forensic physics

Speed is usually measured

It is a measurement of how much distance you cover every second.

written as m/s

speed calculations

= distance / time

distance in meters

time in seconds

km/h to m/s

divided by 3.6

m/s to km/h

times by 3.6

Distance

is a scalar quantity

It is a measurement of how far you have travelled in any direction from your starting point.

distance travelled calculations

= speed x time

scalar

only have magnitude

can be added or subtracted easily

examples: speed, mass, distance

vector

have magnitude and direction

direction needs to be taken into account when combining

represented by arrows on diagrams

examples: velocity, displacement, force, acceleration

Velocity

is the speed in a particular direction.

its a vector

velocity calculation

= displacement / time

The displacement of an object

is the distance travelled in a particular direction from a starting point.

what quantities is force

are vector quantities – the direction they act in is important.

Forces are measured

in newtons (N). Larger forces may be measured in kilonewtons (kN) or even meganewtons (MN).

contact forces definition

When two objects touch, they can exert forces on each other.

examples of contact forces includes

friction

air resistance

normal reaction

friction

When two objects touch, they can exert forces on each other.

air resistance

When two objects touch, they can exert forces on each other.

normal reaction

an object rests on surface

Some forces do not require contact to operate.

non–contact forces.

examples of non-contact forces

gravity

electrostatic force

magnetic force

The starting point of a non–contact force arrow

is usually at the centre of the object.

On diagrams, a contact force arrow should start at the point

where the two objects touch each other

the resultant or net force

the vector sum of all the forces.

The Net force on an object

is the overall effect of the forces acting on an object.

The normal force is the

force that balances the force of gravity.

It is a contact force

Weight

is the force of gravity acting on the mass of an object as it pulls it towards the centre of the planet.

mass

is the amount of particles in a given volume of an object. Usually measured in grams or kilograms.

Weight is measured in

NEWTONS (N) because it is a force.

The following formula is used to calculate Weight.

Weight (N) = mass(kg) x gravity(m/s2) (acceleration)

W=mg

Earth has a gravitational pull of approximately 10 m/s/s towards its core (9.81 m/s2 to be exact).

Mass is

constant, but weight is variable

An object’s weight will depend

on the gravitational pull that is acting on its mass.

An object with the same MASS, may have a

different Weight (N) on different planets due to the difference in gravity.

In space, an object has no Weight (N) as

there is ZERO gravity.

Weight is the gravitational force pulling the car ( and its occupants)

towards earth

In a collision, this vertical force combines

with horizontal forces (like friction and impact) to determine motion.

A car’s weight determines

the normal force (upward support force from the road).

Friction (and traction) depends on the

normal force.

Heavier cars usually have more grip, affecting braking distance.

cars with greater mass

have greater weight

cars with greater mass have greater weight which increases

 inertia (resistance to changes in motion), meaning heavier vehicles require more force to slow down or stop.

In a crash, both vehicles experience the same force, but the lighter car

accelerates more (F = ma), so it suffers greater damage.

Because lighter vehicles accelerate more in collisions,

they are pushed back more violently, which increases the risk of damage compared to heavier vehicles

Newton’s first law

the tendency of objects to resist changes in motion

Inertia
Sir Isaac Newton described it in his first law, which states:

An object at rest will remain at rest unless acted on by a net force

Inertia 

is an object’s tendency to remain in its current state; an object at rest will tend to remain at rest

How do we measure inertia?

Inertia is not measurable as it is a concept, but it can be used to make comparisons as the greater the mass of an object, the more inertia it has. That is, more force is required to move or stop a more massive object than a less massive one.

inertia and momentum

are not the same

momentum is the

measure of how much motion object has

momentum takes into account

both the object’s mass and its velocity

more momentum =

less momentum =

heavy and moving fast

lighter or moving slower

when an object is in motion it has___
and when its at rest, its momentum is___

momentum

zero

frictional forces oppose 

motion

two types of frictional forces

static friction
kinetic friction

static friction

is friction which is experienced when an object is placed on a surface.

kinetic friction

is due to the movement of an object on a surface

Isaac Newton described the relationships between acceleration, force and mass in his Second Law.

The acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass.

Newton’s Second Law is usually written as:

force=mass x acceleration

F = ma

force is measured in newtons (N)

mass is measured in kg

acceleration is measure in m/s²

the more force you apply

the greater the acceleration

the greater the mass

the less the acceleration

The third law describes how

forces always act in pairs when objects interact, with one of the forces acting on each object and in opposite directions.

whenever one object exerts a force on a second object

the second object exerts an equal and opposite force on the first object

one is called the ____ and the other is called the ____

action force
reaction force

forces always come in

pairs

some of the safety features of modern cars are designed to absorb the

energy of a car in a controlled way and decrease the forces acting on those in the car

Crumple zones at the front and rear of a car

absorb the kinetic energy of a collision. They are designed to collapse in a controlled way, slowing the impact and reducing forces.

Air bags also reduce the force applied to the body by

the sudden negative acceleration experienced in a collision. The airbag increases the time over which the velocity of the head changes so that the acceleration is reduced → force is reduced.

Velocity is the

rate of change of displacement

The change in velocity can be calculated

by subtracting the initial velocity from the final velocity.