Mechanics Flashcards
What is the difference between mass and weight?
Mass is scalar and not dependent on gravity acting upon it
Weight is vector and dependent on gravitational field strength
If an object is in equilibrium, it means the object is….
Not accelerating so is either:
-stationary
-moving at a constant velocity
How can forces acting on an object be shown to be in equilibrium?
-Adding horizontal and vertical components of the forces acting on it, showing they equal 0
-If there are 3 forces acting on the object draw a scale diagram -> closed triangle = equilibrium
What is a moment?
A turning force: force multiplied by the perpendicular distance from the point to the line of action of the force
What is meant by a couple?
A pair of equal and opposite coplanar forces
What is meant by the centre of mass?
The point at which all the mass of an object acts
For a uniform object the centre of mass is the centre of the object
If you have a uniform object, where would its centre of mass be?
At the centre of the solid
What is the area under a velocity-time and acceleration-time graph?
The displacement travelled and the velocity respectively
As speed increases, air resistance…
Increases (proportional to the square of speed)
A ball is projected off a castle at 6 m/s, how does its horizontal velocity change from its launch until it hits the ground?
Horizontal velocity stays the same -> no acceleration in that direction
How do SUVAT equations reflect that all objects fall at the same rate?
Mass is not included in SUVAT -> mass of an object doesn’t affect its speed or acceleration
In projectile motion, what is vertical acceleration?
Vertical acceleration + gravitational field strength
What is meant by terminal velocity?
When forces acting on a falling object become balanced
Acceleration becomes 0, and object is moving at max. velocity
What is meant by friction?
A resistive motion between an object and a surface or an object moving through a fluid
Acts in the opposite direction to the movement
What is Newton’s 2nd law?
Acceleration is proportional to force
F=ma
What is Newton’s 1st law?
An object stays moving at a constant velocity until an external resultant force acts upon it
What is Newton’s 3rd law?
If an object A exerts a force on object B, Object B will exert an Equal but opposite force on object A
What is the principle of moments?
When a body is balanced
total clockwise moments = total anticlockwise moments (about the same point)
How do you find a resultant vector?
How you do it in maths. Add the two vectors and use Pythagoras.
You need a bearing too.
A Remote controlled aeroplane flies with a velocity 14ms-1 east with an 8.0ms-1 wind north. Find resultant vector
142 + 8.02 = R2 so root answer = 16.124
Use tan-1(8.0/14) to find angle of 30º
Barings are measured from the north so 90-30 = 060
What is a free-body diagram?
A single body on its own showing all the forces acting on IT but not the forces it exerts on the rest of the world.
Define equilibrium
If an object is in equilibrium there is no resultant force
There are also no resultant moments
Explain how a lever works
A lever increases the perpendicular distance from a turning point / pivot so with the same force you get a larger moment.
What is a couple? What makes them special?
a pair of forces equal in size but acting in opposite directions. A couple causes no resultant linear force but does produce a moment.
Unit for a moment
Nm
Define displacement
How far an object’s travelled from its starting point in a given direction
What is the gradient of a Velocity-Time graph?
Acceleration
(Rate of change of velocity)
What is shown by the area under a speed-time graph?
distance
What is shown by the area under a velocity-time graph?
displacement
What are the advantages of using a data logger over a traditional method to calculate velocity-time graphs?
Higher sampling rates
More accurate data - no reaction times
real-time data display
What is the area under an acceleration time graph?
total ∆V
What is freefall?
When gravity acting on an object and nothing else
All objects fall to the ground _____ assuming no drag or contact friction.
At the same rate
Describe the experiment to determine g using freefall
- As you flick switch electromagnet lets go and timer starts.
- its a ball baring so we can ignore air resistance as it is minimal
- ball hits trapdoor and timer stops
- repeat each height 3 times, take an average
Plot a graph of H against T2 gradient = ∆s/∆t2 = 1/2g so x 2
s = 1/2at2
What is a projectile?
Any object with an initial velocity free to move under gravity.
Horizontal velocity remains unchanged
Verticle decreases due to g
What would considering air resistance do to the path of a projectile?
Causes drag = opposite direction to motion.
If it has a verticle component it decreases the max height
if it has a horizontal component it decreases the max X displacement
3 key ideas about friction
-Always opposes motion
-Never speeds up only slows down
-Ke becomes heat
What’s lift? What causes it?
An upwards force on an object moving through a fluid. It happens when the shape of an object causes the fluid flowing over it to change direction
What is terminal speed (velocity)?
The speed that an object reaches when it falls through a gas or liquid and the forces acting on it are balanced
( no longer accelerating )
Compare elastic and inelastic collisions
Inelastic = Ke NOT conserved
Elastic = Ke perfectly conserved
In an inelastic collision, Ke is _____ and Momentum is ________
Not conserved / Conserved.
Define impulse
The product of force and time
What is under a force-time graph?
Impulse
How can we reduce the force of an impact.
Force = rate of change of momentum.
So if we increase the time of the impact we reduce the force.
How does a car take advantage of force being the rate of change of momentum in terms of safety?
- Crumple zones - increase time of impact decreasing overall force felt
- Seat belt - stretch slightly increasing the time taken for the wearer to stop reducing force
- Air bag - same thing
Define work
W = Force x distance measured in joules.
Power =
Rate of change of energy transfer, 1 watt = 1 joul per second.
What’s under a force-displacement graph?
WORK DONE
Link power force and velocity
Power = Force x velocity
Link power force and velocity
Power = Force x velocity
State the principle of conservation of energy
Energy cannot be created or destroyed. Only transferred from one form to another. Total energy in a system doesn’t change.
Apply Newton’s 1st and 2nd laws on shooting a bullet (projectile motion)
1st - no significant forces acting in the horizontal direction -> bullet will continue to have the same horizontal v
2nd - only force acting on the bullet is weight (acting downwards)
Weight = m x a
m g = m a
g = a = 9.81 ms-2
- falls -> accelerates downwards, gaining speed with each second
- increase in speed -> increase in air resistance
-As falls faster and faster, air resistance increases until it = the force of gravity
-Once air resistance = gravity, forces balanced and skydiver no longer accelerates. The skydiver is said to have reached a terminal velocity.
Why is impulse more useful that only knowing the force?
Because impulse = Δ momentum
f t = mv - mu
In an elastic collision, KE is ___________
In an inelastic collision, KE is ___________
In an explosion, KE is ___________
conserved / lost (transferred to another energy store) / gained (from another store)
Momentum is a _______ and therefore has ______
vector / direction
When a falling object loses GPE, it gains _____.
Therefore _____ lost = __________
gains KE
GPE lost = KE gained
Power =
energy transfer / time
