physics unit 2: dynamics (explaining motion)
DESCRIPTION
PHYSICS UNIT 2: DYNAMICS (Explaining Motion). FORCES. Force : a "push" or a "pull“ unit: Newtons, N (1 N is about ¼ lb) vector - includes direction contact forces and field forces (act over a distance) net force : total effect of all forces acting on an object. FORCES. Typical Forces - PowerPoint PPT PresentationTRANSCRIPT
PHYSICS UNIT 2: DYNAMICS(Explaining Motion)
FORCES Force: a "push" or a "pull“
unit: Newtons, N (1 N is about ¼ lb) vector - includes direction contact forces and field forces (act
over a distance) net force: total effect of all forces
acting on an object
FORCES Typical Forces
gravity, FG: object’s weight, always directed toward center of earth (FG=mg mass × acceleration due to gravity)
normal force, FN: supporting force a surface exerts on an object, always directed upward perpendicular to the surface
tension, FT: force transmitted by a rope or chain, directed along the rope, constant throughout the rope
FORCES Free body diagrams: show just one object &
the forces acting on the object (NOT forces the object is exerting on other things) example: car hitting a wall
Examples Apple on a table Rock under
water Block on a hill Water skier Child pulled
forward at an angle on a sled
NEWTON’S LAWS OF MOTION
The Law of Inertia (1st Law): an object’s velocity stays constant unless acted upon by a net external force inertia: resistance to change in
motion (mass is a measure of inertia, more mass = more inertia)
Example of Newton’s 1st Law
NEWTON’S 2nd LAW OF MOTION
The Law of Acceleration (2nd Law): a net force causes an acceleration proportional to the force, in the same direction, and inversely proportional to mass. Fnet = ma
Fnet: sum of all forces or net force (N), m: mass (kg), a: acceleration (m/s2) 1 N = 1 kg·m/s2
Second The greater the force, the greater the
acceleration The greater the mass, the greater the
force needed for the same acceleration Calculated by: F = ma (F = force, m = mass, a =
acceleration)
NEWTON’S 2nd LAW OF MOTION
NEWTON’S 3rd LAW OF MOTION
The Law of Interaction (3rd Law): for every action force from one object on another, there is an equal magnitude, opposite direction reaction force from the 2nd object back on the 1staction:
hammer hits anvilreaction: anvil hits hammer
NEWTON’S 3rd LAW OF MOTION
Law of Interaction (3rd Law) action & reaction forces do not
balance each other - they are on different bodies (ex: car pulling a trailer)
equal force does not mean equal acceleration - depends on mass (ex: person jumping off the ground)
Examples of Newton’s 3rd law
FORCES Finding the Net Force (total of all forces on an
object) draw a free body diagram identify & label x & y axes separate forces into x and y parts – Fx=Fcos
Fy=Fsin add all x forces, add all y forces equilibrium: no net force – x forces add up to
zero, y forces add up to zero
Example
LAB 2.3 – Elevator Scene 1
LAB 2.3 – Elevator Scene 2
LAB 2.3 – Elevator Scene 3
LAB 2.3 – Elevator Frame 1
LAB 2.3 – Elevator Frame 2
LAB 2.3 – Elevator Frame 3
LAB 2.3 – Elevator Frame 4
LAB 2.3 – Elevator Frame 5
LAB 2.3 – Elevator Frame 6
LAB 2.3 – Elevator Frame 7
QUIZ 2.1 Joe rolls a ball down a hill. The ball has a
mass of 0.500 kg. The force pulling the ball down the hill is 6.00 N. The hill is 100.0 m long. (a) What is the ball’s acceleration? (b) How fast is the ball going at the bottom of the hill, if it started at rest at the top? (c) If the force on the ball doubled, what would happen to the ball’s acceleration? (d) If instead the mass of the ball doubled, what would happen to its acceleration?
12.0 m/s2
49.0 m/s
doubles (24 m/s2)
halves (6 m/s2)
PHYSICS
UNIT 2: DYNAMICS(Explaining Motion)
NEWTON’S LAWS OF MOTION
Law of Inertia (1st Law) objects slow & stop, or require
continued force to keep moving, due to friction
FRICTION Friction Force, Ff:
resistance to motion between objects in contact with each other acts parallel to contact
surface, opposite to motion
caused by uneven surfaces, molecular attraction
FRICTION
static friction: resistance to starting motion (at rest) beneficial (walking, building, eating, wheels rolling)
kinetic friction: resistance to continued motion (sliding) undesirable (machines, moving furniture, wheels
skidding)
kinetic friction < static friction
FRICTION coefficient of friction,: constant
that depends on type of surfaces in contact s: coefficient of static friction k: coefficient of kinetic friction Ff = FN (friction force = ×
normal force)
FRICTION
Ff
FRICTION on horizontal surface:
mg
FN
FN = mg
(normal force = body weight) so Ff = mg
FRICTION on tilted surface:
mgmgcos
FN Ff
FN = mgcos so f = mgcos
PHYSICS
UNIT 2: DYNAMICS(Explaining Motion)
QUIZ 2.2 A 1200 kg car sits on a horizontal
road. (a) How much force does Joe need to push the car at a constant speed if the coefficient of kinetic friction is 0.600? (b) How much will the car accelerate if Joe uses a force of 10,000 N?a) 7060 N
b) 2.45 m/s2
PHYSICS
UNIT 2: DYNAMICS(Explaining Motion)
PROJECTILE MOTION Projectile motion: parabolic
trajectory (path) Two dimensions of motion: horizontal
(x), vertical (y)vy
vx
v
vx = vcos
vy = vsin
if a bullet was fired horizontally, andanother bullet was dropped from thesame height at the same time, whichwould hit the ground first?
PROJECTILE MOTION Vertical
Motion
constant vertical acceleration due to gravity(2nd Law)
PROJECTILE MOTION A monkey hangs from a
tree branch. A hunter aims his tranquilizer gun barrel straight at the monkey. When the hunter fires his gun, should the monkey keep holding on to the branch, or let go?
PROJECTILE MOTION Vertical Motion
position: y = h + visinit – ½gt2
a. for ground launch, h=0, y = visinit – ½gt2
b. for horizontal cliff launch, 0=0, y = h – ½gt2
speed: vy = visini – gt flight time, T: t when y=0
ground: cliff:gsinv2
T ii
gh2
T
A tank moving at constant speed fires ashell straight up into the air. Where willthe shell come back down?
PROJECTILE MOTION Horizont
al Motionconstant horizontal speeddue to no horizontal force(1st Law)
PROJECTILE MOTION A snowmobile fires a
flare, then slows down. Where does the flare land? If the snowmobile speeds up instead, where does the flare land?
PROJECTILE MOTION Horizontal Motion
position: x = vicosit
for horizontal cliff launch, i=0, x = vit
speed: vx = vicosi
range, R: x when t = T ground: cliff:
g)2sin(v
R i2i
gh2
vR i
PROJECTILE MOTION Example: A projectile is launched
from ground level with a velocity of 50 m/s at an angle of 30 degrees. What is its position and velocity 2 seconds later? What is its flight time? What is its range?
PHYSICS
UNIT 2: DYNAMICS(Explaining Motion)
A plane moving at constant speeddrops a flare. Describe the path ofthe flare.
RELATIVE MOTION Referenc
e Frames:
projectile motion in one reference frame can be vertical free fall in another reference frame (and vice versa)
PHYSICS
UNIT 2: DYNAMICS(Explaining Motion)
QUIZ 2.3Circle your answers! Watch sig. fig's & units!1. Joe throws a ball from ground level at an angle
of 41º and a speed of 19 m/s. (a) Find the ball's vertical position after 1.5 seconds. (b) Find the ball's horizontal speed after 1.5 seconds.
2. Jane throws a ball off a 95-m tall building horizontally at 19 m/s. (a) Find the ball's flight time. (b) Find the ball's range.
y = h + visinit – ½gt2 vy = visini – gt
x = vicosit vx = vicosi
7.67 m14.3 m/s
4.40 s 83.6 m
PHYSICS
UNIT 2: DYNAMICS(Explaining Motion)
UNIT 2 REVIEW Newton's Laws (Memorize!):
1st Law: velocity stays constant unless acted upon by a net force
2nd Law: net force = mass x acceleration
3rd Law: for every action force, there is an equal and opposite reaction force
UNIT 2 REVIEW
F = ma FG = mg
Ff = FN
vf = vi + at
x= vit + ½at2
vf2=vi
2 + 2ax
y = h + visinit – ½gt2
x = vicosit
vy = visini – gt
vx = vicosi
g)2sin(v
R i2i
gh2
vR i
gsinv2
T ii
gh2
T