physics chapter 4

is acceleration proportionalto net force, or does acceleration equal net force

acceleration is proportional to net force, not equal to it
a= Fnet/m

how great is friction on a crate that doesnt slide

equal to the applied push, in the opposite direction

as you increase your push, does friction increase?

yes, until maximum static friction is reached

once the crate is sliding, how hard do you push to keep constant velocity

push with a force equal to sliding friction

which is normally greater: static or sliding friction

static friction

how does sliding friction vary with speed

it is approximately independent of speed

does fluid friction vary with speed

yes. it generally increases as speed increases

which is more fundamental: mass or weight? which varies with location

mass is more fundamental. weight varies with location

shake something to and fro

measuring its mass

lift something again gravity

measuring its weigh

Si unit for mass and force

kilogram and newtons

approximate weight of a quarter pound hamburger

about 1 n

weight of a 1-kg brick

w= mg = 1(10)= 10 N

graudal pull makes the top string break. weight or mass

weight

sharp jerk makes the bottom string break. weight or mass

mass, because of inertia

is acceleration directly or inversely propotional to mass

inversely proportional
more mass —> less acceleration for the same force

state newtons second law

an objects acceleration is directly proportional to net force and inversly proportional to mass, and acceleration is in the direction of the net force
Fnet= ma

does directly proportional mean two quantities are equal

no. it means they change in a constant ratio

net force is tripled. what happens to acceleration

acceleration triples

mass is tripled while force stays constant

acceleration triples

mass is tirpled while force stays constant

acceleration becomes 1/3 as large

mass and net force are both tripled

acceleration stays the same

directino of acceleration compared with net force

same direction

condition for free fall

gravity is the only force acting

force/mass ratio for freely falling bodies

g is equal to 10 N/kg or 10 m/s²

why doesnt a heavy object accelerate more than a light object in free fall

a heavier object has more gravitational force but also proportionally more mass, so the ratio f/m is the same

net force on a 10-n freely falling object

10 n downward

a 10 n object has 4N of air resistance. what is the net force

10-4 = 6N downward

two principal factors affecting air resistance

speed and size/ shape (surface area)

acceleration at terminal velocity

0 m/s²

why does a heavier parachutist fall faster with the same parachute

greater mass gives greater weight, so the heavier parachutist reaches a greater terminal speed

two same size objects fall at different speeds. which has greater air resistance

the faster moving object

weight of a 50-kg person

W= mg 50(10)= 500 N

weight of a 2000- kg elephant

20,000 N

weight of a 2.6 kg melon

25 N= 5.6 lb

mass of a 1-N apple

m= w/g= 1/10 = 0.1 kg

mass of a 300- n person

m= 300/10= 30 kg

2000 kg airplane with 500 n thrust. acceleration?

a= 500/2000= 0.25 m/s²

300,000 kg jet with 120,000 N thrust

120,000/300,000= 0.4 m/s²

40 kg block with 200 n net force

200/40= 5 m/s²

speed increases 6.0 m/s in 1.2s

a= 6/1.2= 5.0 m/s²

15 n force on a 3 kg cart

a= 15/3 = 5 m/s²

1 kg stone in free fall

10 m/s² downward

84 n force on a 12 kg package

a= 84/12= 7 m/s²

1 n equals hoq many pounds

1/4.45= 0.225 lb

500 n preson weighs how many pounds

500/4.45= 112.4 lb

1n on 1 kg vs. 2n on 2 kg

yes. both =1 m/s²

30,000 kg jet with two 30,000 N engines

total force= 60,000 N
a= 60,000/30,000= 2m/s²

100-kg skateboarder going 9 m/s stops in 0.2s

a= -45 m/s²
force= 100(45)= 4500 n

buss mas m accelerates at a . add m/5 mass

new acceleration = 5/6a
a= 1.2 m/s²
—> 1.0 m/s²

boxes on a friction free table: rank net forces and accelerations

the exact ranking depends on the diagram, which isnt preserved in the text extraction

crate in equilbrium: rank friction

friction balances the applied force, so net force = 0

how long does the force from your hand last after tossing a ball

only while the ball is in your hand

bowling ball slows down. is there a horizontal force

yes, friction acts opposite the motion

constant velocity vs. constant acceleration

constant velocity —> net force= 0
constant nonzero acceleration —> net force is not -

does an object have less inertia on the moon

no. ineretia depends on mass

1 pound vs. 1 kilogram bags on earth/moon

a kilogram measures mass; a pound usually measures weight. weight changes with gravity, mass doesnt

distinguish sand anf feathers in boxes in orbit

push both with the same force. the box that accelerates less has greater mass sand

does astronaunt mass change on ISS

no

why is a massive cleaver affective

greater mass —> greater inertia

crush a car into a compact cube

mass and weight stay the same if no material is removed

10 kg object on moon vs earth

earth: 100 N
moon= 16.7 N
mass: 10 kg in both places

scale reading when tossing a heavy obejct upward

the scale reading decreases temporarily

mass increases by 2 kg. weight chagnes

W= 2(10)= 20 N

you own mass/ weight

weight = your mass x10

groccery bag withstands 300 n. maximum apples

300/10 = 30 kg

newtons first law as a consequence of second law

if Fnet = 0 then a =0 so the object stays at rest or moves at constant velocity

car moving backward and breaking. direction of acceleration

forward, opposite velocity

car moving forward while braking

velocity forward; acceleration backward

why dont heavy objects accelerate faster in free fall

their greater gravitational force is matched by their greater mass

lineman pushes upward on opponent. what happens to friction

normal force increases, so friction increases

race car at constant 200 km/h. horizontal net force?

0 N

skydiver at terminal speed vs. satelite above atmospher

skydiver at terminal speed: not pure free fall because air resistance acts
satellitle: approximately free fall

coin tossed upward

velocity decreases upward; acceleration remains 10 m/s² downwardf

force on tossed coin halfway up and at top?

gravity downward at both points

acceleration of rock at top of trajectory

10 m/s² downward

car at rest: do forces act

yes. forces can balance

constant- velocity car: why does engine still need to run

engine force balances friction and air resistance

1- N apple helf vs. released

held: net force= 0
released: 1 N downward

why deos a shooting star burn

high speed interaction with the atmosphere causes heating and glowing

parachutists floating downward at constatn velocity

upward air resistance = downward gravity

raindrop at constant velocit

air resistance = gravity

parachutists after openning chute

increased air resistance causes downward acceleration to decrease and then reverse to slowing downward

terminal speed before vs. after opening parachute

higher before, lower after

gravity vs. air resistance before/ at terminal velocity

before: gravity > air resistance
terminal: gravity= air resistance

cat falling from 50 stories vs. 20th

once terminal speed is reached, it doesnt keep speeding up, so it can reach the same terminal speed

metal sphere in viscous liquid at equilbrium

upward fluid resistance= downward gravity

same size balls with different masses and air resistance

the heavier ball generally hits first because air resistance has less relative effect on it

tennis ball vs. lead filled ball

lead filled ball hits first; air resistance has a much greater relative effect on the tennis ball

ball tossed upward with air resistance. return speed?

less than its inital speed

ball tossed upward with air resistance: rise vs. fall time

fall time is longer

can velocity reverse while acceleration stays constant

yes
example: ball tossed upward

does dynamite contain force

it contains stored chemical energy, which can produce forces when released

can a curved path occur without force

no. a curved path requires acceleration, which requires net force

rock dropped on moon: what forces act?

moons gravitational force acts downward

dieting: loss mass or weight

primarly mass

heavy crate on accelerating truck: what force accelerates it?

friction between create and truck

wagon moving at constant velocity needs a steady force. why?

the pulling force balances friction, so net force remains zero.

its not the fall that hutrs; its the sudden stop. why?

your body has inertia and resists the sudden change in velocity. a rapid change in velocity creates a large acceleration/ deceleration and therfore a large force

tossed ball at three points. force and acceleratoin

gravity and acceleration are downward at all three points. velocity changes direction