Topic 5 - Forces Flashcards
What is a vector quantity?
Quantities that have magnitude and direction
What is a scalar quantity?
Quantities that have magnitude only
What is magnitude?
Size
Give 3 examples of a vector quantity.
Force
Velocity
Displacement
Give 3 examples of a scalar quantity.
Speed
Distance
Mass
How are vector quantities represented?
Arrows - length represents magnitude and direction shows direction of quantity
Define force.
A force is a push or pull on an object that is caused by interacting with something
What is a contact force?
The objects must be physically touching
What is a non-contact force?
The objects are physically separated
Give 3 examples of a contact force.
Friction
Air resistance
Tension
Give 3 examples of a non-contact force.
Gravitational force
Electrostatic force
Magnetic force
Is force a vector or scalar quantity?
Vector
What happens when two objects interact?
They exert a force on each other
What is an interaction pair?
A pair of forces that are equal and opposite and act on two interacting objects
What is weight?
The force acting on an object due to gravity
Why is there gravity in earth?
Due to the gravitational field around the earth
What does the weight of an object depend on?
The gravitational field strength at the point where the object is
How do you calculate weight?
Weight (N) = mass (kg) x gravitational field strength (N/Kg)
W=mg
Where is the weight of an object held?
The weight of an object may be considered to act at a single point referred to as the objects “centre of mass”
What is the relationship between weight and mass?
Weight and mass are directly proportional
What device can be used to measure weight?
A calibrates spring balance or newtonmeter
What is the resultant force?
The overall force on a point or object - a number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together
How do you calculate resultant force?
Add forces in the same direction and subtract forces in the opposite direction
How do you show all the forces acting on an isolated object?
Free body diagrams
What happens if a resultant force moves an object?
Energy is transferred and work is done
How do you work out work done?
Work done (J) = Force (N) x Distance (m) W=Fs
How do you resolve (split) a force into components?
If a force acts at an awkward angle, you can split it into two components at rig angles to each other (it has the same combined effect as the single force)
How can you draw resolving forces?
Using a scale grid
What does equilibrium mean?
The forces are balanced and will give a resultant force of zero
When is work done?
When a force causes an object to move through a distance
When does a force do work on an object?
When the force causes displacement of the object
When is 1 joule of work done?
When a force of 1 Newton causes a displacement of 1 metre
What does work done against the frictional force acting on an object cause?
A rise in the temperature of the object
What is needed to stretch, compress or bend an object?
More than one force
Why must two forces be applied to make an object, bend, compress or stretch?
Otherwise it will simple move in the direction of the applied force
What is elastic deformation?
If an object can go back to its original shape and length after the force has been removed
What is inelastic deformation?
If the object doesn’t return to its original shape and length after the force has been removed
What is the relationship between the extension of an elastic object and the force applied? (Under what conditions?)
They are directly proportional provided that the limit of proportionality is not exceeded
How can you work out force? (In terms of elasticity)
Force (N) = spring constant (N/m) x extension (m)
F=ke
When is work done in terms of elasticity?
When a force stretches or compresses the object and causes energy to be transferred to the elastic potential energy store of the object
What happens to the energy if the object is elastically deformed?
All of the energy is transferred to the elastic potential energy store, work done is equal to elastic potential energy stored.
What are the linear and non-linear relationships between force and extension?
Force and extension will be equal until the limit of proportionality (straight line) after that extension will be larger than force (curve)
How do you calculate spring constant in linear cases?
Elastic potential energy (J) = 1/2 x spring constant (N/m) x extension (squared) (m)
What causes objects to rotate?
A force or a system of forces
What is a moment?
The turning effect of a force
How do you calculate a moment?
Moment (Nm) = Force (N) x distance (m)
Give an example of a force causing rotation.
Someone pedalling to turn a wheel on the bike
If the object is balanced what is the relationship between the total clockwise and anti-clockwise moments?
The total clockwise moment about a pivot = the total anti-clockwise moment about that pivot
How can gears and levers be used?
Used to transmit the rotational effects of forces
How do levers work?
Levers increase the distance from the pivot at which the force is applied, so less force is needed to create the same moment making it easier
Give an example of a lever.
Spanner
What are the three classes of lever?
First (pivot in middle)
Second (output effort/load in middle)
Third (input effort in middle)
How do gears work?
Gears interlock so turning one gear causes another to turn in the opposite direction, different sized gears can change the moment.
How does a larger gear change a moment?
A larger gear will cause a bigger moment because the distance to the pivot is greater. A large gear will turn slower than a small gear
What states can a fluid be?
Liquid or gas
What is pressure in a fluid?
Force is exerted at a normal line (perpendicular) to any surface in contact with the fluid
How do you calculate pressure at the surface?
Pressure (pascals (pa)) = force (N) / Area (m squared)
P=F/A
How do you calculate pressure at a certain depth to the column of liquid?
Pressure (pa) = height (m) x density (kg/m cubed) x gravitational field strength (N/kg)
What is density?
How close together the particles are
In liquids is the density uniform or can it vary?
Uniform
In gases is density uniform or can it vary?
Vary
What happens to collisions when a particle is more dense?
There are more particles so more collisions are made so the pressure is higher
What happens to the liquid as depth increases?
The number of particles above that point increase and the weight of them adds to the pressure felt at that point
So pressure increases with depth
What happens when an object is submerged in water?
The pressure of the fluid exerts a force on it from every direction
What happens to the size of the forces on top and bottom as pressure increases?
The force exerted on the bottom is larger than the force exerted on the top of the object
What causes resultant force in liquids?
The force at the top of an object being smaller than the force at the bottom of an object causes upthrust
What is the upthrust equal to when an object is submerged under water?
Upthrust = weight of fluid that has been displaced by the object
When does an object float?
If weight = upthrust
What happens if an objects weight in water is more than the upthrust?
It sinks
What factors effect whether an objects floats or sinks?
Density - a less dense object put in water will weigh less than the equivalent volume of fluid so it displaces a volume of water equal to its weight before it is completely submerged so weight = upthrust and it floats
What happens if an object is denser than the fluid it is placed in?
It is unable to displace enough fluid to equal its weight so weight > upthrust so it sinks
What is the atmosphere?
A thin layer of air that surrounds earth
What happens to the atmosphere when the altitude is increased?
The atmosphere gets less dense as the number of molecules above a surface decreases
When is atmospheric pressure made?
Created on a surface by air molecules colliding with it
Define distance.
How far an object moves
Is distance scalar or vector?
Scalar (it doesn’t involve direction)
Is displacement scalar or vector?
Vector
What is displacement?
It measures the distance and direction in a straight line from an objects start to its finish
Is speed scalar or vector?
Scalar (no direction)
Is the speed of a moving object constant?
No it is always changing
What factors effect the speed at which someone can walk, run or cycle?
Age
Terrain
Fitness
Distance
What is the typical value of walking?
1.5 m/s
What is the typical value of running?
3 m/s
What is the typical value of cycling?
6 m/s
What is the typical value of a car?
25 m/s
What is the typical value of a train?
55 m/s
What is the typical value of a plane?
250 m/s
Speed can also vary in what?
Sound and wind
What is the typical value for the speed of sound?
330 m/s
How do you calculate distance travelled?
Distance traveled (m) = speed (m/s) x time (s)
What is velocity?
Speed in a given direction
What happens when an object is moving in a circle?
It has a constant speed but it’s velocity is changing
How can an object moving in a straight line be represented?
Distance time graph
How can you calculate speed of an object from a graph?
Gradient
How do you find acceleration on a distance time graph?
Draw a tangent and measure the gradient
How do you calculate average acceleration?
Acceleration (m/s squared) = change in velocity (m/s) / time (s)
How do you calculate acceleration of an object on a velocity time graph?
Gradient
How do you calculate the displacement from a graph?
Area underneath line
How do you calculate uniform acceleration?
Final velocity (m/s) - initial velocity (m/s) = 2 x acceleration x distance (m)
What is the value of acceleration due to gravity?
9.8 m/s squared
What is terminal velocity?
The maximum speed it can go whilst free falling
What factors effect terminal velocity?
Friction
Weight
Shape
Area
What is 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 will continue to move at the same velocity
Due to Newton’s first law, what happens to a vehicle moving at a constant speed?
The resistive and driving forces are balanced
When will velocity change?
If a resultant force greater than 0 is acting on it
What does a non-zero resultant force create?
Acceleration in the direction of the force
What does acceleration is proportional to resultant force mean?
The larger the resultant force, the more the object accelerates
What is the relationship between acceleration and mass?
Acceleration is inversely proportional to the mass (so an object with a larger mass will accelerate less)
What is Newton’s second law?
The acceleration of an object is proportional to the resultant force but inversely proportional to the objects mass
What formula describes Newton’s second law?
Resultant force (N) = mass (kg) x acceleration (m/s squared)
What is inertia?
The tendency of objects to continue in their state of rest or of uniform motion (tendency of motion to remain unchanged)
What is an objects inertial mass?
Measures how difficult it is to change velocity of an object
How can you find inertial mass?
Using Newton’s second law (inertial mass is 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 equilibrium?
When two forces balance out
Give an example of Newton’s third law.
A man pushing a wall - they both exert the same amount of force against each other and so neither falls backwards
How do you workout stopping distance?
Stopping distance = breaking distance + thinking distance
How does speed effect thinking distance?
The faster your going, the more distance will be covered in the time you take to react
How does reaction time effect your thinking distance?
Longer reaction time equals longer thinking distance
What factors effect braking distance?
Speed (for a given braking force, the faster the vehicle travels the longer it takes to stop) Weather (if it's wet, the tires have less grip and less friction so it takes longer to stop) Tyre condition (if tires are bald the won't have enough grip and can't get rid of water so the skid) Brake condition
What can effect a drivers reaction time?
Tiredness or drugs or distractions
What are the typical values of reaction time?
0.2 to 0.9 seconds
What happens when a force is applied to the breaks of a wheel?
Work is done by the friction force between the brakes and the wheels and reduces the kinetic energy of the vehicle and increases temperature of the brakes
What can large deceleration lead to?
Brakes overheating or loss of control
What are the typical stopping distances from the high way code?
30 mph - 9m thinking + 14m braking (6 car lengths)
50 mph - 15m thinking + 38m braking (13 car lengths)
70 mph - 21m thinking + 75m braking (24 car lengths)
How do you work out momentum?
Momentum (Kgm/s) = mass (kg) x velocity (m/s)
What is the conservation of momentum?
In a closed system, the total momentum before a collision = the total momentum after a collision
When does a change in momentum occur?
When a force acts on an object that is moving or able to move
How do you work out change in momentum?
Change in momentum (kgm/s) = force (N) x change in time (s)
What safety features do cars have?
- crumple zones (crumple on impact, increase time taken for car to stop)
- seat belts (stretch slightly to increase time taken for passenger to stop)
- air bags (inflate to slow passenger down more gradually than just hitting the dashboard)
How are bike helmets safe in connection to the change in momentum?
The contain a crushable layer to lengthen the time taken for your head to stop and reduces impact on brain
How do crash mats help reduce impact in connection to the change in momentum during a collision?
Made from compressible material to increase the time taken for you to stop if you fall on them
