3.2 Forces in action Flashcards
if you apply a net force on an object, what will happen
the object will accelerate in the direction of the net force
what equation relates force mass and acceleration
F=ma
complete the sentence: a force of 1 Newton will give…
a 1kg mass an acceleration of 1ms-2 in the direction of the force
what is the difference between weight and mass
mass: constant for a specific object or particle (kg)
weight: magnitude is variable- depends on the location (N)
what instrument can be used to determine the weight of an object
newtonmeter
what is weight
gravitational force acting on an object through its centre of mass
what is friction
the force that arises once two surfaces rub against eachother
what is drag
the resistance force on an object through a fluid (gas or liquid) (same as friction)
what is tension
the force within a stretched cable or rope
what is upthrust
an upward force acting on an object when it is in a fluid
what is normal contact force
a force arising when one object rests against another object
how are forces represented in a free-body diagram
-each vector is represented by an arrow labelled with the force it represents
-each arrow is drawn to the same scale (larger force= longer arrow)
what is meant by the term fluid
a substance that has no particular shape and yields easily to external pressure (gas or liquid)
what is the magnitude of drag dependent on
speed and cross sectional area
how is drag related to speed
drag is directly proportional to speed^2 (speed doubled - drag increases by 4)
describe how the forces of weight and drag change when an object undergoes a vertical fall
the weight of the object stays constant however drag increases as speed increases
what is the magnitude of the drag force at the instant to object when it first begins to fall
theres no drag on the object- force is equal to the weight
what is terminal velocity
when drag is equal and opposite to its weight
what is centre of gravity
this is the point where we can consider the weight of the object to be acting
what is centre of mass
if one fore acts in a body and its line of action passes through this point it will cause no rotation
what is location of the centre of mass of a uniform metre ruler
50cm (0.5m)
what is meant by equilibrium
when the resultant force is equal to zero
explain why when an object is in equilibrium the forces acting on it must form a closed triangle
the triangle is closed because the net force is zero and so the object is in equilibrium
if an object is in equilibrium what does this mean in terms of the horizontal and vertical forces acting on the object
the resultant force vertically must be zero and resultant horizontal force must also be zero
how do you calculate the horizontal and vertical components of a force
Tcosθ=F Tsinθ=F (respectively)
what is meant by the moment of a force
the turning effect of a force about an axis or point
how is the moment of the force defined
force x perpendicular distance of the line from the point of action
what is the SI unit for the moment of a force
Nm
state the principle of moments
the sum of clockwise moments is equal to the sum of anticlockwise moments
how can you make an object rotate without there being any transitional motion
apply equal but opposite forces to the object
what is a couple
a pair of equal parallel forces acting in opposite directions and along different lines
what is torque
the measure of the force that can cause an object to rotate about an axis
how is torque defined
torque of a couple = one of the forces x perpendicular separation between the forces = Fd
what are the units of torque
Nm
how is density defined
mass per unit volume
how is density defined
ρ=m/V
what are the SI units of density
kgm-3
describe how you would determine the density of a regular shaped solid
mass can be measured directly using a digital balance and volume can be calculated by measurements taken with a ruler or micrometer
describe how you would determine the density of an irregular shaped solid
mass can be measured directly using a digital balance and volume can be determined by displacement of water when placed into a measuring cylinder
describe how you would determine the density of a liquid
mass can be measured directly using a digital balance and volume can be measured directly using a measuring cylinder
how is pressure defined in words
normal force exerted per unit cross-sectional area
how is pressure calculated
p=F/A pressure=force/ area
what is the SI units of pressure
Nm-2 or Pa
what is a fluid
substance that can flow (gases and liquids)
explain why gases and liquids exert pressure on surfaces
because of the constant bombardment by their molecules- liquids are the same
state the equation that can be used to calculated pressure exerted by a vertical column of a liquid
p=hρg
what does the equation p=hρg show
pressure is not dependent on the cross sectional area, pαh so water pressure is increased with depth, denser liquids will exert greater pressure, the pressure is the same in all directions
what is upthrust
a buoyant force- upward force
why do objects submerged in water experience an upward force
the pressure difference at upper and lower surfaces
state Archimedes principle
the upthrust is equal to the weight of the fluid displaced
what condition is required for an object to sink
if the objects weight is greater than the upthrust
what condition is required for an object to float
upthrust must be equal to the weight of the object
