P5 - Forces Flashcards
What are vector quantities
Have a magnitude and direction
E.g velocity p, acceleration , force, momentum, displacement
What’s a scalar quantity
Magnitude and no direction
Speed, distance, mass, temperature and time.
What are vectors shown with
An arrow. Length = magnitude and direction = direction.
What’s a contact force
Two objects touching for a force to act. Force, air resistance
What’s a non contact force
Objects don’t need to touch for the force to act
Magnetic, gravitational, electrostatic
What’s an interaction pair
Pair of forces that are equal and opposite and act on two interacting objects e.g chair exerts a force on the ground, ground pushes back with the same force. Equal but opposite and felt by both
What is gravitational force
The force of attraction between masses. Gravity attracts all masses but only noticeable when mass is very big
What effects does gravitational forces have
On surface of planets, it makes all things fall towards the ground
Gives everything a weight
What is mass
The amount of stuff on an object. The same everywhere in the universe
What is weight
Force acting on object due to gravity (pull of gravitational force on object)
Gravitational forces varies with…. and why
Varies with location. The closer you are to the mass causing the field the stronger it is. Larger mass = stronger
Mass and weight are
Directly proportional
Increase mass = increase weight
Double mass = double weight
Weight = mass x gravitational field strength
What are free body diagrams
Diagrams that describe all forces acting on an isolated object or system
What is resultant force
Overall force on a point or system
What can you replace a number of forces acting at a single point with
Replace with a single force (as long as the single force has the same effect as the other forces) - resultant force
How to find overall effect of all forces acting on same lines
Adding those in the same direction and subtracting those in the opposite
What happens if the resultant force moves an object
Work is done
When a force moves an object through a distance, work is done/energy is transferred to the object
To make something move you need…
A force to be applied, and a source of energy(food/fuel)
Force does work to move the object and energy is transferred
One joule of work is done when
One newton causes an object to move a distance of 1 metre
1 joule = 1 newton metre
How to use scale drawings
Draw all forces on an object to scale
Straight line from start of first force to end of last force
This is the resultant force, measure to find magnitude and angle for direction
What is equilibrium
Object is in an equilibrium if the forces on it are balanced
If all forces on an object give resultant force = 0, the object is in equilibrium
On scale diagram, this means the tip of the last force you drew will end where your first force began
Key information on split forces into components
Some forces act at awkward angles
Split into two components at right angles
Acting together, these components have the same effect as the single force
What happens when you stretch, compress or bend something
You apply a force, need more than one force acting (otherwise it would go in the same direction rather than change shape)
What does elastically deformed mean
Object that can go back to its original shape and length after force removed
What’s inelastically deformed
If object doesn’t return to original shape and length after force removed
When is work done with stretching
Work is done when force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store. If it’s elastically deformed, all energy transferred to the EPES
Extension is
Directly proportional to force
Force = spring constant x extension x spring constant
Also works for compression
There’s a limit to amount of force you can
Apply to an object to keep it increasing proportionally
What is displacement
Measures distance and direction of a straight line
What is velocity
Speed at a given direction
Object moving in a circle at a constant speed has a….
Constantly changing velocity
Typical everyday speeds…. running walking cycling car train plane
Walking 1.5 m/s Running 3m/s Cycling 6m/s Car 25m/s Train 30m/s Plane 250m/s Speed of sound in air 330m/s changes depend on what sound waves are travelling through
What is wind speed affected by
Temperature, atmospheric pressure, large buildings near by
What is acceleration
How quickly you’re speeding up. It’d also the change in velocity over a certain amount of time
Acceleration = change in velocity/time
What is uniform acceleration
Constant acceleration. Like acceleration due to gravity is uniform in free fall.
Final velocity^2 -initial velocity^2 = 2acceleration x distance
Key information on distance time graph
Gradient = speed (speed = distance/time) Flat = stationary Straight uphill = steady speed Curves = acceleration or deceleration Find speed at a point by drawing tangent and finding gradient
Key information of velocity time graphs
Gradient = acceleration
Flat = travelling at steady speed
Stepper = greater acceleration or deceleration
Curve = changing acceleration
Area under graph = distance travelled.
Use tangent gradient to find acceleration at point
What will happen if an object has no force propelling it
It will always slow down and stop due to friction
Friction acts in…
The opposite direction to movement
What is friction through a liquid called
Drag
Drag increases as
Speed increases
Examples of drag (resistance in fluid or gas)
Air resistance
How to reduce drag
Object into streamlined shape
Objects falling through fluids reach a
Terminal velocity
What happens when an object falls through air
As it starts, gravity is bigger than friction so the object accelerates. As the speed increases, so does drag and gradually reduces the acceleration until friction and acceleration are balanced. This is a terminal velocity
Terminal velocity depends on what, and why
Depends on shape and area.
Less streamlined = lower terminal velocity
Objects with larger surface area = lower terminal velocity
E.g marble has a higher terminal velocity than a beach ball. More air resistance on beach ball so beach ball spends less time accelerating so it doesn’t speed up as much - this happens as the air resistance is large enough to quickly equal the accelerating force as the accelerating force is low
Newton’s first law
A resultant force is needed to make something start moving, speed up or slow down
If the resultant force of a stationary object is 0, it won’t move.
If the resultant force on a moving object is 0, it’ll travel at the same velocity
Acceleration is proportional to
The resultant force.
Larger resultant force on object, the more the object accelerates.
Acceleration is inversely proportional to
Mass.
Larger mass = less acceleration
Resultant force = mass x acceleration
What is inertia
The tendency for motion to remain unchanged
Until acted upon by a resultant force, objects will…
Stay at rest or move at a constantly velocity if already moving
What does an objects inertial mass measure
How difficult it is to change the velocity of an object
Mass = force/acceleration
What is Newton’s third law
Equal and opposite forces act on interacting objects
When two forces interact, the forces they exert on each other are equal
If you push something like a shopping trolley what will happen
The trolley will push back against you just as hard. If you stop pushing, so does the trolley -> works as the two forces acting on different objects
Newton’s third law is an
Equilibrium
E.g man pushing against a wall. He pushes against it and there’s a normal contact force acting on him. These forces are the same size
What is momentum
How much ‘oomph’ an object has (a property moving objects have)
Greater mass/velocity = greater momentum
Momentum = mass x velocity
Momentum before =
Momentum after
Conservation of momentum
What’s an emergency stop
Maximum force applied by brakes to stop in shorted possible distance.
Longer it takes for an emergency stop = higher risk of crashing
Stopping distance =
Thinking distance + braking distance
What is the thinking distance affected by
Speed - if it’s faster you’ll travel further in time
Reaction time - longer reaction = more distance
What is the braking distance affected by
Speed - going faster = longer to stop
Weather or road surface - wet, icy, leaves = less grip
Tyre condition - bald tyres can’t rid of water = skidding
Quality of brakes - worn or faulty can’t apply as much force
What does braking rely on
Relies on friction between the brakes and wheels
How does braking a car work
Brake pedal pushes, which causes the brake pad to be pressed onto wheels which causes friction which causes work done. Work done transfers energy from the kinetic energy stores of wheels to thermal energy stores of brakes.
Brakes increase in temperature
Faster vehicle = more energy to KES = more work to be done
Larger braking force = larger deceleration
Large deceleration are dangerous as they cause brakes to overheat and skidding
Typical reaction times
Between 0.2 and 0.9 seconds
What are reaction times affected by
Tiredness, drugs, alcohol or distractions
