Mechanics and material Flashcards
What is the moment of of force about a point?
The force x the perpendicular distance from the line of action of the force to the point.
What is the principle of moments?
For an object in equilibrium, the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about that point.
What is the centre of mass?
It is the point through which a single force on the body has no turning effect.
What is a couple?
Pair of equal and opposite coplanar forces acting on a body but not along the same line
What is a projectile?
An object in motion acted on only by the force of gravity.
Newton’s first law
An object remains at rest or in uniform motion unless acted on by a resultant force.
Newton’s second law
The rate of change of momentum of an object is proportional to the resultant force on it.
Inertia
Resistance of an object to change its motion
What is Newton’s third law?
When two objects interact they exert equal and opposite forces on each other
What is an inelastic collision?
When the colliding objects have less kinetic energy after the collision than before the collision.
What is energy?
The capacity to do work
What is kinetic energy?
The energy of an object due to its motion
What is potential energy?
The energy of an object due to its position.
Hooke’s law
The force needed to stretch a spring is directly proportional to the extension of the spring from its original length.
How to find the effective spring constant when two springs are in parallel
Add the two individual spring constants
How to find the effective spring constant when two springs are in series
1/k = 1/k1 + 1/k2
Elastic limit
The point beyond which a wire is permanently extended so will not return to its original dimensions when the force applied to it is removed.
How to calculate the elastic potential energy stored in a spring from a force against extension graph
The area under the graph
Define elasticity
Property of a solid that enables it to regain its shape after it has been deformed or distorted.
Define tensile stress
Force per unit area of cross-section in a solid perpendicular to the cross-section.
Define tensile strain
Extension per unit length of a solid when deformed
What is the Young’s modulus of a material?
Stress/strain (provided the limit of proportionality between extension and force hasn’t been exceeded. Used to measure the stiffness of a material.
What is the yield point of a wire?
The point at which the stress suddenly drops when the wire is subjected to increasing strain
Define plastic deformation
Deformation of a solid beyond it’s elastic limit
What is ultimate tensile stress? (UTS)
Tensile stress needed to break a solid material. Used to measure the strength of a material.
Force against extension graph for a metal wire
Straight line through zero for loading curve up to elastic limit. Unloading curve is same as loading curve if elastic limit isn’t reached. If it is, the unloading curve is the same but shifted to the right a bit.
Force against extension graph for rubber
Loading: steep then shallow then steep.
Unloading: similar but underneath loading curve.
Force against extension graph for polythene
Loading: steep then shallow then steep
Unloading: very steep straight line down
Define brittle
Snaps without stretching or bending when subject to stress
Define ductile
Stretches easily without breaking so can be drawn into wires.
What is the moment of a couple?
Force (of one) multiplied by the perpendicular distances between the lines of action of the forces.
How is the motion of two objects different immediately after an elastic or inelastic collision?
The objects will rebound in opposite directions. The velocity of the lighter one will be greater than the heavier one. The momentum of each after is the same as the momentum of the other before. Total momentum has same value and direction before and after
Why is there a gap between the loading and unloading curve for rubber?
It represents the difference between energy stored when it is stretched and the useful energy recovered from when it is unstretched. The difference occurs because some of the energy stored becomes the internal energy (potential and kinetic) of the molecules when it unstretches.
How to measure the value of g using motion sensor
Clamp motion sensor to a stand so it is over the edge of the table. Make sure it is pointing perpendicular to the floor. Connect it to a data logger. Drop an object from just under the motion sensor while it is recording data. Calculate gradient of velocity time graph (for when it is falling) produced on data logger to find the acceleration. This is g. Do from maximum height possible to reduce percentage error.
How to find Young’s modulus of a wire
Clamp wire to desk and attach a marker to it a known length from where the wire emerges from the clamp at 0 tension. Attach mass holder to end of wire over a pulley with the marker on the opposite side of the pulley. The weight of the masses is the force on the wire. For each new weight, find distance between clamp and marker to find extension. Use gradient of load against extension graph, cross-sectional area and original length to find YM. Between each result, remove masses and check wire returns to original length and haven’t exceeded elastic limit.
