3.2 Forces in action Flashcards
Net force (N) =
Mass(Kg) x acceleration (ms-2)
Tension
If a string is pulled tight, tension is the force pulling equally on the object at wither end of the string
Normal contact force
if an object exerts a force on a surface, the surface exerts an equal but opposite force on the object. The force acts perp to the surface
Weight=
Mass x Gravity
Frcition
if an object is moving it usually has a friction force acting on it in the opposite direction to motion.
Centre of gravity
The point where the entire weight of an object seems to act
centre of mass
The point where the entire mass of an object seems to be concentrated
How do you find the centre of mass for a regular object?
centre of mass is at the centre where the lines of symmetry cross, halfway through the thickness of the object
How do you find the centre of mass for an irregular object?
- Hang the object from a point freely
- draw a vertical line downwards from the point of suspension (use plumb bob to get exactly vertical)
- Hang object from different point
- Draw another vertical line down
- Centre of mass is where the lines cross
An object will be nice and stable if?
Low centre of mass and wide base area
When does an object topple over?
if a vertical line drawn downwards from its centre of mass falls outside its base area. This is because of the moments about the pivot
What are the 2 main types of friction?
Contact friction between solid surfaces
fluid friction
What factor effect drag for an object travelling through air?
Velocity
density of fluid/ viscosity of fluid
shape and size of object
What are the 3 things to remember about friction?
They act in the opposite direction to the motion of the object
They can never speed things up or start something to move
They convert kinetic energy into heat energy
Describe the 3 main stages of reaching terminal velocity for a car
- The car accelerates from rest using a constant driving force
- As the velocity increases, the resistive forces increases, this reduces the resultant force on the car and hence reduces its acceleration
- Eventually the car reaches a velocity at which the resistive forces are equal to the driving forces. There is now no resultant force and no acceleration, so the car carries on at constant velocity
What’s the graph for terminal velocity like?
Velocity against time graph start of positive steep then levels of. (Like the side of a Christmas pudding)
Acceleration time graph starts up high has negative steep gradient and levels of at the bottom (it has a slight backwards s shape to it)
Describe a parachutist falling?
- Sky diver leaves plane and will accelerate until air resistance = weight
- He will then be travelling at terminal velocity
- They will open the parachute immediately increasing air resistance so it is now n=bigger then than his weight
- This slows him down until his speed has dropped enough for the air resistance to be equal to his weight again. This new terminal velocity is small enough so you don’t die
Describe the velocity time graph for a parachutist
Steep positive gradient slowly levelling off, (Terminal velocity initially) then parachute opens so steep negative gradient and then levels of (New terminal velocity)
How do you measure the terminal velocity of a ball bearing?
- Put elastic bands around a tube of viscous liquid at fixed distances using a ruler
- Drop the ball bearing into the tube, and use a stopwatch to record the time at which it reaches each elastic band. Record results in a table
- Repeat few times to reduce random error effects, Use magnet to remove ball from tube
- Calculate the times taken by the ball bearing to travel between consecutive elastic bands and calculate the average velocity between each pair of elastic bands
- You should find the avg velocity increases at first, then stays constant.
Density=
Mass/volume
A solid object will float on a fluid if?
it has a lower density than the fluid`
Pressure=
Force/Area
Height x density x acceleration due to gravity
Newton
Force required to give 1 Kg mass an acceleration of 1 m/s
Upthrust
upwards force that fluids exert on objects that are completely or partially submerged in the fluid. Caused as the top and bottom of a submerged object are at different depths
Archimedes principle
an object completely or partially submerged in a fluid experiences upthrust = to the weight of the fluid displaced
How do submarines make use of Archimedes principle?
To sink, large tanks are filled with water to increases the weight of the submarine so that it exceeds the upthrust.
To rise to the surface, the tanks are filled with compressed air to reduced the weight so that its less then upthrust.
Moment of a force (Nm)=
Force (N) x Perpendicular distance from pivot (m0
What’s the principle of moments?
For a body to be in equilibrium, the sum of the clockwise moments about a point is = to the sum of the anticlockwise moments about the same point
In a lever a _____force acts against a ____force by means of a ____object rotating around a _____
In a lever a effort force acts against a Load force by means of a Rigid object rotating around a Pivot
A Couple
Pair of forces of equal size that act parallel to each other but in opposite directions.
What does a couple produce?
Torque (a turning force)
Torque of a couple (Nm)=
size of one of the forces (N) x perpendicular distance between the forces (m)
The DVD is dropped from rest. The circular face remains horizontal as it falls. The DVD does
not reach terminal velocity before it hits the ground.
Describe and explain how the acceleration of the DVD varies from the instant it is dropped
until just before it hits the ground
At the start, acceleration = g (because there is no drag)
Drag increases (as its speed increases / accelerates)
net force decreases or net force < weight
(As it falls) acceleration decreases / (As it falls) acceleration < g
why cant f=ma not be applied to particles travelling at speeds close to speed of light?
The mass of particles increases at its speed gets
closer to the speed of light
state 2 conditions that apply when an object is in equilibirum
Net force = 0
Net moment / torque = 0
