Forces Flashcards
What do vector quantities have?
Magnitude and Direction
What are vector qualities usually repented by?
Arrow
What does the length of the arrow represent (vector)?
Magnitude
What does the direction of the arrow represent (vector)?
Direction of quantity
What are scalar quantities?
Have magnitude (and no direction)
What can a force be? (name three things)
- Pull
- Push
- Twist
What is a force?
Push or pull on object caused by it interacting with something
What is an interaction pair?
Pair of forces that are equal and opposite and act on two interacting objects
What 2 important effects does gravity have?
- On the surface, it makes all things fall towards the ground
- Gives everything a weight
What is weight?
Force acting on an object due to gravity
What does the weight of an object depend on?
Strength of gravitational field at location
When is the gravitational field strength stronger? (name 2 scenarios)
- Stronger for larger masses
2. Stronger the closer you are to mass causing field
What is mass?
Amount of “stuff” in an object
Mass and weight are…
Directly Proportional
What do free body diagrams show?
All the forces acting on an object
What is a resultant force?
Overall force on a point or object
When is work done?
When a force moves an object through a distance, energy is transferred and work is done on the object
When you apply force on a object, what may happen? (name 3 things)
It may stretch, compress or bend
To stretch, compress or bend an object, what is needed & why?
More than one force is needed, otherwise it’ll go in direction of force applied
When a force stretches or compresses an object, what is done and why?
Work is done because the force causes energy to be transferred to elastic potential energy store of object
An object that is elastically deformed can…
Go back to its original shape and length after force is removed
What type of objects can be elastically deformed?
Elastic objects
When an object is elastically deformed, what happens to the energy?
ALL energy (from force) is transferred object’s elastic potential energy store
An object that’s inelastically deformed will…
Its shape will change permanently (when force is removed)
What is Hooke’s law?
Extension of an object is directly proportional to force applied to it, provided that the limit of proportionality is not exceeded
What does the spring constant (k) depend on?
Material being stretched
The stiffer the spring…
The greater the spring constant
What is the limit of proportionality?
When the extension is no longer proportional to force
What type of quantity is distance?
Scalar
What type of quantity is displacement?
Vector
What does displacement measure?
Measures distance and direction in a straight line form object’s starting point to its finishing point
What type of quantity is speed?
Scalar
What type of quantity is velocity?
Vector
What is the difference between speed and velocity?
- Speed is how fast you’re going
- Velocity is speed in a given direction
Can objects can travel at a constant speed with a changing velocity & why?
Yes, object changes direction but remains at same speed e.g. car going around a roundabout
What is acceleration?
The change in velocity in a certain amount of time
Can an object accelerate by changing its direction even if it’s going at constant speed?
Yes
What is deceleration essentially?
Negative acceleration i.e. when something slows down, change in velocity is negative
What is centre of mass?
Point at where the whole mass is concentrated
For a uniform shape, where will gravity act from?
At the centre of the object
What does the position of centre of mass affect?
The stability of objects
How can you finding centre of mass with an irregular shape?
- Hang the irregular shape (e.g. card) from a rod secured onto a clamp stand
- Use a plumbline (string with weight on the end) to draw vertical line on card from rod downwards
- Repeat procedure with card suspended from 2nd & 3rd point to give another similar line
- Centre of mass of card is where lines meet
If a card is at rest, freely suspended from a rod, where will its centre of mass be?
Directly below rod
If the line from the centre of mass is within a base of a titling object, what will happen to the object?
Will not topple
If the line from the centre of mass is outside a base of a titling object, what will happen to the object?
There’ll be a resultant moment and it will topple
How can you make an object (e.g. a stool) more stable?
- Make it wider
2. Make it closer to the ground
How can you increase the momentum of an object?
Increase the mass of object OR its velocity
What is a closed system?
When no external forces can act
What is meant by momentum is conversed?
Closed System: Momentum before event = Momentum after event
In an explosion, what is the momentum before and after the event?
0
Why is the momentum after an explosion 0?
Pieces fly off in different directions = total momentum cancels out to 0
What causes a change in momentum?
Forces
The larger the force… (momentum)
the faster the change in momentum
Why does a car crash cause injury?
Bc person’s momentum changes very quickly = forces on body will be very large + cause injury
Why are cars designed to slow people down over a longer time when they crash?
Longer it takes for a change in momentum = smaller the rate of change of momentum = smaller force = injuries less severe
Name 3 safety features cars have
- Air bags
- Crumple zones
- Seat belts
How do air bags help you in a car crash?
Air bags inflate before you hit the dashboard of car = compressing air inside, slows you down more gradually than if you had just hit the hard dashboard
How do crumple zones help you in a car crash?
Crumple zones crumple on impact = increasing time taken for car to stop
How do seat belts help you in a car crash?
Seat belts stretch slightly = increasing time taken for wearer to stop
How do crash mats and cushioned playground flooring keep you safe if you fall on them?
Increase time taken for you to stop if you fall on them because they’re made from soft, compressible materials
Why is it useful to wear a helmet when cycling?
Bike helmets contain crushable layer of foam = helps lengthen the time taken for your head to stop in a crash = reducing impact on your brain
How do you calculate stopping distance?
Thinking Distance + Braking Distance
What is stopping distance?
Minimum distance that a car can safely stop in
What is thinking distance?
How far the car travels during the driver’s reaction time
What is breaking distance?
Distance taken to stop under the braking force
Name 4 things that affect breaking distance
- Weather/Road Surface
- Speed
- Brakes Condition
- Condition of Tyres
Explain how the weather/road surface can affect the breaking distance
If it’s wet or icy, or there’s leaves or oil on road = there’s less grip (friction) between a vehicle’s tyre and road = means tyres can skid
Explain how speed can affect the breaking distance
For a given brake force: faster a vehicle travels = longer it takes to stop
Explain how the conditions of the brakes can affect the breaking distance
If brakes are worn and faulty, they won’t be able to apply as much force as well-maintained brakes, which is dangerous when you need to brake hard
Explain how the conditions of the tyres can affect the breaking distance
If tyres are bald (don’t have any thread left) then they cannot get rid of water in wet conditions = leads to them skidding on top of the water
Name 2 things that affect thinking distance
- Speed
2. Reaction Time
Explain how speed can affect the thinking distance
The faster you’re going = the further you’ll travel during the time you take to react
Explain how reaction time can affect the thinking distance
Longer reaction time = longer thinking distance
When you brake, why does the temperature of the brakes increase?
- When break pedal is pushed = brake pads are pressed onto the wheels = contact causes friction, which causes work to be done
- Work done between brakes and wheels transfers energy from kinetic energy stores of wheels to thermal energy stores of brakes
- Brakes increase in temperature
Why is a greater braking force needed to make a car stop if it is going fast?
Faster a vehicle is going = more energy it has in its kinetic store = more work needs to be done to stop it
The larger braking force…
the larger the deceleration
Why are very large decelerations dangerous?
Brakes can overheat (don’t work as well) or car could skid
What is a moment?
The turning effect of a force around a fixed point (called a pivot)
How do you increase a moment?
Increase the force OR increase the distance
Total Clockwise Moments….
= Total Anticlockwise Moments (about a pivot)
How do levers make it easier for us to do work?
- Levers increase distance from pivot (where force is applied)
- Since M = Fd, means less force is needed to get same moment
- Means levers make it easier to do work
What are gears?
Gears are circular discs with ‘teeth’ around their edges
How do gears transmit rotational effects?
Their teeth interlock = turning one causes another to turn in opposite direction
What are gears used for?
Used to transmit the rotational effect of a force from one place to another
How can different sized gears can be used to change moment of the force?
Force transmitted to large gear = bigger moment as distance to pivot is greater
Fill in the blanks…
______ gear will turn slower than a ______ gear
Larger gear will turn slower than smaller gear
On a distance-time graph, what does the gradient represent?
Speed
On a distance-time graph, the steeper the gradient…
the faster the object is going
On a distance-time graph, what does a flat section represent?
That an object’s stationary - it’s stopped
On a distance-time graph, what does a straight uphill section represent?
Traveling at steady speed
On a distance-time graph, what does a curve represent?
Acceleration or deceleration
On a distance-time graph, what does a steepening curve represent?
Object’s speeding up (increasing gradient)
On a distance-time graph, what does a levelling off curve represent?
Slowing down (decreasing gradient)
If object’s changing speed (accelerating), how can you can find its speed at that point?
By finding of the tangent to the curve at that point
On a velocity-time graph, what does the gradient represent?
Acceleration
On a velocity-time graph, what does a flat section represent?
Travelling at steady speed
On a velocity-time graph, the steeper the gradient…
Greater the acceleration or deceleration
On a velocity-time graph what do uphill sections (/) represent?
Acceleration
On a velocity-time graph what do downhill sections () represent?
Deceleration
On a velocity-time graph what does a curve represent?
Changing acceleration
How do you the distance on a velocity-time graph?
Area under any section of graph
Would gravity make all objects fall at the same rate if it wasn’t for air resistance?
Yes
What causes things to fall at different speeds?
Air resistance
Frictional force depends…
on object’s shape and area
Objects falling through fluids will…
reach a terminal velocity
When falling objects 1st set off, why do they accelerate?
Bc force of gravity is much more than frictional force slowing them down
(Terminal Velocity) What gradually reduces acceleration and what does this then lead to?
As speed increases, friction builds up until eventually frictional force = accelerating force (resultant force is 0)
When the frictional force = the accelerating force, what is this known as?
Maximum speed or terminal velocity
At terminal velocity, an object will travel at…
a steady speed
What is Newton’s First Law?
A resultant force is needed to make something start moving, speed up or slow down
If the resultant force on a stationary object is 0, it will…
remain stationary
If the resultant force on a moving object is 0, it will…
carry on moving at the same velocity (same speed and direction)
When does velocity change?
If there’s non-zero resultant force acting on object
What will a non-zero resultant force on an object always produce?
An acceleration (or deceleration) in direction of force
Name 5 forms the “acceleration” caused by a non-zero resultant force could be
- Starting
- Stopping
- Changing direction
- Speeding up
- Slowing down
What is Newton’s Second Law? (2 things)
- Acceleration is directly proportional to resultant force
2. Acceleration is inversely proportional to mass of object
The larger the resultant force acting on object…
The more the object accelerates
What is Newton’s Third Law?
When two objects interact, the forces they exert on each other are equal (in magnitude) and opposite
If you push something (e.g. Trolley), what will the trolley do?
The trolley will push back, just as hard
What does friction do?
Slow things down
Friction acts in opposite direction to….
movement
When do you get friction? (name two scenarios)
- Between 2 surfaces
2. When object passes `through fluid (drag)
What is drag?
Resistance you get in a fluid (gas or liquid)
What is air resistance?
Type of drag: frictional force produced by air acting on moving object
What is the best way to reduce drag?
Keep object streamlined (object is designed to allow fluid to flow easily across it)
As drag increases…
speed increases
Why a car’s engine at 70 mph have to work much harder to maintain a steady speed than a 30 mph?
Car has more friction to work against when travelling at 70
mph than 30 mph
