeded for motion to occur ?
The maximum static friction force needs to be exceeded
So essentially as a gradually increasing force is exerted in an attempt to slide the two surfaces apart then the static friction force will increase, matching the applied force until the maximum possible static friction force is reached. Above this maximum static friction force sliding will begin.
Refer to pic on pg. 6 force unit, mechanics binder
What is the coefficient of static friction equal to ?
It is equal to the ratio of the maximum friction force to the force acting normally to press the two surfaces together (refer to the equation)
When does sliding friction occur ?
It only exists when sliding occurs between two surfaces
What is the measure of sliding friction?
This is the coefficient of sliding friction which is (as with static friction) the ratio of the sliding friction force to the force acting normally to press the two surfaces together
During sliding what happens to the friction force (sliding) opposing the motion?
It remains constant
Is sliding friction force greater than static friction force?
No! sliding friction is less than static friction; more force is required to start a body moving against a frictional force than to keep it moving
e.g. try sliding your hand along the surface of a table with and without pressing downwards - notice how much more difficult it was to slide your hand when pressing down (as the force acting ormally to press the surfaces together increases, static friction force increases). You may have also noticed that once you’re hand began sliding the resistance was reduced.
When does rolling friction occur ?
This arises between an object and the surface over which it is rolling
Why does rolling friction occur ?
It arises because of the deformation of the two surfaces caused by the force acting normally to the two surfaces
e.g. the deformation of a car tyre on a road, the rolling friction force is given by the coefficient
What is the rolling friction force given by?
The coefficient of rolling friction
Is the friction force acting on a rolling object higher or lower than the friction force acting on a sliding one ?
Friction force acting on a rolling object is lower than on a sliding one hence the use of ball bearings in machinery to reduce friction
Does lubrication lower rolling friction like in static friction?
No but it may reduce wear
