Temp 3,4,5,6, motion and momentum Flashcards
What is the name for forces that produce an extension in an object?
Tensile forces
What is the name for forces which compress an object?
Compressive forces
What happens to a spring when it experiences tensile and compressive forces?
Tensile deformation
Compressive deformation
What happens to the force-extension graph of a spring when it reaches its elastic limit? And what does the spring experience past this point?
It starts to flatten out and experience plastic deformation
What is elastic deformation?
When the shape of an object is deformed but it will still return to its original shape
What is plastic deformation?
When the shape of an object is deformed but it will not return to its original shape
When does a spring generally obey Hooke’s law?
When it has not yet reached its limit of proportionality.
What is the equation for force in terms of extension? (Hooke’s law)
Force = spring constant x extension
What is the spring constant of a spring actually measuring?
The stiffness of a spring
How can the spring constant be interpreted?
If the spring constant is high, the spring is difficult to extend
What is the gradient of a force - extension graph?
The spring constant
What happens to the work done on a material that is extended?
If it has not gone beyond its elastic limit, the work done on a material that is extended can be fully recovered
What is the equation for work done in a material that has been extended?
Change in W = Force x Change in distance
What is the area under a force - extension graph?
The work done on the spring
What is the work done on a spring in order to extend it transferred into?
Elastic potential energy within the spring
What is the equation for elastic potential energy and where has it come from?
E = 1/2 * F * extension
This is the area under a force extension graph since it will be a triangle
What is the equation for elastic potential energy in terms of the spring constant?
E = 1/2 * k * extension^2
What will doubling the extension of a spring do to its elastic potential energy? Why?
Multiply it by 4, since elastic potential energy is directly proportional to extension^2
What is the name of the ‘loop’ formed under the force-extension graph of rubber?
A hysteresis loop.
What does the area inside of a hysteresis loop represent?
The thermal energy released in the loading of the rubber material.
What is the equation for tensile stress?
Stress = force / cross-sectional area
What is the equation for tensile strain?
Strain = Extension / original length
What is it called when a material is stretched and begins to get thinner?
Necking
What are the 6 major points on a stress strain graph (for steel)? (In order)
The limit of proportionality
The elastic limit
Yield points 1 and 2
UTS point
Breaking point
What is the limit of proportionality?
The point when a material stops obeying Hooke’s law
What could define a strong material?
One with a high ultimate tensile strength
What is the equation for young modulus?
Young modulus = Stress / strain
When does the young modulus apply?
Up to the limit of proportionality of a material
What is the unit for the young modulus?
Pa or Nm^-2
How can you obtain the young modulus of a material from its stress strain graph?
By finding the gradient
How can young modulus be interpreted?
A higher young modulus makes for a stiffer material
How can you identify a brittle material from a stress strain graph?
A brittle material does not have a curve, the line simply ends when the material snaps
What is the elastic limit?
The point at on a force-extension graph where if exceeded, a spring will no longer return to its original shape
What is the equation for spring constant in a series of springs?
1/k = 1/k1 + 1/k2 (the same equation for resistance in parallel)
What is the equation for spring constant for springs arranged in parallel?
K = k1 + k2
What is the ultimate tensile strength of a material?
The maximum amount of stress it can endure before necking and breaking
How can you measure the young modulus of a wire?
Stretch it between to points and mark its length at any point, then find the cross sectional area and total length of the wire. Then add weights to one end and as the wire stretches measured its new weight with each new mass added. Then calculate with this the stress and strain then the young modulus.
What is the unit for stress?
Nm^-2
What is the unit for strain?
There isn’t one
Def conservation of energy
The total energy of a closed system remains constant. Energy cannot be created or destroyed, only transferred from one form to another.
Define energy
The capacity that something has to do work (since one Joule is equal to one Nm)
Def elastic potential energy
The energy stored in an object as a result of reversible changes in its shape
Def electrical potential energy
Energy of electric charges due to their position in an electrical field
Def internal (heat or thermal) energy
The sum of all the random potential and kinetic energies of the atoms in a system
What is important to remember about KE and GPE which is useful in an exam?
Loss of GPE = gain in KE, therefore:
mgh = 1/2mv^2 (Cancel the mass)
What is the equation for power?
P = W / t
What other ways could you find the work done in the power equation?
Find the change in kinetic energy,
Find the change in GPE
Find the regular work done (force x distance)
What is the second equation for power? (In terms of speed)
Power = force x speed
What is the equation for efficiency percentage?
Efficiency = (useful output energy / total input energy) x 100
What is the main equation for work done?
W = force x distance
Define the centre of mass
The centre of mass of an object is the mean point of all the mass of an object
Define the centre of gravity and where is it?
The centre of gravity is the point where the entire weight of an object can be thought to be concentrated. (Appears to act). It is in the same place as the centre of mass.
How can you find the centre of gravity of an object?
By using 2 points of suspension on a 2D object (flat object) you can use a plumb line to draw a vertical line across the object. The point of intersection of these two lines is the centre of gravity.
What are the 6 main forces to be considered in almost any free-body diagram?
Weight
Friction
Drag
Tension
Upthrust
Normal
What are the two main factors in causing an objects terminal velocity?
The objects speed and cross-sectional area
What is drag directly proportional to?
Speed^2
What is the equation for the moment of an object?
Moment = force x distance perpendicular to the direction of the line of action of the force
What is the unit for a moment?
Nm (Newton metre)
Define a couple of forces
When two equal forces parallel to each other but on different lines that are of equal magnitude affect two different sides of an object causing it to spin without moving (rotating and not translating)
What is the torque of a couple?
The torque of a couple is the moment between the pair of forces
What is the equation for the torque of a couple?
Torque (Nm) = Force (N)(of one of the forces) x distance (m)(between the two forces not the centre)
What needs to be included alongside the answer and units for a question that wants the torque of a couple?
The direction of motion of the torque (clockwise or anti-clockwise)
What is an easy way to help visualise and thus solve a triangle of forces question?
Draw the completed triangle of forces
What is the equation for pressure? (Basic)
P = F / A
What is the unit for pressure and what is its unit in SI units?
Pascals (Pa) or (Nm^-2)
What is the equation for the pressure of a vertical column of liquid?
P = height x density x g
What unusual property is true about the pressure in a fluid?
The pressure at any particular depth is the same in all directions.
What is the equation for the upthrust of an object underwater and how is this obtained?
Upthrust = x (depth of the object) * cross sectional area * density of the fluid * g
This is obtained by subtracting the pressure acting at the top of any object from the pressure acting at the bottom of the object. This is what gives x as distance and not depth since x is the distance between the two places that the pressure is being measured.
What is the Upthrust of an object in a fluid equal to?
The weight of the the fluid displaced by the object
What is archimedes principle?
The Upthrust exerted on a body immersed in a fluid wether fully or partially submerged is equal to the weight of the fluid that the body displaces.
What could you change in the ball and tube of liquid experiment to affect the balls terminal velocity? (4)
Change the liquid
Change the size of the ball
Change the shape of the item being dropped
Change the mass of what’s being dropped
What is Archimedes principle?
When a body is partially or fully submerged in a fluid, it experiences an upthrust equal to the weight of the fluid displaced
What is newton’s 1st law?
An object will continue to move with constant velocity or remain at rest unless a resultant force acts upon it.
What is Newton’s second law of motion?
The rate of change of momentum is directly proportional to the force acting on an object
What is Newton’s 3rd law of motion?
If object A exerts a force on object B, object B will exert an equal and opposite reaction force on object A, where both forces are of the same type and are along the same line.
What is the unit for momentum?
Kgms^-1
What is the unit for impulse?
Ns
What’s the equation for momentum (use the generally accepted symbols)
P = mv
What is the law of the conservation of momentum?
Momentum is conserved in any collision provided that no external force acts on the objects.
What is the area under a velocity-time graph?
Displacement (not distance!)
What are thinking distance, breaking distance and stopping distance?
Thinking - The time from seeing the obstacle and applying the breaks
Breaking - The time from applying the breaks to coming to a complete stop
Stopping - The time from noticing the obstacle to coming to a complete stop
What is the equation for thinking distance?
Thinking distance = speed x reaction time
Give the two main practical ways of finding the speed of gravity in a lab
Using electromagnets and a timer
Using light gates
What does the gradient of a displacement-time curve look like when an object is decelerating?
It is curving downwards
How do you get velocity from a displacement time graph?
Find the gradient. Remember v is the differential of s and a is the differential of v
What 4 things can affect thinking distance?
Tiredness
Alcohol/drugs
Illness
Distractions such as children
What 3 things can affect breaking distance?
Bad brakes so reduces friction
Wet or icy roads (anything reducing the tyres friction with the road)
The weight of the car (a lighter car has less momentum)
