topic 2 motion and forces

what does a vector have?

a vector has magnitude and direction

what do scalars have and can they be positive/ negative?

a scalar has just magnitude and tends to be positive

name if these are vector or scalar

• speed

• force

• distance

• momentum

• time

• velocity

• mass

• displacement

• energy

• acceleration

these are:

• scalar

• vector

• scalar

• vector

• scalar

• vector

• scalar

• vector

• scalar

• vector

name some things about distance time graphs

• gradient is velocity

• sharp gradient is fast

• negative gradient means going back to original position

• horizontal line is stationary

• distance = 0 means its at the start point

• area under line = nothing

• curved line means velocity is changing (acceleration)

name some things about velocity time graphs

• gradient is acceleration

• sharper gradient is greater acceleration

• negative gradient is deceleration

• horizontal line = constant speed

• 0 velocity = stationary

• area under line = distance traveled

• curved line means change in acceleration

what is average speed for and how is it calculated?

• this is for when the speed changes during the motion

• worked out using overall distances and timings

describe three methods for calculating speed

1. for constance speeds

- measure distance traveled

- use stopwatch for time taken

- speed = \frac{distance}{time}

2. for average speed

- work out total distance traveled

- find total time taken

- speed = \frac{distance}{time}

3. light gates

- set up one at start and one at end

- measure distance between

- speed = \frac{distance}{time}

• more accurate than stopwatch as no reaction time

what is the acceleration due to gravity?

g = 10ms^{-2}

what is newtons first law?

An object has a constant velocity unless acted on by a resultant force

If a resultant force acts on the object it accelerates

If the resultant force is zero it wont accelerate.

what is newtons second law?

force = mass x acceleration

if an object moving in a circle has constant speed, is it accelerating?

yes, the direction is changing constantly so its velocity is always changing so its accelerating

what is the name of a force supplying acceleration, directed to the centre, in motion in a circle?

centripetal force

what is inertial mass?

a measure of how difficult it is to change the velocity of an object (including from rest)

how is inertial mass calculated?

inertial mass = \frac{force}{acceleration}

what is newtons third law?

every force has an equal and opposite reaction

e.g. a book on a table

• the weight of the book on the table = the reaction force on the book from the table

fill in the blank.

momentum is always _______ in a collision.

momentum is always conserved in a collision

what is the equation for momentum?

momentum = mass x velocity

in a collision,

total momentum before = ____________________

total momentum before = total momentum after

how else can force be calculated referring to momentum?

force = \frac{change.in.momentum}{time} x \frac{(mv-mu)}{time}

what is the equation is relation to thinking distance, stopping distance, and braking distance?

stopping distance = thinking distance + braking distance

what is the average human reaction time?

0.25 seconds / 250 milliseconds

what are some factors of thinking distance?

anything that is because of you

• concentration

• tiredness

• drugs/alcohol

• distractions

what are some factors of braking distance?

anything that is because of the environment / vehicle

• poor road conditions

• bald tires

• worm brake pads

• car mass (including passengers)

what is the danger of large decelerations?

force = mass x acceleration, so a lot of force is on passengers and car. this can cause an injury.

what is the danger of large decelerations in terms of momentum?

before crash momentum = high

after crash momentum = 0

force = \frac{change.in.momentum}{time}

a great force is felt

what is the average mass of a car?

~ 1500kg

what is the work done to stop equal to?

the initial kinetic energy