Unit 2: Mechanics Flashcards
What are scalar quantities?
They have magnitude (size) but have no direction associated with them.
What are some examples of scalar quantities?
Temperature, mass, distance, speed, energy and power.
What are vector quantities?
Quantities that have both direction and magnitude.
What are some examples of vector quantities?
Velocity, force, displacement, acceleration and momentum.
How can two vectors be added together?
By drawing a scale diagram showing the effect of one vector followed by the other i.e. by drawing them ‘nose to tail’.
How can two vectors be added together?
By drawing a scale diagram showing the effect of one vector followed by the other i.e. by drawing them ‘nose to tail’.
What is the sum of a number of vectors known as?
The resultant.
What is the parallelogram law?
When the resultant of two vectors can be found by constructing a parallelogram, the parallelogram is constructed using the two vectors as adjacent sides and the resultant will be the diagonal of the parallelogram.
How can the magnitude of the resultant be found of two vectors at right angles?
Using Pythagoras’ theorem.
How can the magnitude of the resultant be found of two vectors at right angles?
Using Pythagoras’ theorem.
How can you find the angle of the resultant from the horizontal?
By using the sine and cosine rules.
How do you subtract a vector quantity?
You can think of it as a adding a negative vector - it is reversed in direction.
How do you subtract a vector quantity?
You can think of it as a adding a negative vector - it is reversed in direction.
What is the process of resolving vectors into their components?
A single vector can be replaced by a combination of two or more vectors that would have the same effect.
What is weight?
The force that acts on a mass due to the gravitational attraction of the Earth.
What is the gravitational field strength (g) on earth?
9.81 N/kg
How can you calculate the weight of an object?
weight (N) = mass (kg) x gravitational field strength (N/kg)
How can you calculate the weight of an object?
weight (N) = mass (kg) x gravitational field strength (N/kg)
What is the centre of gravity?
This is when the weight of an object can be treated as a single force acting at a single point in the object.
Where is the centre of gravity of a regular shape with uniform density?
The geometric centre.
What force is known as the reaction?
Whenever two solid surfaces touch, they exert a contact force on each other.
What are the two components that a contact force is usually split into?
The normal contact force acting perpendicularly to the two surfaces and the frictional force acting parallel to the surfaces.
When does a frictional force act between two surfaces?
Whenever there is relative motion between them, or when an external force is trying to slide them past each other.
When is an object said to be in tension?
When a force is acting to stretch the object.
What is air resistance?
The drag an object faces as it moves through the atmosphere as it has to push air out of the way.
What is buoyancy?
Any objects that are full/partly submerged in a fluid are subject to an upthrust from the surrounding fluid.
What are free body diagrams?
They are used to show all the external forces acting on an object as they attempt to model the situation so that we can analyse the effects of the forces.
What are free body diagrams?
They are used to show all the external forces acting on an object as they attempt to model the situation so that we can analyse the effects of the forces.
What does the size of air resistance acting on an object depend on?
It depends on the area of the object and the density of the air.
What will happen to air resistance if the relative speed between the object and the air increases?
The air resistance will increase the faster you travel.
What will happen to air resistance if the relative speed between the object and the air increases?
The air resistance will increase the faster you travel.
What can be said about an object if the resultant force is not zero?
The object will accelerate in the direction of the resultant force.
When is an object said to be in equilibrium?
If it is stationary or moving at a constant velocity.
When is an object said to be in equilibrium?
If it is stationary or moving at a constant velocity.
What are one of the conditions of equilibrium?
All the external forces that act upon an object must add up to zero.
If three forces are acting on an object in equilibrium, what must the three forces form?
A closed triangle.
If three forces are acting on an object in equilibrium, what must the three forces form?
A closed triangle.
When is an object in equilibrium if the vectors are resolved into their components?
The sum of the horizontal components must be zero and the sum of the vertical components must be zero.
When is an object in equilibrium if the vectors are resolved into their components?
The sum of the horizontal components must be zero and the sum of the vertical components must be zero.
What is the moment of a force about a point?
It is the magnitude of the force, F, multiplied by the perpendicular distance of the force from the pivot, s.
What is the equation for a moment?
moment (Nm) = F (N) x s (m)
What are the units for a moment of a force?
Newton metres
What is torque?
Another word for moment.
How can you increase the torque of a spanner on a nut?
By exerting a larger force of by getting a longer spanner.
What does the principle of moments state about equilibrium?
If an object is in equilibrium then the sum of the moments about any point must be zero.
What does the principle of moments state about equilibrium?
If an object is in equilibrium then the sum of the moments about any point must be zero.
In terms of the work done to lift an object and the energy the object gains due to being lifted by a lever, which quantity will be greater?
The work done will always be greater than the energy required to lift the object with a lever.
What is a couple?
Two parallel forces which act in opposite directions and are equal in magnitude.
What is the effect of a couple?
They tend to make an object rotate.
What is the torque of a couple?
Fs, where F is the magnitude of one of the forces and s is the perpendicular distance between the forces.
What is an example of a couple?
In an electric motor where the force on each side of the coil is equal but opposite.
What happens if the resultant force on an object passes through the centre of mass?
It will accelerate without rotating.
What happens if the resultant force on an object does not pass through the centre of mass?
The object will spin.
When will the centre of mass and the centre of gravity be equal to one another?
In a place where gravity is uniform.
When will the centre of mass and the centre of gravity be equal to one another?
In a place where gravity is uniform.
What is displacement?
The distance travelled in a given direction.
What is the formula for speed?
speed (m/s) = distance travelled (m) / time taken (s)
What is the instantaneous speed?
The speed at a certain time.
What is the formula for velocity?
velocity (m/s) = displacement (m) / time (s)
What is acceleration?
The rate at which velocity changes.
What is the formula for acceleration?
acceleration = change in velocity / time taken for change
What are the units for acceleration?
m/s^2
Does acceleration always take place in the same direction as velocity and give an example to support your answer?
No as a ball thrown in the air which rises and then falls again is always accelerating downwards due to gravity.
What does a straight line on a displacement-time graph represent?
A constant velocity.
What does the gradient of a displacement-time graph represent?
The instantaneous velocity.
How can the average velocity on a displacement-time graph be obtained?
Through drawing a tangent from the start of the journey to the end.
What does a negative value on a displacement-time graph represent?
Travelling in the opposite direction.
What does a straight, horizontal line on a displacement-time graph show?
It shows that the object is stationary.
What does a curved line on a displacement-time graph suggest?
A non-uniform velocity i.e. acceleration.
What does the gradient of a velocity-time graph represent?
The instantaneous acceleration.
What does the gradient of a velocity-time graph represent?
The instantaneous acceleration.
What does a straight line on a velocity-time graph represent?
Constant acceleration.
How can a negative acceleration be shown on a velocity-time graph?
Through a negative gradient.
What does a negative acceleration mean?
An object is slowing down or it could mean that the object is speeding up in the opposite direction.
What are synonyms of slowing down?
Deceleration and retardation.
What are synonyms of slowing down?
Deceleration and retardation.
What does the area below a velocity-time graph represent?
The total displacement.
What are the four constant acceleration formulae?
v = u + at, s = (u + v)/2 x t, s = ut + (1/2 x at^2) and v^2 = u^2 + 2as
Why does a falling object accelerate towards the Earth?
Due to the Earth’s gravity.
What is the acceleration due to gravity independent of?
The object’s mass.
What is the value of the acceleration due to gravity on Earth?
9.81m/s^2
Describe the journey of an object falling in terms of the forces.
When the object first begins to fall, the weight is larger than the air resistance so the object accelerates, then the air resistance increases as the velocity increases so the acceleration will decrease, eventually air resistance will be equal to weight so their is no acceleration and the object falls at constant velocity.
Describe the journey of an object falling in terms of the forces.
When the object first begins to fall, the weight is larger than the air resistance so the object accelerates, then the air resistance increases as the velocity increases so the acceleration will decrease, eventually air resistance will be equal to weight so their is no acceleration and the object falls at constant velocity.
What is terminal velocity?
When the weight of a falling object is equal to the air resistance and so there is no acceleration so it falls at a constant velocity.
What does terminal velocity depend on?
The surface area of the object, as well as its mass, and on the density of the air.
What is projectile motion?
When an object thrown in the air follows a parabolic path.
Why can the constant acceleration formulae be used for projectile motion?
The horizontal motion does not affect the vertical motion.
Will an object thrown horizontally and an object dropped vertically fall at the same rate?
Yes
What are the vertical and horizontal components of the initial speed of a projectile?
The horizontal component is ucosθ and the vertical component is usinθ.
What happens to the vertical velocity of an object when it reaches its maximum height?
Its vertical velocity will be zero.
What is the time of flight equal to?
Twice the time taken to reach the maximum height.
What is Newton’s first law of motion?
Every object will continue to move with uniform velocity unless it is acted upon by a resultant external force.
What is inertia?
The reluctance of an object at rest to start moving and its tendency to keep moving once it has started.
What is inertia?
The reluctance of an object at rest to start moving and its tendency to keep moving once it has started.
What is the formula for momentum?
momentum (kgm/s) = mass (kg) x velocity (m/s)
What is the momentum of a body a measure of?
How difficult it is to stop it.
What are the other units of momentum?
Ns (Newton seconds)
What is Newton’s second law of motion?
The rate of change of an object’s linear momentum is directly proportional to the resultant external force. The change in momentum takes place in the direction of the force.
What is the Newton (N)?
The S.I. unit of force and one Newton is the force that will accelerate a mass of 1kg by 1m/s.
What is the formula for force?
force (N) = mass (kg) x acceleration (m/s^2)
When is F = ma true?
When the mass of an object is constant.
What is the point of seatbelt, crumple zones and air-bags in cars?
They are all ways of increasing the time taken to come to a stop during a crash which reduces the force.
What form should Newton’s second law be applied in when the mass changes?
F = Δ(mv)/Δt
What is Newton’s third law of motion?
If an object, A, exerts a force on a second object, B, then B exerts an equal but opposite force back on object A.
What is Newton’s third law of motion?
If an object, A, exerts a force on a second object, B, then B exerts an equal but opposite force back on object A.
Why do the two forces involved in Newton’s third law never cancel each other out?
They act on different bodies.
What is the reaction pair of the Earth pulling down on a object i.e. its weight?
The object pulling back up on the earth.
When is work done?
Whenever a force moves through a distance in the direction of the force.
What is the definition of work done?
The work done is equal to the force multiplied by the distance through which the force moves, in the direction of the force.
What is the formula for work done?
work done, W (J) = F (N) x s (m) x cos
What are the vertical and horizontal components of the initial speed of a projectile?
The horizontal component is ucosθ and the vertical component is usinθ.
What form should Newton’s second law be applied in when the mass changes?
F = Δ(mv)/Δt
What is the formula for work done?
work done, W (J) = F (N) x s (m) x cosθ
When is one joule of work done?
Whenever a force of one Newton moves through one metre.
What is work done measured in?
Joules
What is energy?
The ability to do work.
What is energy measured in?
Joules
What are some examples of the different forms of energy?
Electrical, chemical, kinetic, gravitational potential, electric potential, thermal, electromagnetic, nuclear or sound.
What is power?
The rate at which energy is transferred.
What does a power of one watt mean?
1 joule of energy is transferred every second.
What can power be thought of as?
The rate at which work is done.
What is the formula for power in terms of work done?
P = ΔW/Δt
What is the formula for power in terms of work done?
P = ΔW/Δt
What is the formula for power in terms of velocity?
P = Fv
What is a condition of the formula P = Fv?
The force and the velocity must be in the same direction.
What happens when energy is transferred from one form to another?
The total amount of energy does not change i.e. energy is conserved.
Only when does the conservation of energy apply?
In a closed system.
When will the total energy not remain constant?
If energy has been transferred to another object.
What is kinetic energy?
The energy that a moving mass has because of its motion.
What is the formula for kinetic energy?
E = 1/2 x mv^2
What happens to the kinetic energy of a car if it doubles its speed?
Its kinetic energy will go up by a factor of 4.
What is gravitational potential energy?
The energy that an object has because of its position in a gravitational field.
What is the work you need to raise a mass to a greater height above the surface of the Earth stored as?
Potential energy.
What is the formula for power in terms of work done?
P = ΔW/Δt
What is the formula for kinetic energy?
Ek = 1/2 x mv^2
What is the work you need to raise a mass to a greater height above the surface of the Earth stored as?
Potential energy.
If a mass falls, what will the potential energy be transferred to?
Kinetic energy.
What is the formula for the change in GPE?
ΔEp = mgΔh
When does the equation for GPE apply?
Where the gravitational field strength is constant.
What happens to the value of the gravitational field strength as the distance from the Earth’s surface increases?
It decreases.
What is ignored when we state that the change in KE is equal to the change in GPE of a falling object?
The energy losses due to air resistance.
What is efficiency?
The proportion of input energy that is transferred to useful energy.
What is the formula for efficiency in terms of energy?
Efficiency = useful energy output / total energy input
What allows for the efficiency formula to be written in terms of power?
energy = power x time
What is the formula for efficiency in terms of power?
Efficiency = useful output power / input power
What value can efficiency never be greater than?
1
