PHY 5 - Work Flashcards
If displacement is zero, what is the work done ?
0.
Work done = Force x distance moved in direction of force
Why would you include Cos Ø to the work equation ?
To take into account that the force applied is not always at the same angle to the displacement. By using Cos Ø you seperate the force into vetical and horizontal components using the adjacent side.
What are the units of work ?
1 Joule (J) = 1 newton over 1 metre
Describe energy in terms of physics ?
Energy is a scaler, it is a physical quantity capable of producing work.
Name 8 different types of Energy
- Mechanical
- Electrical
- Chemical
- Potential
- Kinetic
- Sound
- Atomic
- Nuclear
What happens to the work done if the displacement is at 90º to the force ?
0 Work done because Cos 90 = 0.
What is the work done by the force normal ?
It is a counter-active force, therefore no work is done.
Describe the work done by friction ?
Always acts opposite to the direction of motion, Cos 180 º always = a negative value.
What equation summarises kinetic energy ?
What equation is described by the work-energy theorem ?
Change in kinetic energy + change in potential energy = Work
What is the equation for potential energy derived from the gravitation forces ?
Potential Energy = mgh
m = mass
g = gravity
h = height
Explain mechanical energy in relation to frictional forces ?
They are non-conservative forms of mechanical energy
What causes friction ?
Molecular adhesion between tangential surfaces- it is independent of the area of the surfaces touching each other.
What direction do frictional forces work on ?
They always oppose the motion
Name the two types of friction
- Static
- Kinetic
What is static friction ?
When the object is not moving, static friction is the force that must be overcome in order to move it.
Describe friction ?
A contact force that acts to oppose sliding motion between surfaces.
Explain when friction would be high or low ?
Increased if surfaces are dry and rough
Decreased if surfaces are wet and smooth
What is the Force Normal ?
The force perpendicular to the surface, typically equal and opposite to the gravitational force.
What is the coefficicent of friction ?
It is defined by the materials from which the two surfaces are made, and the relative roughess of the surfaces in contact.
What factors effect the amount of friction experienced by an object ?
- The force normal
- The coefficient of friction (µ)
Sketch a diagram to demonstrate static friction
What is kinetic friction ?
The force of friction when two surfaces are already moving relative to each other.
It is also the force that slows down a moving object when the force of static friction has been overcome.
What is a normal value for a friction coefficient ?
Usually < 1
µ Kinetic is usually < µ Static
What are the units for the coefficient of friction ?
There are no units, it is just a numerical value.
Sketch a graph of frictional force vs. Applied force
How would you calculate Fmax ?
What happens if F > µN ?
The object must accelerate because the maximal static frictional force has been overcome.
Describe uniform circular motion
A particle moving in a circle at a constant speed.
Why does uniform circular motion occur ?
Acceleration is due to a change in velocity. For a particle moving in a circle at a constant speed (uniform circular motion), the velocity vector changes continuously in direction, but the magnitude remains the same.
What is centrepetal acceleration ?
The velocity is always tangent to the circle, the direction is always changing but the magitude remains the same. This creates an acceleration directed radically inward (ac) (Centre-seeking).
How would you calculate the magnitude of ac ?
V2/R
Where r = radius of the circle
What causes velocity ?
Velocity is the result of a net force acting on an object.
How would you calculate the circumference of a circle ?
2πr
How would you calculate the radius of a circle ?
If a particle is moving in a circle, with non-uniform speed, what forces and accelerations will it experience ?
- Cetripetal
- Tangential
What is the difference between the Centrifugal force and the Centripedal force ?
What is Newton’s Second law ?
What is the SI unit for friction ?
Kg/m/s2
Compared to the force needed to start sliding an object across a rough, flat floor, the force required to keep the object in motion is… ?
Less !
Due to inertia, it is more difficult to get an object moving (overcome static friction) than it is to keep it moving.
Describe mg in terms of physics forces
Mg = Mass x Gravity.
It is the force (weight) exerted by the object on the earth - this is the normal force that acts against gravity.
What is potential energy ?
Energy accumulated by a system that contains it
Define conservation of energy in terms of physics ?
The mechanical energy of a system (Er) = the sum of its Kinetic energy and it’s potential energy.
Er = Ek + Ep
Describe the theorem of mechanical energy
The variation of the mechanical energy of a system is equal to the work of exterior forces acting on the system.
What are the 3 components of the definition of mechanical force ?
- After a round trip, the Ke of a particle on which a force acts, must return to its initial value
- After a round trip, the work done on a particle by a force must equal zero
- The work done by the force on a particle depends on the initial and final positions of the particle and not on the path taken.
Give an example of a non-conservative force ?
Friction
Give an example of a conservative force ?
Gravity
What is the SI unit for power ?
Watt (W) = 1 joule per second
What equation describes power ?
