Forces 1 new version Flashcards
What is the difference between scalar and vector quantities?
Scalar quantities have magnitude only, whereas vector quantities have both magnitude and an associated direction.
How might we represent a vector quantity?
With an arrow.
When representing vector quantities with an arrow, what does the length of the arrow represent?
The magnitude.
When representing vector quantities with an arrow, what does the direction of the arrow represent?
The direction of the vector quantity.
List some vector quantities
-velocity
-displacement
-force
-weight
-momentum
-acceleration
List some scalar quantities
-speed
-distance
-time
-length
-mass
-power
-temperature
What is a force?
A push or pull that acts on an object due to the interaction with another object.
What are the two types of forces between objects?
Contact and non-contact
What is the difference between a contact and non-contact force?
Contact - the objects interacting are physically touching.
Non-contact - the objects interacting are physically separated.
Examples of non-contact forces (hint: there are only three!)
-gravitational force
-magnetic force
-electrostatic force
Examples of contact forces
-friction
-air resistance
-normal contact force
-tension (pulling force through a rope)
(-water resistance)
(-lift)
Is force a vector or scalar quantity?
Vector.
Give examples to describe the interaction between pairs of objects which exert a force on each other.
The force of a ground on a box and the force of a box on the ground.
The force of an object on a surface and the force of the surface on the object.
Weight
The force acting on an object due to gravity.
What does the gravitational field around the Earth cause?
The force of gravity close to the Earth.
What two things does the weight of an object depend on?
-The gravitational field strength at the point where the object is
-The mass of the object
What is the formula for weight?
W = mg
What is the unit for weight?
Newtons, N
What is the unit for mass, m?
kg
What is the unit for gravitational field strength, (g)?
N/kg
or m/s²
Centre of mass
A single point through which the weight of an object may be considered to act.
Describe the relationship between the weight and mass of an object.
Directly proportional.
How can we measure the weight of an object?
Using a calibrated spring-balance (a newtonmeter).
If the distance between two objects were to increase, what would happen to the strength of a non-contact force between them?
It would decrease.
How can we find the centre of mass of an irregular object?
By hanging a plumb line from a bung between the arms of a clamp on a clamp stand.
Draw crosses where the plumb line falls on the object.
Repeat hanging the object (we used whiteboards) from different points.
A gravitational force is always what?
Attractive
Acting between all masses
Magnetic force
-Acts between magnetic poles
-Unlike poles attract and like poles repel
Electrostatic force
-Between + and - charges
-Unlike charges attract and like charges repel
How can the interaction between pairs of objects which exert a force on each other be represented?
as vectors
What is displacement as a vector quantity?
It gives the distance travelled and in what direction, for example travelling 50 miles East.
Reaction force
Force exerted in the opposite direction to an action (original )force.
Normal contact force
The reaction force experienced by an object at rest on a surface.
This reaction force is at 90° to the surface, acting upwards in opposition to the weight of an object.
(For example, a box on a table).
Tension force
A pulling force exerted by each end of an object such as a string or rope when it is being stretched.
Friction force
A force that acts in opposition to the pushing force that is trying to change an object’s motion.
It always acts to slow a moving object down.
It converts kinetic energy into heat.
(two objects sliding past each other experience this force e.g. a box sliding down a slope).
Air resistance
A force of friction produced when an object moves through the air.
What is another name for the normal contact force?
-Reaction force
-Normal reaction force
-Normal force
Resultant force
A single force that describes all of the forces acting on a body.
How do we calculate the resultant of two forces that act in a straight line? (so from a scaled vector diagram?)
-Work out the difference in the two forces.
If a car has more force (N) acting on the way it is facing than backwards, what is the car doing?
Accelerating
If a car has equal force (N) acting on the way it is facing as backwards, what is the car doing?
Moving at a constant velocity
or
Is stationary
Speed vs Velocity
Speed - only has magnitude
Velocity - has both magnitude and direction
If a car has more force (N) acting backwards than the way it is facing, what is the car doing?
Decelerating
When describing resultant forces we must …
state the resultant force
-state in which direction the force is acting
-to get e.g. 25N to the left
When would we say an object is in a state of equilibrium?
When all the forces that act upon the object are balanced i.e. the resultant force is zero.
A cyclist pushes on his pedals moving north with a force of 30N against a wind pushing him east with a force of 40N.
Calculate the resultant force and his direction.
-use a vector diagram
-find a scale factor (e.g. 1cm = 10N)
-in this case, draw a line with an arrow pointing north (3cm)
-from the TOP of this line draw another arrow pointing east (4cm)
-turn this into a triangle and measure the line (5cm)
-use Pythagoras to check your answer
-scale back up and state the direction
How would we find the bearing from a vector diagram?
-Draw an angle in a clockwise direction from the (north) facing line to the one you drew (inside the triangle)
-Use a protractor to measure the angle
Bike man is an example of what?
How a single force can be resolved into two components acting at right angles to each other.
The two component forces together have the same effect as the single force.
What is the equation for resultant force?
mass x acceleration
What is the equation for acceleration
a = ∑force ÷ mass
What are free body diagrams?
Diagrams showing all the forces acting on an object using force arrows.
They can be used to describe qualitatively how several forces lead to a resultant force on an object.
This includes balanced forces when the resultant force is zero.
Picture the Cognito plane neowww
What is the gravitational field strength of the Earth?
9.8 N/kg
What is the gravitational field strength of the moon?
1.6 N/kg
What is the gravitational field strength of the Jupiter and Mars?
Jupiter - 26
Mars - 3.75
When using vector diagrams, how might we CALCULATE an angle? (rather than just measuring it with a protractor)
Using trigonometry
If the angle of a resultant force is below the horizontal, what must we do?
-Measure from the horizontal to the drawn line
-See what angle is the same in the triangle
-Use trig
-Change answer to a minus
Instead of saying e.g. ‘North West’ what is fancier/more appropriate for calculations?
e.g. 67.4° to the horizontal, acting left.
What words can you use when describing the motion of an object?
-Accelerating
-Decelerating
-Upwards
-Downwards
-To the left
-To the right
-Stationary/accelerating at a constant velocity
How to tell which force is resultant?
Not one of the main two forces arranged tip-to-tail.
Give examples of the forces involved in stretching an object.
Stretching requires two forces of tension, acting away from each other.
Explain why, to change the shape of a stationary object (by stretching, bending or compressing), more than one force has to be applied.
If only one force acts on an object the object will accelerate/ rotate instead of deforming.
Describe the difference between elastic deformation and inelastic deformation caused by stretching forces.
Elastic deformation means that a material will return to its original shape when the deforming force is removed.
Inelastic deformation means that the material will not return to its original shape when the deforming force is removed. (exceeding limit of proportionality?)
What is the unit for force, F?
N (newtons)
What is the symbol for spring constant?
k
What is the unit for spring constant?
N/m
What is the unit for extension, e?
metres, m
What is the equation that describes linear elastic deformation?
F = k e
Force = spring constant x extension
Describe the difference between a linear and non-linear relationship between force and extension. (using the example of a spring)
For a linear relationship, the extension is directly proportional to the force applied, so the spring exhibits elastic deformation.
After the limit of proportionality is exceeded (shown as a point on a graph- which you can label P btw) the relationship is no longer linear. This means that for forces applied, the spring will not return to its original shape - exhibiting inelastic deformation.
How do you calculate a spring constant? (in linear cases)
Rearrange F = k e
or Ee = 1/2 k e²
How do you calculate work done in stretching (or compressing) a spring (up to the limit of proportionality)?
using the equation:
Ee = 1/2 k e²
elastic potential energy = 0.5 x spring constant x (extension)²
Because Ee is equal to Work done
Give examples of the forces involved in bending an object.
Bending requires two forces, one acting clockwise and one acting anticlockwise (pick up ya ruler and try).
Give examples of the forces involved in compressing an object.
Compression involves two equal forces acting towards each other.
Describe the relationship between the extension of an elastic object, such as a spring, and the force applied.
The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.
Provided a spring is not ___________deformed, the ____ ____on the spring and the _______ _________ ______ stored are equal.
inelastically
work done elastic potential energy
What goes on the axis of a force - extension graph?
-Extension (m) on the x-axis
-Force (N) on the y-axis
What is meant by ‘deformation’
The change in shape of an object as a result of forces being applied to it.
Extension
The increase in length of a spring when it’s stretched (caused by a weight added)
Why doesn’t a spring fall down when the force acting on it is increased e.g. by adding more mass?
The solid support exerts an equal and opposite force upwards.
What is Hooke’s law?
F = k e
i.e. the directly proportional relationship between the extension of an elastic object, such as a spring, and the force applied.
What does a force that stretches (or compresses) a spring do?
Work
What is stored in a spring?
Elastic potential energy.
What does applying multiple forces to an object cause it to do?
Compress, Stretch, or Bend
Spring constant
An object’s spring constant is a measure of how many Newtons of force it would require to stretch (or compress) the object by 1 metre.
You can think of it as a measure of how stiff the object is. Objects with a higher spring constant are more stiff, so they require more force to stretch.
What does a lower spring constant mean?
An object is more elastic/less firm so easier to stretch.
When a spring is stretched, energy is transferred to its ________ _____ energy store.
Then when the spring is released, most of that energy is transferred to _______ energy.
elastic potential
kinetic
What is the formula for elastic potential energy?
Ee = 1/2 k e²
Another way of saying limit of proportionality?
Elastic limit.
What is extension?
The increase in the length of a spring when it’s stretched
or
The decrease in length of a spring when it’s compressed
What does the gradient of a force-extension graph tell you?
The value of the spring constant, k (when linear)
What does the area beneath the straight line on a force-extension graph tell you?
The energy transferred to the spring/ the elastic potential energy.
What is elastic potential energy?
The energy transferred to an object as it’s stretched.
What is meant by a ‘moment’ of a force?
The turning effect of a force.
What causes an object to rotate?
A force at an angle to an object.
What our our examples for how forces can cause rotation?
-Levers
-Gears
How do we work out the size of a moment?
Moment of a force = force x distance
[M = F d]
Unit for the moment of a force, M ?
newton-metres, Nm.
In the equation M = Fd, what does the d stand for?
The perpendicular distance from the pivot to the place where the force is being applied.
In metres, m.
What is a pivot?
The central point which an object rotates about when a force is applied.
If an object is balanced, what must be true?
Total clockwise moment about the pivot = Total anticlockwise moment about that pivot.
How do we calculate the size of a force, or its distance from a pivot, acting on an object that is balanced? e.g. a see-saw?
Use the equation M = F d
Remembering that for an object to be balanced, total clockwise moment about pivot = total anticlockwise moment about that pivot.
(a heavier item needs to be closer to the pivot on a see saw…)
The greater the moment, the greater the…
rotational effect of the force.
Another word for ‘moment’?
Torque.
What is needed to achieve the same turning effect if closer to the pivot?
A greater force.
Because of M = F d, if distance is small, the force must be great to achieve the same turning effect.
When looking at see-saws, what force are we referring to for M = F d
Weight.
You may have to calculate this first using W= mg.
Which direction does weight act?
Downwards.
What must be done to lift an object?
An upwards force greater than the object’s weight must be applied.
How do we decrease the force needed to lift an object?
Go further away from the object using a lever/plank/spanner etc!!!
What allows two people of different weights to balance on a see-saw.
The total clockwise moment equalling the total anticlockwise moment.
Explain why a longer spanner is easier to use than a shorter one.
Less force is required to generate the same turning effect when using a longer spanner, because the perpendicular distance from the pivot is greater.
M = F d.
Explain how levers transmit the rotational effects of forces.
By using the principle of moments, they take an input force and multiplying it at the output.
Hence they are known as ‘force multipliers’.
What are both levers and gears?
Mechanical components which transmit rotational motion.
How do levers multiply an input force at the output?
The output is closer to the pivot.
Distance is therefore decreased.
Because of M = F d, force must have increased.
What happens if levers are on different sides of a pivot, like in scissors?
The output forces will act in different directions.
What happens if levers are on the same side of a pivot, like in a wheelbarrow?
The output forces will act in the same direction (e.g. turning the wheel.
What is the role of both a simple lever system and a simple gear system?
To transmit the rotational effects of forces.
Explain how gears transmit the rotational effect of forces in a car.
-Interlocking gears allow the turning effect of an engine to be transmitted to the wheels
-It works because the engine provides a turning force to rotate gear A, which rotates gear B, which rotates the wheels
What is the ratio of the turning effect of two gears in proportion to?
The radius of the two gears.
How is the turning effect of an engine doubled as it passes to the wheels of a vehicle?
The radius of gear B is 2x the radius of gear A
Therefore, the moment (turning effect) of Gear B is 2x that of gear A.
(M = F d/distance doubles/force from engine remains constant).
What must a person do to lift a rock that generates an anticlockwise moment of 160Nm?
Generate a clockwise moment greater than 160Nm.
-if in an equation, start by making distance the subject!
What two things can a fluid be?
-A liquid
-A gas
What does the pressure in fluids cause?
A force normal (at right angles) to any surface.
How do we calculate the pressure at the surface of a fluid?
pressure = force at right angles to a surface ÷ the area of that surface
P = F ÷ a (but a is just the one surface!)
What is the unit for pressure, p?
Pascals, Pa
What is pressure?
Force per unit area
What is the unit for area?
metres squared.
What is ‘fluid pressure’?
A measure of the force exerted by a fluid per unit area of the surface in contact with it.
This surface could be the walls of a container, air at the surface, etc.
What is fluid pressure caused by?
The collisions of fluid molecules with an object’s surface.
Fluid pressure acts in all directions.
When calculating pressure in fluids, why do we want the NORMAL force (the one at right angles to a surface the particles are colliding with)?
Because the entire force generates pressure.
If the particles came at a right angle, only a small component of force would be used to generate pressure.
How can the pressure due to a column of liquid be calculated?
Pressure = height of the column x density of the liquid x gravitational field strength
[p = hρg]
In p = hρg, what is the unit for density, ρ?
kg/m³
What do we mean by ‘height of the column’?
The depth of water above a specific point.
What is earth’s gravitational field strength?
9.8 N/kg
Explain why, in a liquid, pressure at a point increases with the height of the column.
Due to the increasing weight of the liquid exerting a downwards force on that point.
Explain why, in a liquid, pressure at a point increases with the density of the liquid.
-Denser liquids have more mass per unit volume
-This results in a greater weight above a specific point
-This increases the force exerted and thus the pressure.
How do you calculate differences in pressure at different depths in a liquid?
Use p = hρg
Which states can exert pressure?
Solids, liquids, and gases.
What factors affect the pressure at a point in a liquid?
-height of column
-density of the liquid
-gravitational field strength
Upthrust
A resultant force upwards.
What causes upthrust?
When a partially/totally submerged object experiences a greater pressure on the bottom surface than on the top surface.
Describe the factors which affect floating and sinking.
-The density of the object
-The depth of the liquid
-Gravitational field strength
(-If the upthrust or the buoyant force (acting upwards) is larger)
How does the depth of a liquid affect sinking/floating?
The grater the depth, the more pressure is exerted upwards increasing the buoyant force acting on an object.
How does gravitational field strength affect sinking/floating?
Yes, increasing the gravitational field strength directly increases the downward force as W = mg
How does density affect sinking/floating?
If the object is:
Less dense than the fluid, it will float because buoyant force > object’s weight.
More dense than the fluid, it will sink as object’s weight > buoyant force.
An equal density to the fluid, it will remain suspended as buoyant force = object’s weight.
What is the atmosphere?
A thin layer of air round the Earth.
(Thin relative to the size of the Earth)
As altitude increases, the atmosphere gets…
…less dense.
What is atmospheric pressure?
The total force of all air molecules colliding with surfaces.
(from all sides…constantly!)
Explain why atmospheric pressure varies with height above a surface.
As the height of a surface above ground level increases, the number of molecules above the surface decreases.
So, the weight of air above the surface decreases.
So atmospheric pressure decreases with an increase in height.
As height increases, there is always ____ ___ above a surface than there is at a lower height.
less air
Atmospheric pressure __________ the further from Earth’s surface you get.
decreases
Why is atmospheric pressure greater at sea level than at a higher elevation?
-The air is more dense at sea level
-So there are more gas particles to collide with objects/the surface
What are the main gases that make up Earth’s atmosphere?
Nitrogen
Oxygen
Greenhouse gases
O₃ - ozone (making up the ozone layer)
