topic 5 - forces Flashcards
What is moment?
the turning effect of a force
What is equation to calculate moment?
moment (Nm) = force (N) x distance (m)
How is the distance measured in moments?
the perpendicular distance from the pivot to the line of action of the force
if an object is in equilibrium, what can be said about the moments acting on the object?
the sum of the clockwise moment about the pivot is equal to the sum of the anticlockwise moment about the pivot
What is the function of a lever?
A lever allows us to lift a heavy object by applying a relatively small amount of force
Explain how a lever can act as a force multiplier
- Force is applied further from the pivot.
- Using a lever means less force is needed to get the same moment
Describe what a gear is and what function it serves in machinery.
A gear is a circular disc with teeth
- in machinery, it is interlocked with other gears in order to transmit the rotational effects of a force from one place to another
what is a fluid?
either a liquid or a gas
in any fluid, at what angle do the forces due to pressure act on a given surface?
pressure in fluids causes a force normal (at right angles) to any surface
what is the equation for pressure?
pressure = force/area
what happens to the density of the atmosphere with increasing altitude?
At increasing altitudes, density decreases
why does atmospheric pressure decrease with an increase in height?
- the pressure is created by collisions of air molecules
- the quantity of molecules decreases as the height increases
- this means atmospheric pressure decreases with an increase of height
what is the Earth’s atmosphere?
a thin layer of air around the earth
How does the pressure in fluids increase with depth?
- as the depth increases, the mass of liquid above that depth also increases
- this means that the force is due to the mass increase
- since the force has increased whilst the area has remained constant
why does the pressure in fluids increase with density?
- As the density of a fluid increases, the number of particles in a given volume increases
- consequently, the weight of the fluid is greater
- this means that the force from the fluid above a certain point is larger. Since the force has increased, the pressure also increases
what equation shows the magnitude of pressure in liquids at different depths?
pressure = height x density x gravitational field strength
what is upthrust always equal to?
the weight of the fluid that the object displaces
what factors influence whether an object will sink or float?
- upthrust
- weight
- density of fluid
what is the equation used to calculate an object’s moment?
moment = mass x velocity
what is the unit used for momentum?
kgm/s
what is meant by “momentum is conserved”?
total momentum before = total momentum after
what is the equation for change in momentum?
change in momentum = force x time
how does a seatbelt improve a passenger’s safety during a collision?
- passenger must decelerate from the vehicle’s velocity at impact to zero, meaning they undergo a fixed change of momentum
- the force they experience is equal to the rate of change of momentum
- seatbelts increase the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced
why is momentum not always conserved in a collision?
external forces may act on the object too
how do protective pads help to reduce injury when two people collide?
- protective pads increase the time taken to come to a stop during the collision
- the rate of change of momentum decreases, reducing the force applied onto the people
other than collisions, what is another type of event where the conservation of momentum applies?
an explosion
what are scalar quantities?
scalar quantities have magnitude only
what are vector quantities?
vector quantities have magnitude and an associated direction
what is force?
a push or pull that acts on an object due to the interaction with another object
what are examples of scalar quantities?
- mass
- temperature
- speed
- energy
- distance
- time
what are examples of vector quantities?
- displacement
- weight
- force
- velocity
- acceleration
- momentum
what is the unit of force?
Newtons (N)
what are the two categories forces can be divided into?
- contact forces
- non-contact forces
what are contact forces?
forces that need to be physically touching to interact
what are non-contact forces?
forces that do not need to be physically touching to interact
what are examples of contact forces?
- friction
- air resistance
- tension
- normal contact force
what are examples of non contact forces?
- gravitational force
- electrostatic force
- magnetic force
what is weight?
weight is the force acting on an object due to gravity
why is the force of gravity close to the Earth?
- the force of gravity close to the Earth is due to the gravitational field
what does the weight of an object depend on?
- the weight of an object depends on the gravitational field strength at the point where the object is
what is the equation to calculate the weight of an object?
weight (N) = mass x gravitational field strength
what is meant by an object’s centre of mass?
the single point where an object’s weight can be considered to act through
what piece of equipment can be used to measure an object’s weight?
a calibrated spring-balance (a newtonmeter)
what is the relationship between the weight and mass of an object?
- the weight of an object and the mass of an object are directly proportional
what is the resultant force?
- resultant force is the single force that is equivalent to all the other forces acting on a given object
what does it mean if a force is said to do “work”?
- the force causes an object to be displaced through a distance
what is the equation used to calculate work done?
work done = force x distance
what are the units for work done?
Joules
what distance must be used when calculating work done?
it must be the distance that is moved along the line of action of the force
what occurs when work is done against frictional forces?
- a rise in temperature of the object occurs
- kinetic energy is converted to heat
what are the relationship between the force applied and the extention of an elastic object?
- the extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded
what is meant by an inelastic deformation?
- a deformation which results in the object being permanently stretched
- the object doesn’t return to its original shape when the force is removed
what is the equation relating force?
force = spring constant x extension
what are the units for spring constant?
(N/M)
what type of energy is stored in a spring when it is stretched?
elastic potential energy
what can extension be replaced with in the equation for spring force?
compression
explain why different people will go at different speeds on the same zip wire
- different people have different surface area
- and hence different air resistance
