Section 3 (mechanics and materials) Flashcards
What is Hooke’s law?
The extension of a stretched wire (🔺l) is proportional to the load or force (F).
F=k🔺l
A graph that obeys Hooke’s law will be a straight line through the origin.
What is meant by the elastic limit of a material?
If you increase the force past the elastic limit the material will be permanently stretched.
What is meant by plastic behaviour?
If a deformation is plastic the material is permanently stretched so when the load is removed the atoms don’t move back to their original position.
What is tensile stress, tensile strain and breaking stress?
Tensile stress= F/A
Tensile strain= 🔺l/ l
Breaking stress- the stress that’s big enough to break the material.
How do you work out the elastic strain energy stored in a stretched material?
Use the area under a force-extension graph.
Energy stored=1/2F🔺l
How is energy conserved in elastic stretches?
Work done stretching the material, elastic strain energy, another energy
If it’s in a spring… Kinetic energy, gravitational potential energy
How is energy conservation used in transport design?
Cars have crumple zones that deform plastically in a crash. The car’s kinetic energy is transferred into changing the shape of the vehicle’s body so less transfers to the people inside.
What is the formula for the Young modulus?
E= resolve stress/ tensile strain= FL/A🔺L
What is the yield point?
Where the material begins stretching and plastically deforming without any extra load.
Why are brittle materials brittle?
They have a rigid structure so the atoms within them can’t move to prevent cracks.
What is density?
Measure of the ‘compactness’ of a substance.
density (kg/m^3)= m (kg)/ v (m^3)
What is a vector?
What is a scalar?
A scalar has no direction (mass, time).
A vector has magnitude and direction (velocity, displacement, acceleration).
What conditions are needed for a body to be in equilibrium?
The forces must all be balanced so there is no resultant force. The object can be at rest or moving with a constant velocity.
What equation do you use to calculate the moment of a force about a point?
M (in Nm) =Fd
What is the principle of moments?
For a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anti-clockwise moments. If they are unbalanced there will be a resultant force and the object will turn.
What is a couple?
A couple is a pair of forces of equal size that act parallel to each other but in opposite directions. They don’t cause a resultant linear force but produce a moment.
What is meant by the centre of mass of an object?
It is where the mass of the object is thought to be concentrated.
Where is the centre of mass in a uniform regular solid?
In the centre.
What is meant by displacement, velocity and acceleration?
Displacement- how far an object has travelled from its starting point in a direction.
Velocity- speed in a given direction.
Acceleration- the rate of change of an object’s velocity.
What are the equations for uniform acceleration?
v=u+at
s=(u+v)t
2
s=ut+1/2at^2
v^2=u^2+2asDescribe a displacement-time graph.
d| | | | / | / |_/\_\_\_\_\_\_\_\_\_\_t The gradient is the acceleration. The steeper the gradient, the bigger the acceleration.
Describe a velocity-time graph.
v|
| /
| /
| /
|_/__________t
The gradient is the acceleration. The steeper the gradient, the greater the acceleration. The area underneath represents the displacement.
If the acceleration is non-uniform the gradient will be a curve.
Describe an acceleration-time graph.
a|\ acceleration | \ | \_\_\\_\_\_\_\_\_\_\_\_ t | \ | \ deceleration
The area is the velocity.
What are Newton’s three laws of motion?
The velocity of an object will not change unless a resultant force acts on it.
The acceleration of an object is proportional to the resultant force acting on it.
Every action has an equal and opposite reaction.
What is meant by freefall?
It is where there’s gravity acting Oman object and nothing else.
All objects in freefall will fall to the ground at the same acceleration.
How does air resistance affect projectile motion?
Air resistance causes a drag force acts in the opposite direction to motion and affects the trajectory of a projectile. Drag reduces the horizontal and vertical distance the projectile travels.
What 3 things do you need to know about frictional forces?
They always act in the opposite direction to the motion of the object.
They can never speed something or start something moving.
They convert kinetic energy into heat.
What is lift?
An upwards force perpendicular to fluid flow.
What is meant by terminal velocity?
It happens when the frictional forces equals the driving force.
Air resistance increases with speed- this affects the top speed of vehicles.
An object travelling through a gas or liquid will reach terminal velocity.
What is the equation for momentum?
p=mv
___ momentum is always conserved in a collision when no ____ ____ __.
Linear momentum is always conserved in a collision when no external forces act.
When is kinetic energy conserved? In an elastic or inelastic collision?
Kinetic energy is conserved in elastic collisions but not in inelastic collisions.
What is impulse?
It is equal to the change in momentum.
It is the area under a force-time graph.
What are the equations for work done?
What is the rate of work done?
W=Fs
W=Fscos0
The rate of doing work is the rate of energy transfer from one form to another.
The area under a force-displacement graph represents work done.
What is the principle for the conservation of energy?
Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change.
