P2 - forces Flashcards
define speed
the speed of an object is the distance travelled in a given amount of time
what are the units for speed?
m/s
when does acceleration occur?
when an object changes velocity
what are the units for acceleration?
m/s^2
what is the gradient of a distance-time graph?
the speed (velocity)
what do flat sections show on a distance-the graph?
where the object has stopped
what does a steeper distance-time graph mean?
the object is going faster
what do curves represent on a distance-time graph?
acceleration
what does a steepening curve mean on a distance-time graph?
the object is speeding up (increasing gradient)
what does a levelling off curve mean on a distance-time graph?
the object is slowing down (decreasing gradient)
what is the gradient of a velocity/speed-time graph?
the acceleration
what do flat sections represent on a velocity-time graph?
steady velocity
what does a steeper velocity-time graph mean?
the steeper the graph, the greater the acceleration or deceleration
what are uphill sections of a velocity-time graph?
acceleration
what are downhill sections of a velocity-time graph?
deceleration
what does a curve mean on a velocity-time graph?
changing acceleration
what is the area underneath the line on a velocity-time graph?
the distance
what are scalars?
scalar quantities have size (magnitude) only
what are vectors?
vector quantities have magnitude and direction
what are some examples of scalars?
distance, speed, mass, temperature etc
what are some examples of vectors?
displacement, velocity, acceleration etc
why is distance a scalar?
distance is how far someone has travelled from a starting point; it has magnitude only so it is a scalar
why is displacement a vector?
displacement is how far in a straight line a person is from that point; it has magnitude and direction so it’s a vector
what is kinetic energy?
the energy an object has when it is moving
how many kg are there in 1 tonne?
1000kg = 1 tonne
what is a free body force diagram?
a diagram of an object with arrows drawn to show the direction and size of the forces acting on the object
what do we use to represent forces?
- forces act on objects and we can represent these forces as arrows
- the arrow shows the direction the force acts and can represent the size of the force
what are the two types of forces?
- non-contact forces
- contact forces
what are some example of contact forces?
- friction
- normal contact force
- push
- upthrust
what are some examples of non-contact forces?
- magnetism
- gravitational force (weight)
- electrostatic
describe friction
- occurs when surfaces slide over each other due to the atoms in each surface interacting
- opposes motion, causes wear and tear
describe normal contact force
- a force that is exerted by a solid surface on an object
describe push
- a force which acts to move an object away from something
describe upthrust
- the force on an object in a fluid due to the difference in pressure over the area of the object
describe magnetism
- force on a magnetic object in a magnetic field
- for two magnets; same poles = repel, opposite poles = attract
describe gravitational force (weight)
- force on a mass in a gravitational field
describe electrostatic
- force on a charged object in an electric field; same charges = repel, opposite charges = attract
are forces vectors or scalars?
forces are vectors; they have direction and magnitude
what is the unit for forces?
N (newtons)
what does weight depend on?
the gravitational field you’re in
what is the resultant force?
the overall force, taking into account the two or more forces acting on an object
what does a non-zero resultant force mean?
the forces are unbalanced
what do you do when you resolve a force?
- when you resolve a force, you split the force into two forces that are at right angles to each other
- the two forces have the same overall effect as the original force
how can you resolve a force?
- you can resolve a force using a scale drawing
- just draw two lines so that the original force becomes the longest side of a right angled-triangle
why is resolving forces useful?
resolving forces is useful when you need to see the effect of a force along a particular line
what is Newton’s first law?
- if there is no net force acting on an object it will remain at rest or travel with uniform velocity
- when the net force is zero, we say the object is in equilibrium
what is Newton’s second law?
- acceleration is proportional to the resultant force applied
what equation is linked to Newton’s second law?
F = ma
what is inertia?
this is a measure of how difficult it is to accelerate an object; the more massive the object, the more inertia it has
what is Newton’s third law?
- for every action there is and equal and opposite reaction
(for every force there is an equal and opposite force)
what is the law of conservation of momentum?
in a collision when no other external forces act, momentum is conserved
what are the units of momentum?
kgm/s
what causes change in momentum?
forces
is momentum a scalar or vector?
momentum is a vector since it has both magnitude and direction
what is the total change in momentum during a collision?
zero
momentum before a collision = momentum after a collision
explain momentum and recoil in relation to a cannon
- before the cannon is fired the total momentum of the system is zero
- the cannonball is fired and now moves at high velocity
- the cannonball now has momentum
- therefore the cannon must also have momentum, this is known as recoil
(total momentum of the system must remain zero, so the cannon must move with equal and opposite momentum to the ball)
what is terminal velocity?
constant speed
what does terminal velocity of moving objects depend on?
their drag
what does a greater drag mean?
the greater the drag (or air resistance or friction), the lower the terminal velocity of an object
what does drag depend on?
the object’s shape and area
describe and explain why a falling object reaches terminal velocity
- when an object starts to fall (for a split second) there is no air resistance, just weight
- so the resultant force is equal to the weight and so the object accelerates
- as the velocity increases, so does the air resistance
- the air resistance is still less than the weight (which remains the same throughout) so there is a resultant force and the object continues to accelerate, but there is not as much as at first
- the velocity is increasing but at a slower rate
- when the air resistance becomes equal to the weight, the resultant force will be 0
- Newton’s first law states that if there is no resultant force acting on an object, it will remain at constant speed
- so at this point, the object has reached terminal velocity
what is gravitational potential energy?
the energy an object has due to its position in a gravitational field
what does an object at height have energy in?
its gravitational potential energy store
what does a moving object have energy in?
its kinetic energy store
what is the gravitational field strength on earth?
10 N/kg
what is the law of conservation of energy?
energy cannot be created or destroyed, only transferred into other forms
what can we say when energy is transferred from one form to another?
work is done
what is power?
the rate at which work is done
what is deformation?
when you apply forces to an object it can be stretched, compressed or bent (this is deformation)
what is needed to deform an object?
at least two forces
what are the two types of deformation?
- plastic deformation
- elastic deformation
what is a plastic deformation?
the material will remain distorted when the force is removed
what is an elastic deformation?
the material will return to it’s original length when the force is removed
how is extension related to force for an elastic deformation?
extension is directly proportional to force for an elastic deformation
what is the relationship between the extension of a spring and the force called?
Hooke’s law
what does Hooke’s law state?
the extension of a material is directly proportional to the force applied
how can spring be arranged?
- in series
- in parallel
do springs in series stretch more or less than a single spring?
springs in series stretch more than a single spring
do springs in parallel stretch more or less than a single spring?
springs in parallel stretch les than a single spring
what is the spring constant?
measure of how stretchy something is
if the spring constant of one spring is k, what would the spring constant of springs in series be?
k/2
if the spring constant of one spring is k, what would the spring constant of springs in parallel be?
2k
what does how much an elastically deforming object stretches for a given force depend on?
its spring constant
what does the spring constant depend on?
the material that you’re stretching (the stiffer the material, the larger the spring constant)
what happens to Hooke’s law when the force is great enough?
it stops working
why does Hooke’s law stop working when the force is great enough?
there’s a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally
what is the limit of proportionality?
where the line stops being straight and starts to curve on an elastic band graph
what is a moment?
- when a force is applied to an object that is pivoted, it can cause a rotation; this is called a moment
- a moment is the turning effect of a force
what is the principle of moments?
when an object is in equilibrium (there is no resultant force or resultant moment) the sum of the anticlockwise moments = the sum of the clockwise moments
what is created when a small force is applied at a large distance from the pivot?
a large turning effect
what is ‘load’?
the force the lever exerts on the load
what is ‘effort’?
the force you exert on the lever
what are levers an example of?
simple machines
what do levers act as?
force multipliers
how do levers work?
- they transfer the turning effect of a force; push one end of a lever down and the rotation around the pivot causes the other end to rise
- moment due to a force depends on distance of force from pivot
- levers increase distance from pivot that force is applied, so less input force is needed to get the same moment
- this moment provides an output force to a load
why are levers known as force multipliers?
they reduce the force needed to get the same moment
what are some examples of when levers act as force multipliers?
- long sticks or bars
- wheelbarrows
- scissors
what do gears fit together to do?
gears fit together to transfer turning effects
what are gears?
circular cogs with ‘teeth’ around their edge
how do gears work?
the teeth of different gears can interlock so that turning one gear causes another to turn as well; because of how they are linked together, a gear spinning clockwise will make the next gear spin anticlockwise (this then alternates as you go from gear to gear)
what does a small force applied to a small cog create?
a small moment
what does the small moment of a small cog do the the big cog?
it applies the force to the larger cog which creates a larger moment
how can you work out how speeds and moments will change between gears?
by looking at the gear ratios
how does an ice skater benefit from ice skates having a thin blade?
- because ice skates have a thin blade, there is a small area which forces are acting on causing there to be high pressure
- an ice skater benefits from this because it helps to cut into the ice and would help with stability
in a liquid, what happens to the pressure with depth?
pressure increases with depth
why can gases be compressed?
the particles are really far apart so there’s room to push them closer together
why can’t liquids and solids be compressed?
- the particles are close together
- because the particles cannot be forced to move any closer, therefore solids and liquids are more dense than gases
how do particles in fluids behave?
the particles in fluids move randomly and in all directions
how is pressure created by fluids in a container?
pressure is created due to collisions with the walls of a container
what do fluids trapped in containers create?
a net force at right angles to all surfaces
what can the pressure in liquids be used in?
hydraulic systems
what is a key point for hydraulics?
the pressure in a trapped liquid will be the same everywhere throughout that liquid
what are hydraulic systems used as?
force multipliers; they use a small force to produce a bigger force
what are some applications of hydraulic systems?
- car jacks to lift cars
- diggers
- heavy duty cutting tools
why do things float in terms of density?
an object will float if it is less dense than the fluid it is in
why do things float in terms of forces?
an object will float if its upthrust is equal to (or greater than) its weight
what is Archimedes Principle?
the upthrust exerted on a body immersed in a fluid, is equal to the weight of the fluid that the body displaces
explain the Cartesian Diver Experiment
- squeezing the bottle compresses the air trapped inside the diver and therefore reduces the diver’s volume
- therefore, the diver now displaces less water and so the weight of the displaced water becomes less
- this means the upthrust becomes smaller
- the weight of the diver has not changed and is now greater than the upthrust; therefore, the diver sinks
