# How an object in translatory motion comes to rotary motion

1. Jun 27, 2011

### srivi

In a car race two motar vehicles are moving with very high speed , they have collided got coneected each other and started rotating together .
Becuase they are in motion according to newtons first law they are continued with motion,
but how did they got rotary motion ?

2. Jun 28, 2011

### Staff: Mentor

You mean this accident?

Don't forget friction and think - where does the smoke around wheels comes from?.

3. Jun 28, 2011

### tiny-tim

hi srivi!

as you know, in every collision, https://www.physicsforums.com/library.php?do=view_item&itemid=53" is conserved

also, https://www.physicsforums.com/library.php?do=view_item&itemid=313" is conserved …

if the two cars do not collide exactly head on, then before the collision they have a non-zero angular momentum about their combined centre of mass …

so they still have that angular momentum after, when they're connected

(and if they don't stay connected, then each car will rotate unless the line of the https://www.physicsforums.com/library.php?do=view_item&itemid=340" of the collision is through its own centre of mass)

Last edited by a moderator: Apr 26, 2017
4. Jun 28, 2011

### HallsofIvy

If one of the cars, off to one side of the other, is moving faster, then the system, that is, the two cars together, already has a rotary motion about their center of mass.

In Borek's clip, at least one of the motorcycles, probably both, still has its throttle on.

5. Jun 28, 2011

### rcgldr

The green bike is the one with it's rear tire touching the pavement and spinning the bikes.

For collisions in space, angular momentum is preserved. For collisions on the ground, the earth is part of the system, and angular and linear momentum are conserved only if you take into account the effect on the earth, due to friction between the tires and the pavement causing some tiny effec on the earth. If the collision occured on ice, or very slick pavement, then friction would take much longer to slow down the movements after a collision.