5. Mechanics Flashcards
What are the two categories all quantities fall into?
scalar, vector
What is the difference between a scalar quantity and a vector quantity?
scalar quantities have magnitude only; vector quantities have magnitude and direction
State 7 scalar quantities
- time
- mass
- temperature
- distance
- length
- speed
- energy
State 6 vector quantities
- weight
- acceleration
- force
- displacement
- velocity
- momentum
When is an object in equilibrium?
when there are no resultant forces and no resultant moments
What is a moment?
the force multiplied by the perpendicular distance from the pivot to the line of action of the force
What is the principle of moments?
if a system is in equilibrium, the sum of the clockwise moments = the sum of the anticlockwise moments, at any point
What is a couple?
a pair of forces of equal size which act parallel to each other but in opposite directions
What is the centre of mass?
the single point that you can consider all the weight to act through
What is displacement (s)?
distance and direction from starting point in a straight line
What is speed (v)?
how fast something is going regardless of direction
What is velocity (v)?
how fast something is going with direction
What is acceleration (a)?
the rate of change of velocity
What does the gradient of a displacement-time graph represent?
velocity
What does the gradient of a velocity-time graph represent?
acceleration
What does the area under a velocity-time graph represent?
change of displacement
What does the area under an acceleration-time graph represent?
change in velocity
What is Newton’s first law of motion?
if there are no resultant forces acting on a object it will either:
- stay at rest
- travel at a constant speed in a straight line (constant velocity)
What is Newton’s second law of motion?
“the acceleration of an object is proportional to the resultant force acting on it”
What is the equation representing Newton’s second law?
F = ma
What is Newton’s third law of motion?
“if object A exerts a force on object B, then object B exerts an equal but opposite force on object A”
How can you find the centre of mass of a flat object?
- hang the object freely from a point
- draw a vertical line downwards from the point of suspension
- use a plumb bob to get the line exactly vertical
- hang the object from several different points, and draw vertical lines
- the centre of mass is where the line cross
When will can object topple over?
if the line of action of the objects weight falls outside its base area
What is freefall?
the motion of an object undergoing an acceleration of ‘g’
What is friction?
- a force that opposes motion
- turns kinetic energy into thermal energy
What are the two main types of friction?
- contact friction
- fluid friction
When does contact friction happen?
between solid surfaces
What is fluid friction also known as?
- drag
- fluid resistance
- air resistance
What is fluid friction caused by?
a gas or a liquid
What three factors affect drag?
- viscosity of the fluid
- speed
- surface area of profile
What is the relationship between drag and speed?
drag is directly proportional to speed squared
What is lift?
an upwards force on an object moving through a fluid
How does lift occur?
lift happens when the shape of an object causes the fluid flowing over it to change direction
What direction does lift act in comparison to the direction of fluid flow?
perpendicular
What direction does drag act in comparison to the direction of fluid flow?
parallel
When does terminal velocity occur?
when the frictional forces on an object are equal to the driving forces on an object
What are the two main ways of increasing a vehicle’s maximum speed?
- increasing the driving force
- reducing the frictional force
How are momentum, mass and velocity related?
momentum = mass x velocity
Is momentum a scalar or vector quantity?
vector
What is the principle of conservation of linear momentum?
the total momentum before a collision = the total momentum after a collision, provided no external forces act
What is an elastic collision?
a collision where both momentum and kinetic energy are conserved
What is an inelastic collision?
a collision where momentum is conserved but kinetic energy is not conserved
Are most real life collisions elastic or inelastic?
inelastic
What must be applied for a change in momentum to occur?
a force
What is the relationship between force, change in momentum and time?
force = change in momentum / time
What is impulse?
the product of force and time
What is impulse equal to?
change in momentum
What are the units of impulse?
Ns or kgms^-1
What does the area under a force against time graph equal?
- the impulse
- the change in momentum
What three things in a car increase the duration of impact and hence decrease the size of the force when change in momentum is conserved?
- crumple zones
- seat belts
- air bags
How do crumple zones decrease the size of the force on a passenger?
deform plastically (permanently) on impact, causing the car to take longer to stop, increasing the impact time and decreasing the force on the passengers
How do seat belts decrease the size of the force on the passenger?
stretch slightly, so they increase the impact time for the wearer, reducing the force acting on their chest
How do air bags decrease the size of the force on the passenger?
slow down the passengers more gradually, and prevent them from hitting hard surfaces inside the car
When is work done?
when energy is transferred
What is the equation linking work done, force and distance?
work done = size of force in direction of travel x distance travelled
How is work done calculated when the force and direction of travel aren’t parallel?
work done = force x cos(angle between force and direction of travel) x distance
What is the area under a force against distance graph equal to?
work done
What is power?
rate of doing work
What are the units of power?
Watts
How is power calculated?
power = change in work done / time
What is the equation linking power, force and velocity?
power = force x velocity
What is the principle of conservation of energy?
- energy cannot be created or destroyed
- it can only be transferred from one form to another
- but the total amount of energy in a closed system will not change
What is efficiency?
a way to quantity how much of the energy (or power) put in is transferred into useful energy (or power)
How is efficiency calculated?
efficiency = ((useful energy/power out) / (total energy/power in)) x 100
How do you calculate kinetic energy?
1/2 x mass x velocity^2
How do you calculate gravitational potential energy?
mass x gravity x height
