3.4 Mechanics and materials Flashcards
What is a scalar quantity?
A quantity with only a magnitude
What is a vector quantity?
A quantity with both magnitude and direction
What are some examples of scalars?
Speed, mass, distance, temperature
What are some examples of vectors?
Velocity, force/weight, acceleration, displacement
What is the moment of a force about a point?
The force multiplied by the perpendicular distance from the line of action of the force to the point
What is a couple?
A pair of coplanar forces where the forces are equal in magnitude but act in opposite directions
What is the principle of moments?
For an object in equilibrium, the sum of anticlockwise moments about a pivot is equal to the sum of clockwise moments
What is the centre of mass of an object?
The point at which an object’s mass acts
What does it mean if an object is uniform?
Its centre of mass is exactly at its centre
What is speed?
The scalar quantity describing how quickly an object is travelling
What is displacement?
The vector quantity describing the overall distance travelled from the starting position
What is velocity?
The rate of change of displacement
What is acceleration?
The rate of change of velocity
What is instantaneous velocity?
The velocity of an object at a specific point of time
What is uniform acceleration?
Where the acceleration of an object is constant
What does the area under an acceleration-time graph represent?
The change in velocity
What does the gradient of a velocity-time graph represent?
Acceleration
What does the area under a velocity-time graph represent?
Displacement
What does the gradient of a displacement-time graph represent?
Velocity
When can you use SUVAT equations?
When an object is moving at uniform acceleration
What does it mean if an object is in free fall?
It is experiencing an acceleration of g (9.81ms^-2)
What is friction?
A contact force that opposes the motion of an object
What do frictional forces do in terms of energy transfer?
Convert kinetic energy into other forms, such as heat and sound
When does terminal velocity occur?
When the frictional forces acting on an object and driving forces are equal (no resultant force/ acceleration)
What components of a projectile’s motion are affected by air resistance?
Both the vertical and horizontal components
What is Newton’s 1st law?
An object with no resultant force will remain at rest or will be travelling at a constant velocity
What is Newton’s 2nd law?
The acceleration of an object is proportional to the resultant force experienced by the object, F=ma
What is Newton’s 3rd law?
For each force experienced by an object, the object exerts a force of equal and opposite magnitude
Why are free-body diagrams called ‘free-body’?
The diagram concerns only the object and nothing else
What is momentum?
The product of the mass and velocity of an object
What does the conservation of linear momentum state?
Momentum is always conserved in any interaction where no external forces act, so the momentum before and after an event is equal
What is force?
The rate of change of momentum
What is impulse?
The change in momentum
What does the area under a force-time graph represent?
The change in momentum
What are the purpose of crumple zones, seat belts and air bags?
To decrease the force exerted on passengers, by increasing the time of the collision
What is conserved in an elastic collision?
Momentum and kinetic energy
What is conserved in an inelastic collision?
Only momentum
Why is kinetic energy not conserved in inelastic collisions?
It is converted into other forms of energy
What type of collision is it when the objects stick together after the collision?
Inelastic
What type of collision is an explosion?
Inelastic
What is work done defined as?
The energy transferred as a force causing a motion multiplied by the distance travelled in the direction of the force
What is the rate of doing work the same as?
The rate of energy transfer
What does the area under a force-displacement represent?
Work done
What is efficiency?
A measure of how efficiently a system transfers energy
What is the principle of conservation of energy?
Energy cannot be created or destroyed, but can be converted from one form into another
What is Hooke’s Law?
The force needed to extend/compress a spring is proportional to the extension, provided the elastic limit has not been exceeded, F = ke
What is the elastic limit?
The maximum extent a material can be stretched without permanent alteration of its form
What is tensile force?
The force applied to a material that makes it stretch
What is tensile stress, and what are the units?
The tensile force exerted per unit cross-sectional area, in Nm^-2 or Pa
What is tensile strength?
The tensile stress at which the material breaks
What is tensile strain?
How much a material stretches per original length, no units as it is a ratio of length
What is the Young modulus of a material, and what are the units?
The ratio of stress to strain, in Nm^-2 or Pa
What is the gradient of a stress-strain graph?
The Young Modulus of the material
On a stress-strain graph, what is are the two consecutive points called where the extension is no longer proportional to the force (where Hooke’s Law is no longer obeyed)?
The proportionality limit, followed by the elastic limit
What happens to the material at the elastic limit?
The material becomes permanently deformed (plastic, no longer elastic)
What is the difference between elastic deformation and plastic deformation?
- Elastic deformation: after the load is removed, the material can return to its original form (reversible)
- Plastic deformation: after the load is removed, the material cannot return to its original form (irreversible)
What is the point after the elastic limit on a stress-strain curve?
The yield point
What is the yield point?
Where there is a large increase in extension for a small increase in force (steeper gradient)
What is the region after the yield point on a stress-strain curve?
The plastic region
What is happening in the plastic region?
Extension is rapidly increasing, making ductile materials into wires
What is happening in the region after the plastic region?
The material is necking
What is the point at which the curve ends on a stress-strain graph?
The ultimate tensile strength - the material’s fail point
What is the area under the linear region of a stress-strain curve?
Stress energy density
What is the point at which the line is no longer linear on a force-extension graph?
The elastic limit, where the material becomes plastic
What type of materials have a large plastic region on a stress-strain curve?
Ductile materials - form wires after plastic deformation
What type of materials have no plastic region on a stress-strain curve?
Materials with a high tensile strength
Strong but brittle
What do highly elastic materials look like on a stress-strain curve?
- Difficult to stretch at first, but then easier before becoming difficult again (hysteresis curve)
- Steep -> shallow -> steep
What is the area under a force-extension graph?
Work done
What is the gradient of a force-extension graph?
Spring constant
What is a ductile material?
Forms wire after plastic deformation
What is a brittle material?
Strong but not stretchy (opposite to ductile)
What is a tough material?
Resists high stresses
What is a stiff material?
Stretches a small amount for a large stress
What are the 2 equations for EPE?
EPE = 1/2Fe
EPE = 1/2ke^2 (not given)
What is spring energy transformed to?
Kinetic and gravitational potential energy
