p8 - forces in balance

what is a force

a push or pull that acts on an object because of its interaction with another object

name 4 contact forces

friction

name 3 non-contact forces

gravity

give an example of a force changing shape

squeezing a tube of toothpaste

give an example of a force changing motion

pushing a car

newtons third law

when two objects interact, they exert equal and opposite forces on each other

a boxer punches a bag with 100 n, what force does he feel

100 n in the opposite direction from the bag

why does a car move forward even though friction forces are equal

friction of road on tyre acts on car and causes acceleration, tyre friction on road acts on road, not car

weight definition

the force of gravity on an object due to earth

force of object on earth

equal and opposite to weight of object

compare the force of mud on a car and mud on a tractor to pull the car out

mud on tractor must be greater than mud on car

why are the mud forces on car and tractor not equal and opposite

they act on different objects and are part of separate interactions

friction forces when a car moves forward

friction of road on tyre forward, friction of tyre on road backward, equal and opposite

what type of force causes tyres to push the car forward

friction force of road on tyre

contact force vs non-contact force

contact requires touching, non-contact acts at a distance

tension in a rope

pulling force transmitted along the rope

what happens if you apply too much force on toothpaste

toothpaste comes out too fast due to excessive squeezing

newtons third law example with rope

two roller skaters pull on opposite ends of rope, they move towards each other with equal and opposite forces

application: explain how a tractor pulls a stuck car

tractor pulls rope, rope pulls car, equal and opposite forces on rope and car, net motion occurs if tractor overcomes mud resistance

application: explain why a car accelerates from rest

engine produces driving force, friction of road on tyre pushes car forward, forces on tyre and road are equal and opposite but act on different objects

application: why does a box not move when pushed lightly

applied force is less than friction, forces are unbalanced, no acceleration occurs

symbol equation for weight

w = m x g

define newton

the unit of force

why does squeezing a tube of toothpaste change its shape

force applied deforms the tube material

compare friction and tension

friction resists motion at contact, tension transmits pulling along rope

what is a resultant force

a single force that has the same effect as all the forces acting on an object

what happens if the resultant force is zero

forces are balanced, object stays at rest or moves at same speed and direction

define balanced forces

forces acting on an object with a resultant force of zero

define unbalanced forces

forces acting on an object with a resultant force greater than zero

newtons first law

if forces on an object are balanced, it stays at rest or keeps moving at same speed and direction

if only two forces act on an object and resultant is zero, what must be true

forces are equal in size and opposite in direction

example of balanced forces in real life

glider on air track moves at constant speed because friction is absent

example of balanced forces with friction

pushing a crate at constant speed, push force equals friction, forces are balanced

what happens when a jet plane takes off in terms of forces

thrust > air resistance, resultant force > 0, plane accelerates

what happens when brakes are applied

braking force > engine force, resultant force acts opposite to motion, car slows down

how to calculate resultant force when forces act in opposite directions

difference between the two forces in direction of larger force

how to calculate resultant force when forces act in same direction

sum of the forces

why does a crate move at constant speed when push equals friction

forces are balanced, resultant force is zero, velocity does not change

why does a plane accelerate

forces are unbalanced, resultant force > 0, changes speed in direction of larger force

why does a car slow down when braking

forces are unbalanced, resultant force acts opposite to motion, reduces speed

what is a free-body force diagram

a diagram showing all forces acting on a single object with arrows representing magnitude and direction

why must a free-body diagram show only one object

to isolate forces on that object and calculate resultant accurately

application: a person standing on the floor, what forces act on them

gravity downwards, normal contact force upwards, forces balanced, person stationary

application: a box pulled across floor at constant speed

tension in rope equals friction, forces balanced, box moves at constant speed

application: a rope tug-of-war where one side pulls harder

resultant force towards stronger side, box accelerates in that direction

compare balanced and unbalanced forces

balanced: resultant 0, motion constant

how do arrows in a free-body diagram represent forces

length = magnitude, direction = direction of force

why do unbalanced forces cause acceleration

net force not zero, changes speed or direction according to newtons second law

what is a moment of a force

the turning effect of a force on an object

formula for moment

M = F x d

what does d represent in m = f x d

perpendicular distance from the line of action of the force to the pivot

how can you increase the moment of a force

increase the size of the force (f) or increase the perpendicular distance (d)

why is a long-handled spanner easier to use than a short one

longer handle increases d, so less force needed for same moment

what is a lever

a rigid bar that can turn around a pivot to exert a force

define pivot

the point about which a lever turns

define load

the weight of the object being lifted or moved

define effort

the force applied to the lever

why can a lever lift a heavy object with less effort

lever acts as a force multiplier, moment from small effort produces larger turning effect on load

compare load and effort in a force multiplier

effort is smaller than load, but turning effect (moment) is sufficient to lift load

what is the line of action of a force

the line along which the force acts

application: loosening a tight wheel nut

apply force to spanner, longer handle increases moment, nut turns with less effort

application: using a crowbar to lift a heavy object

effort applied at end of crowbar, pivot at fulcrum, load lifted with small force

why does increasing f increase the moment

moment = f x d, larger f multiplies d, turning effect increases

why does increasing d increase the moment

moment = f x d, larger d multiplies f, turning effect increases

why can’t you undo a tight nut with your fingers

fingers apply too small a force, moment insufficient to turn nut

compare short-handled and long-handled spanners

long handle: larger d, less effort needed

how can a lever be used to increase a force

by applying the effort further from the pivot than the load, increasing the turning effect

why does a bottle opener act as a force multiplier

force applied further from pivot than edge of cap, moment increases, cap experiences larger force

why do scissors cut string easily

force applied at handles further from pivot than cutting edge, turning effect increases, string is cut with small effort

define force multiplier

a device that increases the effect of a force so a small effort produces a larger force on the load

compare pivot location and force in a force multiplier

pivot is nearer to load than to applied force, small effort produces larger load force

what is the line of action of the applied force in a lever

the direction along which the force acts, perpendicular distance to pivot affects moment

how do gears multiply turning effect

small gear turns larger gear, force acts further from output shaft, moment of output shaft increases

what is the effect of low gear on a car

small gear drives large gear, output shaft turns slower but with higher turning effect, high moment, low speed

what is the effect of high gear on a car

large gear drives small gear, output shaft turns faster but with lower turning effect, low moment, high speed

compare low gear and high gear

low gear: high turning effect, low speed

why do the edges of gears exert equal and opposite forces

forces act at contact point tangentially, newtons third law, gears push back on each other

application: explain using a bottle opener to remove cap

apply small effort far from pivot, pivot near cap edge, moment multiplies force on cap, cap pops off

application: using scissors to cut string

apply small force at handles, pivot near cutting edge, turning effect multiplies force on string, string cuts easily

application: explain low gear advantage

small gear drives large gear, output shaft has greater turning effect, useful for climbing or heavy load

application: explain high gear advantage

large gear drives small gear, output shaft turns faster, lower turning effect, useful for high speed on flat road

why does a small gear driving a larger gear increase turning effect

moment = force x distance from shaft, larger gear radius increases distance, moment increases

why does a large gear driving a small gear reduce turning effect

force acts closer to shaft of small gear, distance smaller, moment reduced

compare turning effect in levers and gears

both multiply force by increasing distance from pivot/shaft, turning effect depends on effort distance and load distance

if an object at rest does not turn what does that mean for its sum of anticlockwise and clockwise moments

the sum of clockwise and anticlockwise moments are the same

in the parallelogram of forces what is the resultant force

the diagonal of the parallelogram

what does it mean to resolve a force

finding perpendicular components that have a resultant force that is equal to the force

for an object at equilibrium what is the resultant force

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