3. Moving vehicles have kinetic energy 3. Moving vehicles have kinetic energy and energy transformations are an and energy transformations are an

important important aspect in understanding motionaspect in understanding motion

When we do work on an object by applying a force over a distance, we cause an energy transfer to take place

ENERGY CANNOT BE CREATED NOR DESTROYED, ONLY CHANGED FROM ONE FORM TO ANOTHER

It is transformed into heat energy,sound energy, deformation of the car and

kinetic energy of flying parts or other car/wall

4. Change of momentum relates to the 4. Change of momentum relates to the forces acting on the vehicle or the driverforces acting on the vehicle or the driver

Recall Newtons Third Law

Miss Egg’s car

During the crash momentum will change to zero due to a force, F, applied over a short period of time, t.

The change in momentum p= impulse = F.t

SO FOR A GIVEN CHANGE IN p, THE BIGGER t IS, THE SMALLER F WILL BE!

Crumple zone: Bubble wrap, tissue paper, foam, whatever

Miss Egg

Total momentum is always conserved in a collision.

(total p[before] = total p[after])

because any force applied to one object is applied to the other so the momentum that one object loses is gained by the other!

( i.e. same change in p, different direction, so total change = zero)

Energy is also conserved, but not necessarily as kinetic energy - it may change to other forms.

An elastic collision is one in which kinetic energy is conserved (a rare event).

Crash testingCrash testingRemember:

Crash testingCrash testing

m 3mv v

WHAT IF THEY JOIN ON COLLISION?

Total momentum before collision = mv - 3mv = -2mv

Two masses headed for a crash:

so, Total momentum after collision = -2mvTotal momentum is always conserved in a collision.

(total p[before] = total p[after])

m 3mV

combined mass after the collision is 4m

so what velocity gives total momentum after collision = -2mv ??

-2mv = 4mV

-2v = 4V

4m

-0.5v = V

Crash testingCrash testing

WHAT IF THEY JOIN ON COLLISION?

Total momentum before collision = 20 - 60 = -40 kg.m/s

e.g. Two masses headed for a crash:

so, Total momentum after collision = -40 kg.m/sTotal momentum is always conserved in a collision.

(total p[before] = total p[after])

10 30V

combined mass after the collision is 40kg

so what velocity gives total momentum after collision = -40 kg.m/s ??

-40 = 40V

-1 = V

40kg

V= -1 m/s

10 kg 30 kg2 m/s 2 m/s

5. Safety devices are utilised to reduce 5. Safety devices are utilised to reduce the effects of changing momentum.the effects of changing momentum.

An object will continue in its state of rest or uniform motion in a straight line unless acted upon by an unbalanced force

Newton’s first law is not always apparent.Newton’s first law is not always apparent.

Friction and air resistance are everywhere, so it’s hard to Friction and air resistance are everywhere, so it’s hard to see it in action.see it in action.

You don’t always realise you’re movingYou don’t always realise you’re moving– you are moving very fast right now!you are moving very fast right now!

You can get a false sense of security in a car - you may feel You can get a false sense of security in a car - you may feel quite safe and comfortable moving at constant high speedquite safe and comfortable moving at constant high speed

All the objects in the car are also moving at a high speed. If All the objects in the car are also moving at a high speed. If the car stops suddenly, those objects keep moving - possibly the car stops suddenly, those objects keep moving - possibly colliding with the driver who is stopped by the seatbelt.colliding with the driver who is stopped by the seatbelt.

Stationary objects are stationary because forces are present Stationary objects are stationary because forces are present and balanced - easy to think there are no forces acting but and balanced - easy to think there are no forces acting but there are!there are!

Source: http://www-aig.jpl.nasa.gov

6. The models applied to motion and forces 6. The models applied to motion and forces involving vehicles can be applied to a wide involving vehicles can be applied to a wide variety of situations.variety of situations.