forces Flashcards
Define scalar quantities
- Quantities that have magnitude only
Define vector quantities
- Quantities that have magnitude and an associated direction
How can a vector quantity be represented?
- By an arrow
- The length of the arrow represents the magnitude
- The direction of the arrow represents the direction of the vector quantity
Examples of scalar quantities
- Distance, mass, speed, time
Examples of vector quantities
- Velocity, displacement, acceleration, force, momentum
What is the difference between scalar and vector quantities?
- Vectors can have direction - can be positive or negative
- Scalars can only be positive
Define a force
- A push or pull that acts on an object due to the interaction with another object
What can all forces between objects be?
- Contact forces; the objects are physically touching
- Non-contact forces; the objects are physically separated
What is the unit of force?
- Newtons (N)
Give examples of contact forces
- Friction, air resistance, tension, normal contact force
Give examples of non-contact forces
- Gravitational force, electrostatic force, magnetic force
What type of quantity is force?
- A vector quantity
How do forces interact between objects?
- There’s an equal but opposite force of attraction between the objects
Define weight
- The force acting on an object due to gravity
Define gravitational field
- The force of gravity close to the Earth
What does the weight of an object depend on?
- The gravitational field strength at the point where the object is
Define the centre of mass
- The single point where the weight of an object may be considered to act
What is the weight of an object directly proportional to?
- The mass of the object
What is used to measured weight?
- A calibrated spring-balance (a newtonmeter)
Define mass
- The quantity that tells us how much matter is within an object
What is the difference between mass and weight?
- Mass is constant no matter where the object is
e.g., if the mass of a person on Earth is 70kg, they will be 70kg in mass no matter where they are in the universe - Weight, however, will vary depending on where the object is (depending on the strength of the gravitational field)
What does a g of 9.8N/kg mean?
- For every kilogram of mass, an object will exert 9.8N of weight downwards onto the earth
Define the resultant force
- The single force that has the same effect as all of the original forces acting on the object (the overall force)
What are the 4 directions in a free-body diagram?
- UP: lift
- DOWN: weight
- LEFT: drag (e.g., air resistance, friction)
- RIGHT: thrust
What can a single force 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 happens when a force causes an object to move through a distance?
- Energy is transferred and work is done on the object; so, a force does work on an object when the force causes a displacement of the object
When is one joule of work done?
- When a force of one newton causes a displacement of one metre
i.e., 1 joule = 1 newton-metre
What does distance mean in the work done equation?
- The line of action of the force
e.g., if someone was going up a flight of stairs, only the vertical distance would matter because that it the line of action of the force; the length of the stairs is irrelevant
What happens when work is done against the frictional forces acting on the object?
- A rise in temperature of the object is caused
How can you use scale drawings to find resultant force?
- Draw all the forces acting on an object to scale, tip to tail
- Draw a straight line from the start of the first force to the end of the last force - this is resultant force
- Measure length to find magnitude and angle to find direction of the force
e.g. bearing of….
What can happen when a force is applied to an object?
- It can either be compressed, stretched or bent
What is the condition for changing the shape of any object?
- By either compressing, stretching or bending it, we must apply more than one force to the object for it to remain stationary; if we only applied one force onto the object, it would simply move
Describe the forces of compressing a box against the floor
- When we are compressing a box against the floor, two forces are being applied; not just the one by your hand, but also the equal and opposite normal contact force exerted by the floor itself
Define deformation
- When an object changes shape (it undergoes deformation)
Define elastic deformation
- When the forces are removed, the object returns back to its original shape; it is called this because elastic objects undergo elastic deformation
- All energy is transferred to the object’s elastic potential energy store
Define inelastic deformation
- When the forces are removed, the object does not return back to its original shape and stays deformed
What is extension?
- The increase or decrease in length of a spring when its stretched or compressed (calculated by doing stretched length subtract natural length
Describe Hooke’s law
- The extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded
i.e. as we double the size of the force acting on a spring, the extension will also double
Define spring constant
- How easy/difficult it is to stretch the spring; this defines how much the spring will stretch
What does a high spring constant mean?
- A high spring constant means it is difficult to stretch it (more force is required to produce the same extension) and thus a low spring constant means it is easy to stretch
What can the spring constant equation also apply to?
- The compression of an elastic object (as well as the stretching of one), where ‘e’ would be the compression of the object
What happens when a force stretches or compresses a spring?
- The force does work and elastic potential energy is stored in the spring; provided the spring is not inelastically deformed, the work done on the spring and the elastic potential energy stored are equal
Define moment
- The turning effect of a force (is called the moment of a force)
Explain moment using a spanner as an example
- The force on the spanner causes a turning effect or moment on the nut - a larger force or longer distance = larger moment
- The get the MAXIMUM moment you need to push at right angles to the spanner.
Describe the moment of a balanced object
- The total clockwise moment about a pivot equals the total anticlockwise moment about that pivot
What can you use a simple lever and a simple gear system for?
- To transmit the rotational effects of forces
Define perpendicular distance
- The shortest distance between the pivot and the line of action of the force
Define a pivot
- A point around which something can rotate or turn
Define an effort
- The force used to move a load over a distance
Define a load
- The overall force that is exerted, usually by a mass or object, on a surface
Give examples of a simple lever
- A solid beam laid across a pivot
How does a solid beam laid across a pivot transmit rotational effects of forces?
- As effort is applied to rotate one end about the pivot, the opposite end is also rotated about the pivot in the same direction
- This has the effect of rotating or lifting the load
Define force multiplier
- Something that increases the effect of a force
What are the advantage of simple levers?
- Levers make use of moments to act as a force multiplier
- They allow a larger force to act upon the load than is supplied by the effort, so it is easier to move large or heavy objects
How can you increase force on a load?
- Make the lever longer
- Increase the distance between the effort and the pivot
e.g. it is easier to push furthest from the hinge when opening a door
What are gears?
- Wheels with toothed edges that rotate on an axle or shaft
- The teeth of one gear fit into the teeth of another gear
What happens as one gear turns (and is connected to another gear)?
- The other gear must also turn; where the gears meet, the teeth must both move in the same direction, meaning the gears rotate in opposite directions
Describe the forces and moments of two connected gears (one larger and one smaller gear)
- The larger gear will rotate more slowly but with a greater moment
- The smaller gear will rotate more quickly but with a smaller moment
Define a fluid
- A liquid or a gas
- Substances that can flow because their particles are able to move around
What does pressure in fluids cause?
- A force normal (at right angles) to any surface
Compare the density of a liquid and a gas
- Density for a given liquid is uniform (does not vary with shape or size
- Density of a gas can vary
Define upthrust (complex definition)
- A partially or totally submerged object experiences a greater pressure on the bottom surface than on the top surface
- This creates a resultant force upwards
- This force is called the upthrust
Define upthrust (simple definition)
- Upwards force exerted by a liquid or gas on an object floating in it
Why does pressure increase with density in a fluid?
- A denser fluid would be more compact therefore it would have more particles in each area; this means there are more particles colliding, which increases pressure
Why does pressure increase with depth (e.g., in a column of liquid)?
- A depth increases, the weight of the particles above that point increases, meaning the pressure increases
Explain how objects in fluids experience upthrust
- When an object is submerged the pressure of the fluid exerts a force in every direction
- Pressure increases with depth so the force exerted on the bottom is larger than the force acting on the top
- Creates a resultant force called upthrust
- Up thrust is equal to the weight of the fluid that has been displaced
When would an object float?
- When the upthrust is equal to the weight of the object
What is the magnitude of upthrust equal to?
- The weight of fluid displaced by the object
When would an object sink?
- When the object’s weight is greater than the upthrust
Does an object with a lower density than the fluid sink/float? Why?
- It floats, because it displaces the amount of fluid equal to its weight, resulting in the upthrust being equal to its weight
Define the atmosphere
- A thin layer (relative to the size of the earth) of air around the earth
Describe an object sinking in terms of its displacement
- If the object only displaces a small amount of water but has a high mass, the upthrust will be small because only a small volume of water was displaced (which has a small weight)
- Upthrust will hence be smaller than the weight of the object, so the object sinks
Describe floating/sinking in terms of an object’s density
- If the object is less dense or the same density as the fluid, it will float
- If an object is denser than the fluid, it will sink
Describe the relationship between altitude and density
- The atmosphere gets less dense with increasing altitude
Describe the relationship between atmospheric pressure and altitude; explain why
- Air molecules colliding with a surface create atmospheric pressure
- The number of air molecules, and so the weight of air, above a surface decreases as altitude increases.
- So as height increases, there is always less air above a surface than there is at a lower height, which means atmospheric pressure decreases with an increase in height
Define distance
- A scalar quantity which tells us how far an object moves; distance does not involve direction
Define displacement
- A vector quantity which measures the distance an object moves in a straight line from the start point to the finish point, and the direction of that straight line
Define speed
- A scalar quantity which tells us the distance an object has travelled in a given time; speed does not involve direction
What factors effect the speed at which a person can walk, run or cycle?
- Age, terrain, fitness, distance travelled
List the typical speed values of walking, running and cycling
- Walking ~ 1.5 m/s
- Running ~ 3 m/s
- Cycling ~ 6 m/s
List the typical speed values of cars, trains and aeroplanes
- Cars ~ 25 m/s
- Trains ~ 50 m/s
- Aeroplanes ~ 250 m/s
Define velocity
- The velocity of an object is its speed in a given direction; velocity is a vector quantity
Why does an object travelling in a circular motion at a constant speed result in a change of velocity but not a change of speed?
- Because velocity is speed in a given direction, a change in direction means a change in velocity
- Therefore an object can be travelling at a constant speed but still have a changing velocity (due to a change in direction)
What is gradient equal to in a distance-time graph?
- Equal to the speed;
- This makes sense because when we calculate the gradient we are doing Δy / Δx which is essentially distance/time
Explain how a distance-time graph displays information
- Straight line with a gradient; constant speed (and acceleration)
- Line with a gradient of 0 (flat line); stationary object
- Curved line; acceleration(increase speed) / deceleration (decreasing speed) taking place
- Acceleration would be an under curve
- Deceleration would be an over curve
What is the condition for representing a moving object on a distance-time graph?
- The object has to be moving along a straight line
How can you calculate the speed at any particular time on a distance-time graph?
- Drawing a tangent and measuring the gradient of it
Define acceleration
- The acceleration of an object tells us the change in its velocity over a given time
What does a negative acceleration mean?
- The object is decelerating; i.e. the object is slowing down
What is the gradient equal to in a velocity-time graph?
- Equal to the acceleration;
- This makes sense because when calculating gradient, we do Δy / Δx which is the same as Δv/t in this context which equals acceleration
How can you calculate distance travelled by an object (or displacement) using a velocity-time graph?
- The area under the graph is equal to the distance/displacement of the object
Explain how a distance-time graph displays information
- A straight line with a gradient; constant acceleration
- Line with a gradient of 0 (flat line); 0 acceleration, meaning constant velocity
- Curved line; acceleration is increasing
How can you calculate distance in a velocity-time graph when acceleration is not constant?
- i.e. when there is a curve
- We count the squares
What is the acceleration of any object falling freely under gravity near the Earth’s surface?
A
* 9.8 m/s²
For the final and initial velocity equation what do u check for if one of the ‘u’ or ‘v’ value seems to be missing?
- Check if it talks about the object being from ‘rest’ or ‘stationary’
- This means that u or v is zero
Describe what happens when an object falls through a fluid
- The object initially accelerates due to the force of gravity
- Eventually the resultant force will be zero and the object will move at its terminal velocity
Define terminal velocity
- The constant velocity reached
Describe what happens when a skydiver falls from a helicopter
- Initially the force acting downwards due to gravity is a lot greater than any upwards force; unbalanced forces means a resultant force and diver accelerates towards ground
- Velocity increases as diver accelerates; as velocity increases, so does air resistance because skydiver is colliding more frequently with air particles
as air resistance increases, eventually there is an equal and opposite force to his weight, causing skydiver to decelerate - As forces balance and resultant force is 0, skydiver will travel a a constant velocity; this is called terminal velocity
- As he deploys parachute, his surface area increases which increases air resistance acting upwards, causing forces to become unbalanced again; skydiver will therefore begin to decelerate and velocity downwards will reduce
- Air resistance decreases with decreasing velocity, as skydiver is colliding less frequently with air particles
- Forces will balance again and skydiver will travel at a much lower terminal velocity which is safe to land at
Define drag
- Resistance you get in a fluid
Define air resistance
- Type of drag which is the frictional force produced by the air acting on a moving object
Describe the relationship between drag and speed
- As speed increases, frictional forces increase
What are the factors affecting terminal velocities when falling through a fluid>
- How much air resistance the object experiences
- The object’s weight
- Causing different objects to reach different terminal velocities
Describe Newton’s first law
- If the resultant force acting on an object is zero and:
- The object is stationary, the object remains stationary
- The object is moving, the object continues to move at the same speed and in the same direction, so the object continues to move at the same velocity
Describe a non-zero resultant froce
- It will always produce acceleration or deceleration in the direction of force
- The acceleration can be starting, stopping, speeding up, slowing down, or changing direction
- The arrows will be unequal on a free body diagram
Define inertia
- The tendency of objects to continue in their state of rest or of uniform motion (as long as no resultant force is being applied)
When will velocity change?
- The velocity of an object will only change if a resultant force is acting on the object
How does a vehicle travel at a steady speed?
- The resistive forces (air resistance and friction) balance the driving force,
- Hence, the overall resultant force is 0
- Meaning the vehicle travels at the same speed
Describe Newton’s second law
- The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object
What happens when you increase resultant force acting on an object and assume mass is constant?
- The acceleration must increase in the same proportion, as there is a directly proportional relationship between acceleration and force (F ∝ a)
What happens when you increase the mass of an object and keep acceleration constant?
- A lower force is required, so there is an inversely proportional relationship between mass and force
Speed of cars on a main road and a motorway; how much would it need to accelerate and how much force would be required to go from a main road to a motorway?
- Main road ~ 13 m/s
- Motorway ~ 30 m/s
- To accelerate from a main road to a motorway, the car needs an acceleration of 2 m/s²
- For a typical family car, it would require a force of ~ 2000N
Define inertial mass
- A measure of how difficult it is to change the velocity of an object
(a bit like spring constant is how difficult it is to extend a spring)
What ratio is inertial mass defined as?
- The ratio of force needed to accelerate an object over the acceleration produced (so F/a)
What does a high inertial mass mean?
- The higher the inertial mass of an object, the more force is required to produce the same acceleration as an object with a low inertial mass
Describe newton’s third law
- Whenever two objects interact, the forces they exert on each other are equal and opposite
Describe pushing off a swimming pool wall using Newton’s third law (100N force)
- The swimmer exerts a 100N force on the wall and so the wall exerts an equal and opposite 100N force on the swimmer
Why when a swimmer pushes off a swimming wall does the swimmer accelerate but the wall doesn’t?
- Because the wall has a much higher mass than the swimmer;
- a = F / m shows that we need a high force or a small mass for an object to accelerate
- So even though both the swimmer and the wall have the same magnitude of force exerted on them due to Newton’s third law, their accelerations will differ due to their varying mass
Name the equation for stopping distance
- Stopping distance = thinking distance + braking distance
Define the stopping distance of a vehicle
- The sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance)
In terms of stopping distance, how is it affected if the vehicle is going faster?
- For a given braking force, the greater the speed of the vehicle, the greater the stopping distance
What is a typical reaction time for a human?
- Although reaction times vary from person to person, typical values range from 0.2 s to 0.9 s
List the factors that affect a driver’s reaction time
- Tiredness
- Drugs
- Alcohol
- Distractions may also affect a driver’s ability to react
List the factors that affect a driver’s breaking distance
- The speed at which the vehicle is travelling (faster speed = longer breaking distance)
- (Poor) conditions of the tyres and brakes
- Adverse road and weather conditions e.g., wet or icy roads
How can you measure reaction time?
- One person holds the ruler and the volunteer places their fingers on either side with the 0 mark on their thumb
- The person then lets go of the ruler without warning and the volunteer should catch the ruler as fast as possible
- Check where the volunteer’s thumb is now
What is dangerous about a braking force being greater?
- The greater the braking force the greater the deceleration of the vehicle
- Large decelerations may lead to brakes overheating and/or loss of control
What happens when a force is applied to the brakes of a vehicle?
- Work is done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases
How does a vehicle going at a faster speed affect braking force?
- The greater the braking force needed to stop the vehicle in a certain distance
What are the typical stopping distances from the Highway Code?
- Speed limits are put in place
- 70MPH = 24 car lengths
- 50MPH = 13 car lengths
- 30MPH = 6 car lengths
Define momentum
- A property all moving objects have
- It’s a vector quantity, meaning it can be positive in one direction and negative in the opposite direction
Define the conservation of momentum
- In a closed system, the total momentum before an event is equal to the total momentum after the event
What should you check for in a momentum question?
- If the object is ‘still’ or ‘stationary’ the momentum is equal to 0 because momentum is a property of MOVING objects
- Conservation of momentum; If the total momentum before the event is 0, then the total momentum after is 0 as well
When does a change in momentum occur?
- When a force acts on an object that is moving or able to move
What is mΔv?
- Change in momentum
in the combined
F = m x a
and
a = (v-u) / t
equations to get
F = (mΔv) / Δt
what does force equal
A
the rate of change of momentum
How does a car crash lead to injury; answer in terms of momentum?
- During a car crash, we lose momentum very quickly; a large change in momentum down to 0 in a short space of time results in large forces acting on us, which leads to injury
?How can we reduce the force that passengers experience in a car crash
- Because we cannot alter the change in momentum, to reduce the force passengers experience, we need to increase the time taken for momentum to decrease and for the change in momentum to take place (i.e., implementing safety features in cars)
How do safety features in cars (e.g., air bags and seat belts) help prevent injury?
- They help to increase the time taken for change in momentum to take place, in order to reduce the forces acting on passengers
Explain how air bags work
- They contain air which is compressible, meaning that when your head hits it, it will take a longer time for your momentum to decrease than if it were to hit a hard surface
Explain how do seat belts work belts work
- They are slightly elastic so increase the time taken for momentum to change, which reduces the forces acting on the passenger
Explain how crumple zones work
- Crumple on impact increase time taken for the car to stop
Why is a faster rate of change of momentum dangerous in a car crash?
- As we can see in the combined equation, force acts as the rate of change of momentum, so if there is a high rate of change of momentum, this equates to a greater force acting on passengers, which leads to injury
Explain other safety measures to reduce momentum
- Helmets contain a crushable layer of foam - lengthen time taken for head to stop in a crash
- Crash mats and cushioned playground flooring made of soft compressible materials
