P5 - forces Flashcards
is force a vector or a scalar?
vector
what’s the difference between a vector and a scalar?
vectors have a magnitude and a direction, scalars only have a direction
name 5 vector quantities
force, velocity, displacement, acceleration, momentum, etc.
name five scalar quantities?
speed, distance, mass, temperature, time
what are vectors usually represented by?
an arrow - the length of the arrow shows the magnitude, and the direction of the arrow shows the direction of the quantity
what is the equation for weight? what are the units for all of the measurements?
weight(N) = mass (kg) x gravitational field strength (N/kg)
what is the relationship between weight and mass?
they are directly proportional
when a chair is sat on the ground, what is the force of the ground on the chair called?
normal contact force
what is an interaction pair?
a pair of forces that are equal and opposite and act on two interacting objects (basically Newtons third law)
name 4 contact forces:
friction, air resistance, tension in ropes, normal contact force
what is a force?
a push or pull on an object that is caused by it interacting with something
what are the two types of forces
contact or non-contact
what is a contact force?
when two objects have to be touching for a force to act
what is a non-contact force?
a force where the objects do not need to be touching for the force to act
name three non-contact forces
magnetic force, gravitational force, electrostatic force
what is gravitational force?
the force of attraction between masses
what is weight?
the force acting on an object due to gravity (the pull of the gravitational force on the object)
what is force measured in?
newtons
where does a force act from on an object?
a single point, called it’s centre of mass (a point at which you assume the whole mass is concentrated)`
what is a uniform object?
one that’s that same density throughout and is a regular shape
where will the centre of mass be on a uniform object?
at the centre of the object
what is weight measured with?
a calibrated spring balance (or newtonmeter)
is mass a force?
no
what is mass measured in?
kilograms
what is mass measured with?
a mass balance
what do you need to know to calculate the weight of an object?
its mass and gravitational field strength
what do free body diagrams show?
all the forces acting on an object
what do the sizes of the arrows show in a free body diagram?
the relative magnitudes of the forces
what do the directions of the arrows show in a free body diagram?
the directions of the forces acting on the object
what is a resultant force?
the overall force on a point or object
what can you do if you have a number of forces acting at a single point?
you can replace them with the resultant force - a single force that has the same effect as all the original forces together
how do you find the resultant force when multiple forces all act along the same line (they’re all parallel)?
you add together those going in the same direction and subtracting any going in the opposite direction
what happens if a resultant force moves an object?
work is done
what happens when a force moves an object through a distance?
energy is transferred and work is done on the object
what must be done to make something move (or keep it moving if there are frictional forces)?
a force must be applied
when a force is applied, what does the thing applying the force need?
a source of energy (like fuel or food)
what happens when a force does ‘work’ to move an object?
energy is transferred from one store to another
can you say that ‘work is done’ even if energy is wasted?
yes
what is the difference between ‘work done’ and ‘energy transferred’?
they are the same
what happens when you push something along a rough surface (like a carpet)?
you are doing work against frictional forces. Energy is being transferred to the kinetic energy store of the object because it starts moving, but some is also being transferred to thermal energy stores due to the friction. This causes the overall temperature of the object to increase
what equation can you use to find out how much work has been done? (include units)
work done (J) = force (N) x distance (m) (w = Fs)
what can you use scale drawings for?
to find the resultant force
how do you use scale drawings to find the resultant force?
- draw all the forces acting on an object ‘tip-to-tale’ (start drawing the arrow of the second force from the point (end) of the first arrow). Make sure it’s to scale (make sure you choose a sensible scale, e.g. 1 cm = 1 N)
- draw a straight line from the start of the first force to the end of the last force (making a triangle). This line is the resultant force
- measure the length of the resultant force on the diagram to find the magnitude of the force (using your scale to convert it back into newtons)
- the direction of the resultant force is measured as a bearing (clockwise from north)
when is an object at equilibrium?
when the forces on it are balanced - if all the forces acting on an object combine to give a resultant force of zero, the object is in equilibrium
what is a scale diagram?
a diagram of all the forces acting on an object, drawn so that one force begins where the previous one ends (the arrow of one force starts at the tip of the arrow of the previous force)
what should happen if you draw a scale diagram of an object in equilibrium?
the tip of the last force you draw should end where the tail of the first force you drew begins (e.g. for three forces, the scale diagram will form a triangle)
why might you want to split a force into components?
not all forces act horizontally or vertically - some act at awkward angles. To make these easier to deal with, they can be split into two components at right angles to each other (usually horizontal and vertical). Acting together, these components have the same effect as the single force
what does it mean to resolve a force?
to split it into components
how do you resolve a force?
by drawing it on a scale grid. Draw the force to scale, and then add the horizontal and vertical components along the grid lines. Then you can just measure them
what does it mean if a force causes an object to stretch, compress or bend?
there must be more than one force acting on the object (otherwise the object would simply move in the direction of the applied force, instead of changing shape)
what is displacement?
a vector quantity that measure the distance and direction in a straight line from an objects starting point to its finishing point
how could an object travel at a constant speed with a changing velocity?
if it is changing direction whilst staying the same speed
what is the formula that links speed, time, and distance travelled?
distance travelled (m) = speed (m/s) x time (s) s = vt
what is the average speed of a person walking?
1.5 m/s
what is the average speed of a person running?
3 m/s
what is the average speed of a person cycling?
6 m/s
what is the average speed of a car?
25 m/s
what is the average speed of a train?
30 m/s
what is the average speed of a plane?
250 m/s
what factors affect the speed a person can walk, run, or cycle?
their fitness, their age, the distance travelled, the terrain, etc.
what is the speed of sound?
330 m/s in air
what affects the speed of sound?
what the sound waves are travelling through
what is wind speed affected by?
things like temperature, atmospheric pressure, and if there are any large buildings or structures nearby (e.g. forests reduce the speed of the air travelling through them)
what is uniform acceleration? give an example.
constant acceleration, e.g. acceleration due to gravity for objects in free-fall
what is acceleration?
the change in velocity in a certain amount of time
what equation can you use to find the average acceleration of an object? (include units)
acceleration (m/s^2) = change in velocity (m/s) / time (s)
a = Δv/t
what is deceleration?
negative acceleration (if something slows down, the change in velocity is negative)
what is the approximate acceleration for objects in free fall?
9.8 m/s^2 - the same value as gravitational field strength
what equation can you use for uniform acceleration?
v^2 - u^2 = 2as v = final velocity (m/s) u = initial velocity (m/s) a = acceleration (m/s^2) s = distance
what does the gradient equal on a distance-time graph?
speed
what do flat sections mean on distance-time graphs?
that it’s stationary
what do straight uphill sections on a distance-time graph mean?
it’s travelling at a constant speed
what do curves represent on a distance-time graph?
acceleration or deceleration
what does gradient represent on a velocity-time graph?
acceleration
what do flat sections represent on a velocity-time graph?
travelling at a constant speed
what do uphill sections mean on a velocity-time graph?
acceleration
what do downhill sections mean on a velocity-time graph?
deceleration
what does a curve mean on a velocity-time graph?
changing acceleration
how do you work out the distance travelled on a velocity-time graph?
by calculating the area under the graph
how do you calculate acceleration on a velocity-time graph?
you find the gradient of the line
why will an object with no force propelling it along always slow down and stop? (unless you’re in space)
because of friction
in which direction does friction act?
the opposite direction to movement
what needs to happen in order for an object to travel at a steady speed?
the driving force needs to balance the frictional forces
where do you get friction?
between two surfaces in contact, or when an object passes through a fluid (drag)
how can you reduce friction between surfaces
using a lubricant
what is drag?
the resistance you get in a fluid (a gas or a liquid). Air resistance is a type of drag.
what is the most important factor in reducing drag?
keeping the shape of the object streamlined
what does streamlined mean?
designed to allow fluid to flow easily across it, reducing drag
what will frictional forces from fluids always increase with?
speed
does a car have more friction to work against when travelling at 70 mph or 30 mph? what does this mean?
a car has much more friction to work against when travelling at 70 mph, so at 70 mph the engine has to work much harder just to maintain a steady speed
what are the four steps to an object reaching its terminal velocity?
- when a falling object first sets off, the force of gravity is much greater than the frictional force slowing it down, so it accelerates
- as the speed increases the friction builds up
- this gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force (so the resultant force is zero)
- it will have reached its maximum speed, or terminal velocity, and so will now fall at a steady speed
what does terminal velocity depend on?
shape and area
will a less streamlined object have a higher or lower terminal velocity?
lower
will objects with large surface areas have higher or lower terminal velocities?
lower
would a marble or a beach ball have a higher terminal velocity? why?
the marble would have a higher terminal velocity, because there is more air resistance acting on the beach ball at any given speed, so the beach ball spends less time accelerating (and so doesn’t speed up as much) before the air resistance is large enough to equal the accelerating force
what is Newton’s first law?
if the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it will just carry on moving at the same velocity.
what does a non-zero resultant force result in?
a non-zero resultant force will always produce acceleration (or deceleration) in the direction of the force. This “acceleration” can take 5 different forms: starting, stopping, speeding up, slowing down and changing direction.
On a free-body diagram, the arrows will be unequal.
what is the relationship between acceleration and the resultant force?
they are directly proportional
what is the relationship between the acceleration and mass of an object?
they are inversely proportional
what is the formula that describes Newton’s second law (linking acceleration, mass, and resultant force)?
(include units)
resultant force (N) = mass (kg) x acceleration (m/s^2) F = ma
what is an estimate for the mass of a car?
1000 kg
how long approximately would it take for a car to accelerate from rest to a typical speed?
typical speed = ~25 m/s
time it takes = ~10 seconds
what is inertia?
the tendency for motion to remain unchanged: until acted upon by a resultant force, objects at rest will stay at rest and objects moving at a steady speed will stay moving at that speed (Newton’s first law).
This tendency to continue in the same state of motion is called inertia
what does an object’s inertial mass measure?
how difficult it is to change the velocity of an object
how can you find an object’s inertial mass?
using Newton’s Second Law of F = ma. Rearranging this gives m = F / a, so inertial mass is just the ratio of force over acceleration
what is Newton’s third law?
when two objects interact, the forces they exert on each other are equal and opposite.
what is an example of Newton’s Third Law in an equilibrium situation?
a man pushing against a wall - as the man pushes the wall, there is a normal contact force acting back on him. These two forces are the same size. As the man applies a force and pushes the wall, the wall ‘pushes back’ on him with equal
a book resting on the ground is in equilibrium - the weight of the book is equal to the normal contact force. Is this Newton’s Third Law?
no - the two forces are different types, and they are both acting on the book
describe an experiment to investigate how mass and force affect acceleration (testing Newton’s second law)
- set up the apparatus so that a trolley of known mass is connected to a piece of string that goes over a pulley and off the side of the bench. The other end of the string is connected to a hook (that you know the mass of and can add more masses to). a light gate (connected to a data logger or computer) is suspended above the string.
- set up the trolley so that it holds a piece of card with a gap in the middle that will interrupt the signal on the light gate twice. If you measure the length of each bit of card that will pass through the light gate and input this into the software, the light gate can measure the velocity for each bit of card. it can use this to work out the acceleration of the trolley.
- The weight of the hook and any masses attached to it will provide the accelerating force, equal to the mass of the hook x acceleration due to gravity
- the weight of the hook and masses accelerates both the trolley and the masses, so you are investigating the acceleration of the system
- mark a starting line on the table the trolley is on, so that the trolley always travels the same distance to the light gate
- place the trolley on the starting line, holding the hook so the string is taut, and release it
- record the acceleration measured by the light gate as the trolley passes through it. This is the acceleration of the whole system.
- repeat this twice to get an average acceleration
- to investigate the effect of mass, add masses to the trolley one at a time to increase the mass of the system. Don’t add masses to the hook, or you’ll change the force. Record the average acceleration for each mass,
- to investigate the effect of force, you need to keep the total mass of the system the same, but change the mass on the hook. To do this, start with all the masses loaded onto the trolley, and transfer all the masses to the hook one at a time, to increase the accelerating force. Record the average acceleration for each force
is momentum a scalar or a vector?
a vector
what is the equation for momentum? (include units)
moment (kg m/s) = mass (kg) x velocity (m/s)
ρ = mv
what are the units for momentum?
kg m/s
what is the conservation of momentum?
in a closed system, the total momentum before an event (e.g. a collision) is the same as after the event. (momentum before = momentum after)
how does an explosion adhere to the conservation of momentum?
in an explosion, the momentum before is zero. after the explosion, the pieces fly of in different directions, so the total momentum cancels out to zero
describe how momentum is conserved by a gun recoiling as it shoots a bullet? (4 marks)
before the gun fires the bullet, the total momentum is zero (neither the gun nor the bullet are moving) [1 mark]
when the bullet leaves the gun, it has a momentum in one direction [1 mark]
the gun moves backwards so it has momentum in the opposite direction [1 mark]
this means that the total momentum after the bullet has been fired is zero. momentum has been conserved [1 mark]
