Forces, Movement, Shape and Momentum (1c)

Force

Push or pull that arises from the interaction between objects

Different Types of Forces

• gravitational

• reaction

• friction

• drag

• air resistance

• thrust

• upthrust

• electrostatic

• magnetic

• tension

Gravitational Force

• gravitational force of attraction between all objects with mass

• more mass = bigger gravitational force

• ex: football is kicked → gravitational force pulls it toward the earth’s center

Reaction Force

• when an object rests on a surface, surface exerts a push on the object

• acts perpendicular to surface

• ex: football rests on the grass’ horizontal surface → grass exerts a reaction force vertically upwards on the ball

Friction

• friction opposes motion of an object, slows it down

• occurs when 2 surfaces move over each other

• ex: box pushed across carpet → carpet exerts frictional force, slows box’s motion

Drag Force

• type of frictional force occurring when an object moves through a fluid

• particles in fluid collide with object moving through and slow the motion

• ex: pebble thrown into water → water molecules flow against solid surface, slows it down

Air Resistance

• type of drag, also a frictional force

• occurs when air particles collide with an object moving through it and slow it down

• ex: skydiver opens parachute → air resistance opposes motion, reduces speed to make landing safe

Thrust

• force produced by engine that speeds up motion

• ex: engine of a car exerts thrust and increases speed

Upthrust

• when an object is fully/partially submerged in a fluid, the fluid exerts upward acting push force on the object

• ex: ball held underwater shoots upwards when released due to upthrust exerted by water pushing it back to the surface

Electrostatic Force

• there’s an electrostatic force between two charged objects

• like charges repel, opposite charges attract

• electron gets close to cation → cation pulls (attracts)

• electron gets close to electron → electron pushes (repels)

Magnetic Force

• there is magnetic force between two objects with magnetic poles

• like poles repel, opposite poles attract

• north pole gets close to south pole → pull (attract)

• north pole gets close to north pole → push (repel)

Tension

• tension occurs in a stretched object (rope or spring)

• when pull force exerted on each end, tension acts across the length of the object

• ex: two people pull a rope in opposite directions → tension acts along rope, pulls back on each person

Effect of Force on an Object

force causes objects to:

• change speed

• change direction

• change shape

Effects of Specific Forces

• thrust of an engine speeds up a car

• friction of brakes slows car down

• gravitational force of sun on a comet changes its direction

• when two opposing forces push on each end of a spring, it changes shape (compresses)

Scalar Quantity

have magnitude but not direction

• ex: mass

Vector Quantities

magnitude and direction

• ex: weight

Distance

how far an object has travelled regardless of direction

• total length of path takes

• has magnitude, no direction

• scalar quantity

Displacement

how far it is between two points in space, including direction

• length and direction of a straight line from starting to finishing point

• has magnitude and direction

• vector quantity

Common Scalar Quantities

• distance

• speed

• mass

• energy

• volume

• density

• temperature

• power

Vector

• displacement

• velocity

• weight

• force

• acceleration

• momentum

Using Arrows to Represent Forces

• length: magnitude

• direction: direction

• scale: proportional to relative magnitudes of forces

• labelled with force name/description

• describe angle with respect to vertical or horizontal

Resultant Force

• single force describing all of the forces acting on the body

• when multiple forces act, they can be combined to produce on net force describing the combined action of all the forces

• this force determines

  • direction of object

  • magnitude of net force

Calculating Resultant Force

• opposite directions: subtract

• same direction: add

• force in opposite direction are equal: no resultant force (forces are balanced)

Friction Cause

• when 2 or more surfaces rub against each other

• at a molecular level, both surfaces contain imperfections (not perfectly smooth)

• these imperfections push against each other

Unbalanced Forces

when forces acting on an object don’t cancel out

• forces are unbalanced

• there’s a resultant force

Unbalanced force, mass and acceleration

F = m x a

• F: resultant force in N

• m: mass in kg

• a: acceleration in m/s²

Weight

force experienced by an object with mass when placed in a gravitational field

Weight vs Mass

Mass is a measure of how much matter is in an object

• magnitude, no direction

• scalar quantity

Weight is a force

• magnitude and direction

• vector quantity

Gravitational Field Strength

• plants have strong gravitational field strengths

  • attract nearby masses with strong gravitational force

• different planets have different gravitational field strengths

  • depends on mass

  • more massive planet = stronger gravitational field

Impacts of Weight

• objects stay firmly on the ground

• objects fall firmly to the ground

• satellites kept in orbit

Weight Equation

W = m x g

• W: weight in N

• m: mass in kg

• g: gravitational field strength in N/kg

g on Earth

• 10 N/kg

• also acceleration of freefall on Earth in m/s²

Weight Depends On:

• mass of object

• mass of attracting planet

mass and weight are __________ proportional

directly

___________ of weight depends on gravitational field strength

magnitude

Stopping Distance

total distance travelled by a car during the time it takes to stop in an emergency

Stopping Distance Formula

stopping distance = thinking distance + braking distance

Thinking Distance

distance travelled in the time it takes to react to an emergency and prepare to stop

• factors:

  • speed of car

  • driver’s reaction time

Reaction Time

measure of how much time passes between seeing something and reacting to it

• average: 0.25s

• increased by:

  • tiredness

  • distractions (ex: phone)

  • intoxication (alcohol or drugs)

Braking Distance

distance travelled under braking force in metres

• for given braking force, greater speed = greater stopping distance

Factors affecting Stopping Distance

• vehicle speed

  • greater speed = greater braking distance

• vehicle mass

  • more mass = more distance

• road conditions

  • ice

  • rain

  • brakes are less effective = vehicle travels further

• driver reaction time

  • thinking distance increased if driver is distracted (phone, satnav, radio, person)

  • thinking distance increased if driver is tired, on medication, under influence of alcohol/ drugs

Terminal Velocity

terminal velocity is reached when fastest speed an object can reach while falling

• reached when upward and downward forces are balanced

• resultant force reaches zero

• object no longer accelerates, constant terminal velocity reached

Falling Object Forces

• weight

• air resistance

Change in Air Resistance in a Falling Object

• increases as speed increases

• because object collides with air particles as it moves through air

• faster object = more collisions with air particles

Change in Weight in a Falling Object

no change in weight

• because W = mg

• m doesn’t change

• acceleration of free fall (g) doesn’t change

Skydiver in Freefall Reaching Terminal Velocity

• skydiver steps out of plane → no support force of plane, only force exerted = Weight

  • downward resultant force acting on skydiver

  • resultant force = weight

  • skydiver accelerates downward at max. acceleration

• skydiver starts falling, speed is very small → air resistance is very small

  • downward resultant force

  • resultant force = weight - air resistance

  • skydiver accelerates downward but acceleration decreases

• as skydiver accelerates, speed increases → air resistance increases

  • downward resultant force on skydiver

  • resultant force = weight - air resistance

  • skydiver accelerates downward but acceleration decreases

• as skydiver’s acceleration decreases, speed increases a slower rate

  • eventually skydiver reaches a speed where air resistance = weight

  • forces are balanced, resultant force is zero

  • skydiver no longer accelerates, constant velocity reached → terminal velocity