A.2 Forces and Momentum Flashcards
Resultant force
Sum of all the forces acting on an object
Resolving forces
A single force can be resolved (broken down) into two components at 90* to each other
Newton’s first law
An object continues in uniform motion in a straight line or at rest unless a resultant external forces acts
Newton’s second law
The resultant force on an object is proportional to the acceleration providing the mass of the object remains constant
Newton’s third law
For every action on an object there is an equal but opposite reaction on another object
Two types of forces
Contact and field
Contact forces
Requires objects to be in contact with each other
Contact forces example
Normal, friction, tension, elastic
Field forces example
Gravitational, electric, magnetic
Field force
Force that can act at a distance
Normal force
When an object pushes on a surface, surface pushes back on the object
Objects on an incline - normal force is calculated how?
Fn = mg + Fn=mgcos0 on an incline
Components of the weight force are split up how in an object on the incline
Going down the slope is mgsin0, acting on an angle is mgcos0, weight is mg
Surface friction
Force that opposes motion on two solid objects
Static friction
When a force is applied to an object but the force is not large enough to move the object, static friction acts
What happens when static friction reaches a maximum value?
It becomes dynamic friction and the object starts moving
What is the magnitude of static friction equal to?
The magnitude of the applied force
What is the coefficient of friction value?
Ratio of two forces (Ff and Fn)
How to calculate coefficient of static friction?
tan0, when the block is about to move
What happens when the force applied to the object exceeds the maximum static friction force?
It becomes dynamic friction and the object is moving
Which is bigger, static or dynamic friction?
Static friction
What is the maximum value for a coefficient of friction?
Usually 1.0
Viscous drag force
The force acting on a moving object due to the viscosity of the fluid which it is moving through
What does the size of the drag force depend on? (6 factors)
Shape, size, surface and cross section of object, viscosity of fluid, speed of object
Why can’t Stoke’s law be used on big objects?
Turbulent flow (laminar) occurs
What does an object need to be to calculate drag force?
A small smooth sphere
Buoyancy
Ability of a fluid to provide a vertical upward force on an object placed on or in it
What is the size of the buoyancy force equal to?
The weight of the fluid displaced
An object will sink until when
When it has displaced its own weight of fluid
If an object is floating, which two forces are balanced
Weight force and buoyancy force
When an object is falling through a fluid, three vertical forces act on it - which three
Weight down
Buoyancy upwards
Viscous drag upward
What are the two constant forces on an object falling through a fluid?
Weight force downward, buoyancy upwards
What is the changing force on an object falling through a fluid?
The viscous drag force will increase as the speed increases
What are the forces on an object falling through a fluid at terminal velocity?
Fb+Fd = Fg
Tension force
A pulling force, may cause on object to stretch
Magnitude of the restoring force is equal to what?
The extension of length
Elastic restoring force (Fh)
The object will exert a restoring force to return the object to the equilibrium position
Spring constant
K, value that represents the stiffness of the object. Larger the force, stiffer the object. (Nm-1)
What direction does a restoring force act in?
Opposite direction to the extension
Object behaving elastically
Returns to original position after being stretched
Maximum extension
After it is stretched past this point, it will be permanently deformed. Elastic limit
Force fields
Region of space where an object may experience a force due to a property such as mass or charge
Gravitational force field
A region of space where an object experiences a force due to its mass
g
Gravitational field strength, 9.8Nkg-1 at earth’s surface
What can gravitational force also be referred to when on earth?
Weight force
Free body diagram
A diagram that shows all the forces acting on an object
Balanced forces
Object remains at rest or continues at constant speed in a straight line
Unbalanced forces
Forces in opposite direction are unbalanced, accelerates in direction of greater force
How heavy we feel depends on what force?
Normal force
If an object is in equilibrium, the sum of all forces…
Will add to 0 so resultant forces are 0
p
Linear momentum, Ns
Linear momentum (p)
The product of mass and velocity of an object. It is a vector so has both magnitude and direction (Ns)
What happens when an impulse is applied to an object with constant linear momentum?
The change in p will be equal to the impulse.
j
Impulse, Ns
Why can’t Newton’s 2nd law be used sometimes?
It assumes mass is constant, if mass is changing use change in p/change in t
How may an impulse be found without a formula?
Area under a force time graph
Law of conservation of momentum
The total linear momentum of a system remains constant provided no resultant external force acts on the system
What does the law of conservation of momentum allow us to determine?
The outcome of collisions and explosions
What happens to total energy in collisions?
It is conserved
Elastic collision
No kinetic energy is lost (occurs between subatomic particles)
Inelastic collision
Kinetic energy is lost (converted to heat and sound)
What happens to kinetic energy in an explosion and why?
It increases and comes from the energy source of the explosion ie chem pot or elastic pot
What is angular velocity?
Angular displacement (angle) over time taken
Angular velocity units and symbol
Symbol w, units rads-1
Centripetal acceleration
Acceleration towards the centre of a circle
Centripetal force
Unbalanced force that acts to create circular motion
How to break down a resolving force?
Vertical is Fsin0 and horizontal is Fcos0
Translational equilibrium
When a resultant force on an object is zero and it is not accelerating
What is the magnitude of static friction equal to?
Magnitude of the applied force
Density
Mass/volume
Angular velocity formula
2 π f
How can an object be moving at a constant speed in a circle but still be accelerating?
Acceleration is rate of change of velocity, direction of motion is constantly moving so velocity is constantly changing.
Centripetal force formulas x2
F=mv^2/r and F=mw^2r
What are the balances of the forces in horizontal circular motion?
Vertical forces are balanced, there is an unbalanced horizontal force to the centre of the circle
Where does centripetal force come from in horizontal circular motion?
Friction between tyres and road
Horizontal circular motion - what happens if the car travels at a speed where the friction force is not large enough to provide a centripetal force?
Car continues in a straight line at a tangent to the bend
Where does centripetal force come from in banked corners?
The horizontal component of the normal force
What forces can the normal force be resolved into on banked corners?
Vertical component - balances weight, horizontal component - provides centripetal force
Two key differences between vertical and horizontal motion
- Vertical motion speed is not constant
- Vertical motion - size of normal force varies throughout the motion
Forces acting at the top of a loop (vertical circular motion)
Fg+Fn=Fc
Formula for minimum speed
v= √rg
What would you feel if normal force was 0?
Weightless sensation
Forces acting on the bottom of a loop (vertical circular motion)
Fn up, Fg down. Fc=Fn-Fg
Kinetic energy in a vertical circular motion loop
Ek (bottom) must = Ek(top)+Ep(top)
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Viscosity
Resistance of a fluid to movement through it (n, Pas)
Density symbol and unit
p, kgm^-3
