Mechanics And Materials Flashcards
What is a vector?
Any physical quantity that has a direction as well as magnitude
What is a scalar?
Any physical quantity that is not directional
What is equilibrium?
An object is in equilibrium if it has a resultant force of zero and a resultant moment of 0 so it is not accelerating or turning
How do we know if forces are in equilibrium?
- We can construct a closed triangle of forces
- One of the forces is equal but opposite in direction to the sum of the other two
What is a free body diagram?
A diagram showing all forces that act on a point object.
What is a moment?
A moment of a force about any point=
Force x Perpendicular distance from the line of action of the force to the pivot
What is the principle of moments?
When a system is at equilibrium the sum of clockwise moments = the sum of anticlockwise moments about any given pivot
What is the centre of mass?
The point through which the line of action of a single force on the body has no turning effect.
What is a couple?
A couple is a pair of equal and opposite forces acting on a body, but not along the same line of action. The couple tries to turn the body.
How do you calculate the moment of a couple?
Force x Perpendicular distance between the lines of action of the forces
What is a stable equilibrium?
A body is in stable equilibrium if it is displaced then released and returns to the equilibrium position.
What is an unstable equilibrium?
A body is in unstable equilibrium if it is slightly displaced from equilibrium it accelerates away from its equilibrium
When does a body tilt?
Fd>Wb/2
Why does the force required to tilt an object decrease as it becomes more tilted?
Because the width of the base decreases
When will an object topple?
When the line of action of the weight of the object is pushed past the pivot
What are the 5 SUVAT equations?
- v=u+at
- s=ut + 1/2at^2
- s=vt - 1/2at^2
- s=(v+u)t/2
- v^2 = u^2 + 2as
What is the relationship between horizontal and vertical motion?
They are independent but linked by time
What equipment can we use to
investigate the acceleration due to gravity?
Use a multi flash photo or a video clip
How can a video be used to investigate acceleration due to gravity?
- A vertical meter ruler is used to provide a scale
- Camera with slow speed shutter records the ball in a dark room illuminated by a stroboscope
- For every image of the ball, the time of decent of the ball and distance covered is measured
- Plotted as a graph to get acceleration
What is a projectile?
Any object acted upon only by the force of gravity
What are the key points when considering projectiles?
- Only acceleration is g downwards
- Horizontal velocity of an object is constant
- Vertical and Horizontal motion are independent of each other
What is projectile-like motion?
Any form of motion where an object experiments a constant acceleration in a different direction to its velocity.
What are the effect of air resistance on projectiles?
- Experience a resistive force opposite to the direction of motion
- Reduces the velocity of the object such that it has a decreased vertical and horizontal displacement
What is thinking distance?
Distance travelled by a vehicle in the time it takes for the driver to react
What is braking distance?
Distance travelled by the car in the time taken for it to stop safely
What is stopping distance?
Thinking distance + Braking Distance
What is the limiting friction force?
The maximum force acted on an object just before sliding occurs
What is thinking distance affected by?
- Distractions
- Drugs: depressants
- Alcohol
- Tiredness
What is braking distance affected by?
- Condition of the road surface
- Condition of the tires
What does ABS do?
It prevents the wheels from sliding uncontrollably when the limiting frictional force between the tires and the ground is reached.
What is the impact time?
The duration of the impact force
How does a crumple zone increase safety?
- Engine compartment is designed to give way in a front-end impact
- Increases impact time
- Reduces impact force as change in momentum is the same
How do safety belts increase safety?
- Prevent occupant from hitting the frame which would result in a much larger impact force
- Does this by gradually slowing them down
- Increases impact time
- Reduces impact force
How does an airbag increase safety?
- Acts as a cushion increasing impact time
- Force spread over a larger area than a seat belt
- Less pressure on the body
What does the drag force depend on?
- Shape of the object
- Objects speed
- Viscosity of the fluid
What is terminal velocity?
When the drag force is equal to but opposite in direction to weight
What is Newtons Second Law?
Force is the rate of change of momentum
What is impulse?
Change in momentum
When is a collisions elastic?
When KEBefore = KEAfter
When is a collision inelastic?
When KEBefore != KEAfter
What are the properties of impulse in a collision?
The impulse is equal for both objects but in opposite directions.
When would you use F=p/t instead of F=ma?
When the mass is constantly changing such as in a launching rocket
How can you find impulse in a F - T graph?
Area under the graph
What can we say about the direction of momenta when two objects of the same mass collide at an angle?
They will be at right angles
What does Hooke’s Law state?
The extension of a spring is directly proportional to the force applied provided its limit of proportionality is not exceeded.
What is the limit of proportionality?
The point at which force is no longer proportional to extension.
What is the limit of elasticity?
Point beyond which the spring will no longer return to its original shape when the force is removed.
What happens when the force is removed from the spring that has gone beyond the limit of elasticity?
The force-extension graph may have the same gradient, but shows a permanent extension for zero force.
What is the elastic region?
Region up to the limit of elasticity
What is the plastic region?
Region past the limit of elasticity
What is the equation for Hooke’s Law?
F = kx
What is the equation for energy stored in a spring?
E = 0.5kx2 OR E = Favgx
What is the equation for springs in parallel?
F = kx where k = k1 + k2 + …
What is the equation for springs in series?
F = kx where 1/k = 1/k1 + 1/k2 + …
What is density?
Mass per unit volume
How do you measure the density of a regular solid?
- Measure mass using balance
- Measure dimensions using vernier callipers or a micrometer and calculate volume using equation
- Calculate density
How do you calculate the density of a liquid?
- Measure mass of an empty measuring cylinder
- Pour as much liquid as possible and measure its volume directly
- Measure mass of cylinder and liquid
- Calculate density
How do you calculate the density of an irregular solid?
- Measure the mass of the object
- Immerse the object on a thread in a liquid-filled measuring cylinder and record the increase in volume
- Calculate density
What is energy used for?
- To accelerate an object
- Change its shape
- Increase its temperature
What is work?
Work is done on an object when a force acting on it makes it move
What is the equation for work?
W = Fd
What is the equation of work if force is in a different direction to motion?
W = Fdcos θ
Where θ is the angle between movement and force
What is the area under a force distance graph equal to?
Work Done
What is the nuisance in GPE?
It is only valid for relatively small h compared to radius of the earth
What is the equation for power of a powered vehicle?
P = Fv
What is motive power?
Energy wasted to surroundings per sec + gain in KE per sec
What are the types of deformation?
- Tensile
- Compressive
What is elasticity?
The ability of a solid to regain its shape after it has been deformed and the forces of deformation are removed
What is tensile deformation?
Deformation that stretches an object
What is compressive deformation?
Deformation that squeezes an object
What is the equation for stress?
T/A
What is the equation for strain?
X/L where x is extension
What is the equation for Young’s Modulus?
E = stress/strain = FL/AX
How do you work out the energy transferred to a spring?
Area between loading and unloading graphs
Why is the centre of mass of a hanging object directly below the point of attachment.
So that there is a zero moment about the pivot which is the attachment point.
State the law of the conservation of energy.
Energy cannot be created or destroyed, but only transferred between energy stores
What is a brittle material?
A material that shows little, or no, plastic deformation before breaking
What is a ductile material?
A material that can be drawn into a wire
What is stiffness?
Proportional to Young’s Modulus and equal to the gradient in a stress-strain graph
What is UTS?
Ultimate Tensile Strength
What is the strength of a material?
The value of UTS
What is strain energy?
Energy required to deform an object
What is tensile stress?
The tension that is applied per unit of cross-sectional area of a wire
What is tensile strain?
The ratio between the extension of the material and the original length
Define the Young’s modulus
The ratio of stress over strain
