Mechanics Flashcards
Name 6 scalar quantities
Speed, mass, distance, time, energy, power
Name 6 vector quantities
Acceleration, force, displacement, momentum, velocity, weight
What is meant by equilibrium?
A state in which the resultant force and resultant moment on an object is zero, in this state the object is either at rest or moving with a constant velocity
What is meant by the moment of a force about a point?
The turning effect of a force about a fixed point (pivot)
What is a couple?
A pair of equal but opposite coplanar forces (forces acting in the same plane), so they act along the same line but in opposite directions
What is the principle of moments?
For a body to be in rotational equilibrium, the sum of the clockwise moments about any point must be equal to the sum of the anticlockwise moments about that same point, so the moments will be balanced and no overall turning effect will occur
What is meant by centre of mass?
It is the single point in which you can consider an objects entire mass to act through/be concentrated (no matter what its orientation), so the object will always balance around this point (no overall turning effect produced)
Describe how you could find the centre of mass of an irregular shape
Hang the object freely from one point using a string with a mass on the end (plumb bob) to get a perfectly vertical line, then draw a vertical line down from the point of suspension, then hang the object from different points then the point where the lines all cross over will be the centre of mass
When will a tilted object topple over?
When the tine of actiong of its weight (drawn down from its centre of mass) falls outside of its base area, resulting in an overall moment and a turning force
What are 2 ways to increase the stability of an object?
1) Lower the centre of mass
2) Make the base wider
What is the difference between distance and displacement?
Distance (scalar) is how far an object has travelled in total and it does not depend on the direction the object is travelling in whereas displacement (vector) is how far something has travelled from a given point and it does consider the direction
What is the difference between speed and velocity?
Speed (scalar) is how fast something is travelling and it does not depend on the direction the object is travelling in whereas velocity (vector) is how fast something is travelling but considering direction (it is the rate of change of displacement)
What is acceleration?
The rate of change of an object’s velocity
What does a negative acceleration mean?
The object is either decelerating (slowing down) or it is accelerating in the opposite (negative) direction
What does the gradient of a displacement-time graph represent?
The velocity
What does the gradient of a velocity-time graph represent?
The acceleration
What does the area under a velocity-time graph represent?
The displacement
What does the area under an acceleration-time graph represent?
The velocity
What does it mean for an object to be in freefall?
It is accelerating due to gravity and nothing else, meaning that gravity is the only force causing its acceleration
What happens to the trajectory of a projectile if air resistance is not negligable?
The trajectory becomes shorter and more curved as it falls faster, hence not travelling as far
What happens to the maximum height of a projectile if air resistance is not negligable?
The maximum height is lower because the upward motion is slowed down more quickly
What is friction?
A resistive force that opposes relative motion between two surfaces in contact
What are the 3 key things to remember about frictional forces?
They always act in the opposite direction to motion, they never speed things up or cause something to move and they convert kinetic energy into heat
Name and describe the two main types of friction
Contact friction (between two solid surfaces)
Fluid friction (also known as drag)
What three things affect fluid friction?
The viscosity (thickness) of the fluid, the speed that the object is travelling at (greater speed = greater frictional force) and the shape of the object (larger area = larger frictional force)
What is lift?
An upwards force acting on an object moving through a fluid due to the objects shape and its interaction with the fluid (it acts perpendicular to the direction of motion)
What is drag?
A frictional force that opposes the motion of an object moving through a fluid
What is terminal speed?
The maximum speed that an object can reach when falling through a fluid, it occurs when the driving force and the opposing force (like air resistance) balance so there is no overall resultant force, meaning the object no longer accelerates and travels at a constant speed
How can a greater terminal speed be reached?
By reducing the air resistance (e.g. making the object more streamline) or by increasing the driving force (e.g. by increasing the weight of an object in freefall or increasing the engine power in a vehicle)
What factors affect the maximum speed of a vehicle?
engine power, air resistance, vehicle shape, road gradient (steepness)
What is Newton’s first law?
It states that a body will stay at rest or continue to move at a constant velocity in a straight line unless an external force acts upon it (so unless there is a resultant force)
What is Newton’s second law?
It states that the acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass (F = ma)
What is Newton’s third law?
It states that when two objects interact with eachother, they exert an equal but opposite force on eachother
What is momentum?
A measure of how much motion an object has and how hard it is to stop it, momentum is dependent on both the mass of the object and how fast it is moving
What does a negative momentum mean?
It means the object has a negative velocity (as mass cannot be negative and momentum is mass times velocity), that means it is travelling in the negative (opposite direction)
What is meant by the conservation of momentum?
The total momentum of a closed system remains constant, provided no external forces act on it, this means that the momentum before an event (like a collision or explosion) is equal to the momentum after the event
What is meant by an elastic collision?
A collision in which both the momentum and kinetic energy are conserved (so the kinetic energy before the collision is equal to the kinetic energy after the collision)
What is meant by an inelastic collision?
A collision in which momentum is conserved but the kinetic energy is not conserved (so some of the kinetic energy is converted into other forms of energy)
Why does a cannon move backwards when a ball is projected forwards following an explosion?
Before the explosion the momentum is zero, so when the ball is fired forwards with a certain momentum the cannon must move backwards with the same momentum to ensure the momentum after the explosion is zero (hence ensuring momentum is conserved)
How is force connected to momentum?
A force is a rate of change of momentum (A change in momentum over a time period)
How do car safety features help protect people in a car crash?
The work to by increasing the time it takes for a change in momentum, so because force is rate of change of momentum, the force exerted on the people will be reduced as the time increases
What does the area under a force-time graph show?
An impulse
What is an impulse?
A change in momentum
Why does ‘following through’ in a sport such as golf result in the ball travelling at a higher speed?
It means that the force is being applied to the ball over a longer period of time, resulting in a greater impulse (force x time) and because an impulse is a change in momentum, the ball will also have more momentum, hence it will have a higher speed (momentum = mass x velocity)
What are the ethical considerations related to momentum conservation in transport design?
Ethically, designers must minimise harm to passengers and others in crashes, so this involves using the principle of conservation of momentum to do so and reduce injuries
What is work done?
The energy being transferred as a force acts on an object over a distance (rate of doing work = rate of energy transfer)
What is the significance of the area under a force-displacement graph?
It shows the work done
What is work done against when an object is pushed across the floor?
Work is being done against friction
(between the floor and the object)
What is work done against when an object is lifted up from the floor?
Work is being done against gravity as the box is moving in the opposite direction to the force due to gravity
What is work done against when an object is falling the the floor?
Work is being done against air resistance
What is meant by power?
The rate of doing work or the rate of energy transfer
What is the principle of conservation of energy?
It states that 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
Derive an equation for the maximum height reached by an object that is thrown at a speed, v, from the ground
Look in booklet for answer
Derive an equation for the maximum speed reached by an object that is dropped from a height, h, above the ground
Look in booklet for answer
What does a high efficiency mean?
It means that a high percentage of the total power input is being usefully transferred (useful power output), with minimal energy being wasted
