chapter 4 - forces in action Flashcards
uniform
weight is the same at all parts if it’s dimensions are the same
free body diagrams
each force is labelled as a vector arrow scaled to the magnitude and direction of the force they represent
newtons 1st law
all objects will continue to be stationary or move at constant velocity unless they are acted on by a resultant force
newtons 2nd law
rate of change or momentum is directly proportional to the resultant force and occurs in the direction of resultant force
p = mv
F = ma
W = mg
newtons 3rd law
if a body exerts force in an object the object exerts and = and opposite force on the body
mass
measure of its resistance to acceleration or deceleration
weight
the gravitational force which acts in the object
g varies due to
- non uniformities in the shape and composition of the planet
- the effect of earths rotation about its axis
drag
opposes motion
size depends on
- speed of object
- shape of object
- viscosity of fluid
motion of an object through a fluid
- released from rest - only force is W
- speed increases so drag increases
- F = W - drag and it decreases so acceleration decreases
- drag = W so F = 0 and acceleration = 0 (terminal velocity)
object falling in a vacuum
- acceleration = g - no drag
- straight line
max speed of a vehicle
depends on
- max forward force of the engine
- how quickly drag increases to = engine force (shape of vehicle)
- F = engine force - drag force
triangle of forces
if an object is in equilibrium under three forces they can form a closed triangle
centre of gravity
point at which weight acts
finding CofG of irregular objects
- suspend the object from a pin through a hole at the edge of it
- hook a plumb line from the pin and mark the line
- change the orientation and mark the line again (repeat 3 times)
- the CofG is the where they intercept
moment of a force
turning effect produced by a force
force * perpendicular distance from the force
principle of moments
- if an object is in equilibrium the sum of clockwise moments = sum of anti-clockwise moments
couple
- consists of 2 = and oppositely directed parallel, coplanar forces, who’s lines of action dont intersect
- produce a rotation
- can’t produce a resultant force so can’t produce linear motion
torque due to a couple
moment of a couple
torque = force * perpendicular distance between the forces
conditions for equilibrium
- no changes to motion - stationary or constant v
- resultant force = 0 so linear acceleration is 0
- resultant torque must be zero - so angular acceleration = 0
density
mass per unit volume
m/ v
- depends in mass of each atom and the way they are packed together
pressure
normal force per unit area on a surface
F / A
unit is Pa = N/m^2
why does a box move down a slope
- slope increases
- angle between W and direction of motion decreases
- component of W acting down the slope increases and becomes bigger than friction so box starts to move
why do heavier objects have a larger terminal velocity
- have a larger mass but same size - so larger W (W= mg)
- will take longer for drag to increase enough (increases with speed) to = W
- will accelerate for longer and reach a higher terminal velocity later
man standing in a lift equation
M(a+ g) = F
density practical
regular
- measure dimensions with a ruler
- measure mass with balance
liquid
- same but convert mL> cm^3
irregular
- eureka can displacement
- water displaced = volume
pressure in liquids
pgh
pressure of a fish = pressure due to water + atmospheric pressure = pgh + 101kPa
atmospheric pressure
101kPa
1.01*10^5 Pa
archenemies principle
the upthrust exerted on an object is = to the W of fluid displaced
upthrust
= weight of th efluid the body displaces
= Axpg
= Vpg
= mg
= to the force when suspended in air - the force when submerged
fraction of a block submerged
amount submerged/ total = density of object / density of water
density of water
1* 10^3 kgm^-3
forces acting on a ball moving in the water
up - upthrust
down - drag and weight
if an object is stationary in water
U = W
p(W)Vg = p(B)Vg
therefore have the same density
if an object is resting on the surface of a liquid (forces)
upthrust and weight are =
average mass of an adult
50- 150 kg
one newton
the force that will give a mass of 1kg an acceleration of 1m/s^2
drag
opposes motion of an object in a fluid
drag is prob to
speed squared
terminal velocity experiment
