Forces Flashcards
What is a force?
A force is a push or pull
What is the scalar force? give 3 examples
A scalar force has magnitude only. For example:
- speed
- distance
- time
What is a vector force? give 3 examples
A vector force has magnitude and direction
For example:
- velocity
- displacement
- acceleration
How do you show forces?
Using arrows on a diagram
What is tension? how is it represented?
Tension is always a pulling force and never a pushing force it is represented by a ’T’
What is friction? How is it represented?
Friction is the force opposing the motion, it is always parallel to the contact surface. It is represented by ‘Ff’
What is normal reaction force? how was it represented?
The force is perpendicular to the contact surface. Represented by ‘R’
What is weight?
The force acting downwards on an object
What diagram do you use to represent arrows?
A free-body diagram
What assumption can we always make with forces?
There is no friction unless told otherwise in the question
Where should the weight arrow be drawn?
Underneath the object, from its centre
A rubber duck is floating on water. What forces are present and what can be said about them?
The forces present:
- Upthrust
- Weight
They are equal forces so the duck stays afloat
What is Newton’s first law?
A body in motion stays in motion at constant velocity and a body at rest stays at rest unless acted on by a net external force
What is Newton’s first law commonly referred to as?
The law of inertia
What would the free-body diagram look like for a girl hanging motionless on a rope?
Tension in the up direction and the weight in the down direction at equal lengths
What would the free-body diagram look like for a car parked on a sloped street?
- Normal force perpendicular to the -contact surface
- weight downwards
- friction going up the slope
What would the free-body diagram look like for a car moving to the right and slowing down?
- Normal force down
- Weight up
- Friction to the left
What would the free-body diagram look like for a ball rising in a parabolic trajectory, do not neglect friction and air resistance
- weight down
- air resistance opposing the ball’s direction
What is a scale vector diagram?
When the lines of a free body diagram are joined together from tip to tail
What does it mean if the lines of a free body diagram match up to form a closed polygon?
That means the object is in equilibrium and has a resultant force of 0
How can a force be split?
vertically and horizontally
How can you find the component of the force in the direction required?
Fcostheta
e.g. 8cos20
what is newtons second law?
A body accelerates when acted on by an external force. the rate of change of momentum is proportional to the resultant force (Fnet=ma)
what are the 5 steps to solve 2nd law problems
- draw free body diagrams
- set up 2nd law equations in each dimension (x and y direction)
- identify numerical given data in each direction
- plug in numbers
- solve
in a grocery store, you push a 14.5kg cart with a force of 12N. If the cart starts at rest, how far does it move in 3 seconds?
F/m = 24/29 ms^-2 = a
s = ut+1/2at^2
s = 3.7m
A 5kg salmon is hanging from a fish scale in an elevator. What is the salmons apparent weight when the elevator is:
a) at rest
b) moving upward and slowing at 3.2 ms-2
c) moving downward and speeding up at 3.2 ms-2
d) moving upward and speeding up at 3.2 ms-2
a) W = mg = 5 x 9.8 = 49N
b) -3.2 x 5 = -16N (There is a force downward slowing it down, therefore Weight is greater than the upward force) -> 49-16 = 33N
c) 49N + 16N = 65N
d) 49N +16N = 65N
What is friction?
The force that opposes motion
What happens to the force of friction on a block as the force pushing it increases?
As the force pushing it increases, so will the frictional force to stop it moving. Once the applied force exceeds the maximum frictional force, the block will move. While the block moves the force of friction will remain constant at its maximum value
What is the formula for frictional force
Ff = uR
Ff = frictional force
u = coefficient of friction
R = normal reaction force
why is static friction always larger the dynamic friction?
more forces apply on the body to keep it stationary than the kinetic forces that make it accelerate
Why does weight increase when an elevator goes up?
If you stand on a scale in an elevator accelerating upward, you feel heavier because the elevator’s floor presses harder on your feet, and the scale will show a higher reading than when the elevator is at rest. On the other hand, when the elevator accelerates downward, you feel lighter.
A person with a weight of 600 N stands on a
scale in an elevator.
What is the acceleration of the elevator when the
scale reads 900N?
1) 1.5 m/s² upwards
2) 5.0 m/s² upwards
3) 5.0 m/s² downwards
4) 1.5 m/s² downwards
Answer: C
900 N = mass x 9.8 m/s²
mass = 900 N / 9.8 m/s² = 91.84 kg
The net force in this case is the difference between the reading on the scale (900 N) and the person’s weight (600 N), which is 300 N. Therefore, using the equation:
acceleration = 300 N / mass = 3.25 m/s².
Since the scale reading is greater than the person’s weight, the elevator must be accelerating upwards, so the answer is
C) 1.5 m/s² upwards.
What is Newtons 3rd law
If body A exerts a Force F on body B, hen body B exerts and equal but opposite force on body A
What are the 5 rules for newtons 3rd law pairs
- The same type of force
- Act along the same line
- Act on different objects
- Opposite in direction
- Equal in magnitude
How would you describe the 3rd law pair of an aircraft hovering in Earth orbit
The gravitational force of the aircraft on the earth is equal and opposite to the gravitational force of the Earth on the aircraft
Which of the following is a correct statement of Newton’s second law of motion?:
A. A force acting on a body is proportional to the mass of the body
B. The rate of change of momentum on a body is equal to the net external force acting on the body
C. The momentum of a body is proportional to the net external force acting on the body
D. A force acting on a body is proportional to the acceleration of the body
B
What does a large coefficient of friction versus a small one mean?
A large coefficient = sticky surface
A small coefficient = slippery surface
Describe what happens when a skydiver jumps out of a plane
Immediately on leaving the aircraft, the skydiver accelerates downwards due to the force of gravity. There is no air resistance acting in the upwards direction, and there is a resultant force acting downwards. The skydiver accelerates towards the ground. The force of air resistance slowly increases until it is equal to the force of gravity. Fnet = 0 (terminal velocity). Once the parachute is deployed, the force of air resistance increases, and there is an upwards resultant force on the skydiver. The force of air resistance slowly matches the force of gravity again creating a new but slower terminal velocity that makes it safer for the skydiver to land on the ground.
What does the graph for a skydiver jumping out of a plane look like
a curved line plateauing to show the skydiver slowing down. A sharp curved line downwards to a new terminal velocity
2 identical balls are dropped from a tall building one a few seconds after the other. Air resistance is not negligible. As the balls fall, the distance between the balls will:
A. decrease
B. increase
C. increase and then remain constant
D. remain constant
C
What is inertia
The tendency to do nothing or remain unchanged
What is momentum
a vector quantity where the product of mass and velocity
What does elastic and inelastic momentum mean
elastic = kinetic energy is conserved
(hit and separate)
inelastic = kinetic energy is not conserved (hit and stick)
What is the formula for impulse
I = change in momentum = F x t
A car of mass 2000kg, initially at rest, takes 5000N to move 100m. What is its final velocity at the end of those 100m?
Wd = F x d
5000 x 100 = 500,000J
500000 = 1/2 mv2
v = 22.36ms-1
How can we decrease the force acting on an object?
Increase the time to decrease the force
What is a period
The time interval for one complete rotation
What if frequency
the number of complete oscillations in one second
How many radians are there in one full revolution
2pi
how do you change from radians to degrees
rads = pi/180 x degrees
how do you change from degrees to radians
degrees = 180/pi x rads
What is the formula for angular velocity
w (omega) = 2pi/T
At any given point, an object with circular motion will also have an instantaneous linear velocity. how will this look like to the curve
it will be tangent to the curve
what are the 2 formulas for linear velocity
v = 2pir/T
v = wr
what is centripetal acceleration
the rate of change of velocity and its direction
what is the formula for centripetal acceleration
a = v^2/r
which way will centripetal acceleration always point
the centre
What is the formula for centripetal force
Fc = mv^2/r
True or False:
the net force is always the centripetal force
TRUE
what is meant by tangential velocity
linear velocity
if you swing a ball on a string above your head, and the string breaks, what happens
the ball travels in a straight line, tangent to the circle
if i leave a cup of coffee on the top of a car and then take a sharp right turn, which way will the coffee move
move in a straight line, unless acted on by an outside force
if you swing a ball in a vertical circle where is it most liekly to break and why
At the bottom
At the bottom, Tension is greater than weight. T-W = mv^2/r
T = mv^2/r +W
the greatest tension is at the bottom of the circle, and the greater the tension the more likely the string will break
why is the normal reaction force greater at the bottom of a roller coaster loop
The normal force is large at the bottom of the loop because for the net force to be directed inward, the normal force must be greater than the outward gravity force.
what would the free body diagram for the gravitron
a frictional force going up
same magnitude of weight acting down
normal reaction force = Fc = to the centre
What is providing the centripetal force as a car drives around a bend
friction!
if a car is driving around an elevated curve, what two forces make up the centripetal force
the x-component of the normal force
friction
What is Hooke’s Law?
law of elasticity that relates the size of the deformation of an object to the deforming force or load.
what is the formula that describes hookes law
F = -kx
what does hooke’s law look like on a graph
a straight line through the origin. The graph will curve once the spring reaches its limit of proportionality
What relationship does hookes law state
Force is proportional to extension
What is a fluid
a liquid or a gas i.e. water, oil, air
what is density? the formula?
the measure of the mass per unit volume of a substance
density = mass/volume
When an object is submerged in a fluid, what is the upward force that a fluid exerts on a body floating in it
upthrust
what is archimedes principle
the upthrust on an object in a fluid is equal to the weight of the fluid displaced.
what is the volume of liquid displaced equal to (in terms of the object)
the volume of liquid displaced is equal to the volume of the object itself
float or sink?:
when an object is less dense than water
float
whne floating and object will sing until it has…
displaced an equal weight of water to its own total weight
what will happen is weight is removed from an object submerged in a fluid
the upthrust will now be greater than teh weight and will accelerate upwards, until the upthrust is again equal to its own weight.
what are the 2 factors affecting upthrust
- density of liquid
- volume of object immersed
what is the formula for upthrust
upthrust = Vpg
Volume of ball x density of fluid x g
what is the density of water
1000 kg/m3
what are the 2 types of flow
laminar and turbulent flow
what is viscosity
a fluid’s resistance to flow (think of it like friction but for fluids like honey or water)
what does a high and low viscosity mean
high = thick like syrup
low = thin like water
what is the coefficient of viscocity
n (eta)
units = Pa s
as the temperature of a liquid increases, what happens to the viscosity
as temp increases, viscosity decreases
what is stokes law and formula
an expression describing the resisting force on a particle moving through a viscous fluid
formula: Fdrag = 6 pi nrv
what does weight equal in a fluid at terminal velocity
W = upthrust + viscous drag
write the formula for terminal velocity
v(term) = 2r^2g(ps-pf)/9n
