Mechanics Flashcards
What is a vector quantity?
a physical quantity with direction AND magnitude
What are some examples of vector quantities? (5)
force
acceleration
displacement
momentum
velocity
What is a scalar quanitity?
a physical quantity that is NOT directional and has a magnitude ONLY
What are some examples of scalar quantities? (5)
distance
speed
mass
temperature
energy
What is displacement?
a vector quantity- describes how far an object is from where it started and in what direction
How can we combine vectors?
-use Pythagoras’ theorem (for two vectors acting at right angles to each other)
-use a scale drawing, find angles using a protractor and use the sine/cosine equation (for vectors not at right angles)
What do we mean by coplanar forces?
forces which act in the same plane
How can we resolve vectors? Give the equations to find out the Fx and Fy vectors
use trigonometry
-for the horizontal component, Fx = F cos θ
-for the vertical component, Fy = F sin θ
How are vectors resolved on a slope?
usually need to resolve weight:
- W cos (θ) perpendicular to the slope
- W sin (θ) parallel to the slope
What does it mean if an object is at equilibrium?
-at rest
-no resultant force
-constant velocity
What does it mean if a vector triangle is closed?
the forces are at equilibrium
What is a moment?
the force multiplied by the perpendicular distance from the line of action of the force to the pivot
What is the equation to calculate the moment of a force? Include units
M=Fd
where:
M= moment (Nm)
F= force (N)
d= perpendicular distance from the line of action from the force to the pivot (m)
What do the principle of moments state?
at EQULIBIRBIUM, the sum of the clockwise moments= the sum of the anti-clockwise moments ABOUT A GIVEN POINT
What do we mean by the centre of mass?
the point at which the entire WEIGHT of an object can be considered to ACT THROUGH
What is a couple?
a pair of forces of equal magnitude acting antiparallel either side of a pivot
What mnemonic is used for the equations of motion?
SUVAT
where
s= displacement (m)
u= initial velocity (ms^-1)
v= final velocity (ms^-1)
a= acceleration (ms^-2)
t= time (s)
How can we work out acceleration if we have 2 velocity’s and 2 times?
-same as a gradient of a velocity-time graph
change in velocity/ change in time
How can we work out velocity if we have two displacements and two times?
-same as a gradient of a displacement-time graph
change in displacement/ change in time
What doe we mean by instantaneous velocity?
the velocity of an object at a specific point in time
How can we work out the average speed/velocity?
velocity:
total displacement/ total time taken
speed:
total distance/ total time taken
What is the gradient of a displacement-time graph at a specific point equal to?
instantaneous velocity
What is the gradient of a velocity-time graph equal to?
acceleration
What is the area under a velocity-time graph equal to?
overall displacement
What is the area under an acceleration-time graph equal to?
overall change in velocity
When do we use SUVAT equations?
when the acceleration is uniform and resultant force is constant
Name all the SUVAT equations (4)
v= u + at
s= 1/2(u + v) x t
v^2= u^2 - 2as
s= ut + 1/2at^2
What is something to know when an object is being projected?
the horizontal and vertical components need to be treated independently
What do we mean by time of flight?
how long the projectile is in the air
What do we mean by the maximum height?
the height at which the projectile is momentarily at rest
What do we mean by the range?
the horizontal distance travelled by the projectile
What do you do if a projectile is at an angle and with a velocity acting in that direction?
resolve the vectors
How do you work out time of flight?
2 x time taken to reach maximum height
How can you find the time taken to reach maximum height?
use the SUVAT equations
When working with projectiles, what component is always zero for the x-components?
acceleration
When working with projectiles/free fall, what would acceleration be for the y-component?
if up is the +ve:
if moving up: 9.81
if moving down: -9.81
What are the two types of resistive forces?
-contact friction
-fluid friction/drag
What increases fluid friction/drag? (3)
-increasing viscosity of fluid
-increasing speed of the object relative to the fluid
-increasing the cross-sectional area of object pushing through the fluid
What is lift?
an upwards force on an object moving through a fluid. It is perpendicular to the fluid flow
What is thrust?
direction of motion
What is the effect of having/not having air resistance in projectile motion?
no air resistance:
range and maximum height is increased compared
with air resistance:
lower maximum height, smaller range (proj. diagram would have an asymmetrical and lower parabola compared to no air resistance)
What is terminal velocity? When does it occur?
the maximum velocity an object can have
- occurs when resistive forces balance the driving forces
RP3: describe this practical
-aim of the experiment is to calculate the value of the acceleration due to gravity, g
-done by measuring the time it takes for a ball-bearing to fall a certain distance. The acceleration is then calculated using an equation of motion
RP3: what should the graph be? Write the equation of the line
2h/t= gt +2u
where:
y = 2h/t (m s-1), velocity
x = time
m = g(m s–2), acceleration
y-intercept = 2u, initial velocity
What does Newton’s 1st law state?
the velocity of an object will not change unless acted on by a resultant force
What does Newton’s 2nd law state? Write down the equation with units
the resultant force on an object is equal to its rate of change in momentum
F= change in momentum/change in time
where:
F= resultant force (N)
momentum= kgms^-1
time= s
What equation can you use from Newton’s 2nd law for a constant mass?
F= ma
where
f= resultant force (N)
m= mass (kg)
a= acceleration (ms^-2)
What equation can you use from Newton’s 2nd law for a constant velocity?
F= v(change in mass/ change in time)
where:
F= resultant force (N)
v= velocity (ms^-1)
mass= kg
time= s
What does Newton’s third law state?
when one body exerts a force on a second body, the second body simultaneously exerts a force of equal magnitude and opposite direction to the first body
How can we calculate momentum?
p= mv
where:
p= momentum (kgms^-1)
m= mass (kg)
v= velocity (ms^-1)
What is the principle of conservation of momentum?
the total momentum before a collision = the total momentum after a collision provided no external force acts
What is impulse?
change in momentum
What are the equations to work out impulse? What is its unit?
impulse (Ns) = force x change in time
or
impulse (Ns) = change in momentum
When are the impulse equations used?
when force is constant
How can we work out impulse from a force-time graph?
area of the graph
What is something to know about energy?
total energy is always conserved
What does the principle of the conservation of energy state?
energy cannot be created or destroyed, only be transferred from one store to another
What do we mean by an elastic collision?
when there is no loss of kinetic energy in the collision
What do we mean by an inelastic collision?
when the colliding objects have less kinetic energy than before
What happens to the kinetic energy in an inelastic collision?
some of the energy will be shifted to the surroundings e.g thermal stores
What does it mean if objects couple together after a collision?
they must have undergone an inelastic collision
What is work?
the amount of energy transferred when an external force causes an object to move over a certain distance
How can we calculate the work done?
W=fs
where:
W= work done (J)
f= force (N)
s= displacement (m)
How can you calculate the work done of the force is at an angle?
W = Fs cos θ (or sin θ)
How can you find the work done from a force-displacement graph?
find the area
What is power?
the rate of energy transfer
How can we calculate power?
Power (W) = change in work done/ change in time
How can we calculate power if an object is moving at a constant velocity with a constant force?
P=Fv
where
P= power (W)
F= force (N)
v= velocity (ms^-1)
What does the force need to be in order to use the P=Fv equation?
must be in the direction of the velocity
What is efficiency?
How useful something is/how efficient it is
What are the two equations to calculate efficiency?
(useful power output/total power input) x 100
or
(useful energy output/total energy input) x 100
