Module 3: Forces and Motion Flashcards
When can you use SUVAT equations
When there is constant acceleration.
What is the normal contact force relative to the surface?
Perpendicular.
What is the friction relative to the surface?
Parallel.
What is the relationship between normal contact force and maximum friction?
F=μN
What is tension and what are the assumptions made when answering a question involving it?
Tension is the force exerted by a rope, cable or a similar object.
Tension is the same throughout a cable if you assume that the cable doesn’t have a mass and doesn’t stretch.
What is the centre of mass of an object?
The point on an object where a single force doesn’t have a turning effect.
What is the centre of gravity of an object?
The point on an object where weight appears to act.
What does it mean when an object is uniform?
It’s centre of mass is in the geometric centre of the object.
What is Newton’s first law?
That an object will remain at rest or continue to move with a constant velocity unless acted upon by a resultant force (law of inertia).
What is Newton’s second law?
That the resultant force acting on an object is directly proportional to the rate of change of momentum, where the change in momentum occurs in the same direction as the resultant force.
What is Newton’s third law?
That when one body exerts a force on a second body, the second body simultaneously exerts an equal and opposite force on the first body
What are the conditions for dynamic equilibrium?
Dynamic equilibrium can only occur if the object is in translational and rotational equilibrium.
Define a moment.
The product of the force and the perpendicular distance from the pivot.
Define a couple and torque.
A couple is a pair of equal and opposite forces of magnitude F with their line of action separated by a distance d.
Torque is the moment of a couple, and is defined as the product of one of the forces and the perpendicular distance between them, Torque = Fd.
What is the principle of moments?
For a body in rotational equilibrium, the sum of the anticlockwise moments is equal to the sum of the clockwise moments about that point.
State the principle of conservation of momentum?
Momentum is always conserved in a closed system.
What is the difference between elastic and inelastic collisions?
Both collisions conserve total energy and momentum, but in elastic collisions, kinetic energy is also conserved.
What is impulse?
The product of a force and the time this force acts on an object, therefore, impulse = change in momentum, Δp=fΔt.
What is the relationship between drag and speed?
Drag ∝ speed².
When does an object reach terminal velocity?
Terminal velocity occurs when drag and weight are equal.
Define pressure.
The normal contact exerted per unit cross-sectional area.
What is the equation for pressure in a fluid?
P = ρhg, pressure = density of the fluid x height of object x g.
State Archimedes principle?
The upthrust exerted on an object submerged in a fluid is equal to the weight of the displaced fluid, regardless of whether or not the object is partially or fully submerged.
When fully submerged…
Volume of the object = Volume of displaced fluid.
When partially submerged…
Mass of object = Mass of displaced fluid
What is work done?
The product of the component of the force in the same direction as motion and distance travelled, W=Fscosθ
What is the principle of conservation of energy?
Total energy in a closed system stays constant. Energy can’t be created or destroyed, only transferred.
What is power?
The rate of work done / energy transferred.
Power = …
E/t, energy over time.
Fv, force x velocity, as long as the force is constant.
What are tensile forces?
Forces that cause extension.
What are compressive forces?
Forces that cause compression.
State Hooke’s Law.
The force, F, that is requires to extend or compress a spring by a certain distance, x, are directly proportional up to the elastic limit.
F = kx
This is shown graphically by an F vs x graph which is linear and passes through the origin.
What is the spring constant, k ?
A measure of stiffness.
What is the gradient and area under the graph of an F vs x graph?
Gradient: Spring constant, k
Area under graph: Energy stored
What happens when the elastic limit has been exceeded?
Deformation occurs.
What is the spring constant for two springs in series?
k = (k₁⁻¹ + k₂⁻¹)⁻¹
What is the spring constant for two springs in parallel?
k = k₁+ k₂
Describe the F vs x graph of a metal wire.
Loading: Linear (obeys Hooke’s Law) up until elastic limit, then line begins to curve downwards due to plastic deformation.
Unloading: Linear, same gradient as loading.
Describe the F vs x graph of a rubber band.
Loading: Doesn’t obey Hooke’s law, curved line.
Unloading: Curved line, creates a hysteresis loop. More work is done loading than unloading, the area inside of the loop is the thermal energy lost.
Describe the F vs x graph of polythene.
Loading: Doesn’t obey Hooke’s Law, curved line.
Unloading: Doesn’t create closed loop as permanent extension has occurred, line is linear.
Define tensile stress.
The force applied per unit cross-sectional area of a wire.
σ = F/A
Define tensile strain.
The fractional change in the original length of a wire.
ε = x/L
Define ultimate tensile strength (UTS).
Maximum stress the material can withstand.
Define the Young modulus
Stress over strain. It is a measure of stiffness as it depends on the material
E = σ/ε
What does it mean if a material is strong?
It has a high UTS.
What does it mean if a material is brittle?
Breaks with little plastic deformation / elastic up to breaking point.
What does it mean if a material is ductile?
Can be drawn into wires under tension.
What does it mean if a material is malleable?
Can be hammered into thin sheets under compression.
What does it mean if a material is tough?
Can absorb a high amount of energy before it breaks (greater area under curve).
What does it mean if a material is elastic?
Obeys Hooke’s law, temporarily deformation.
What does it mean if a material is plastic?
It is permanently deformed.
