Skateboarding Physics: Force of Landing from 10ft Roof

[kevshabar Reference]

Which scenario would exert the most force on the skateboarders feet upon landing on the ground?

Scenario 1: Skateboarder rides skateboard 15 mph off of 10ft building roof
Scenario 2: Skateboarder falls directly vertical (0 mph) off of 10ft building roof

Assumptions:
-- no friction
-- maximum vertical fall distance is the same in both scenarios
-- roof and ground are level in both scenarios

UPDATE: (more assumptions)
-- board and rider remain in a horizontal (board) and upright (rider) position
-- rider lands with both feet on the board
-- board lands with all wheels coming in contact with the ground at the same time
-- rider's shock absorbing body movements are the same in both scenarios

 

[Baluncore]

Welcome to PF.
They will be the same, but in scenario 1, the front axle will start to fall first so axle impact with the ground will not be synchronous. The total deceleration time will be longer so the force can be less.

 

[Halc]

Sounds an awful lot like a homework problem, so why not show us your result first?

Meanwhile, there's no assumption that the guy is actually going to land feet-down. I mean, a skateboard under you is going to affect a fall since it cannot all go over the edge simultaneously.

 

[kevshabar Reference]

Sounds an awful lot like a homework problem, so why not show us your result first?

Meanwhile, there's no assumption that the guy is actually going to land feet-down. I mean, a skateboard under you is going to affect a fall since it cannot all go over the edge simultaneously.

It's actually a debate with my brothers, thanks for your concern though.

 

[Baluncore]

The answer will be rider, style, case and assumption dependent.
If the rider lands on their head there will be little force on their feet.
What assumptions are being made?

I would expect the rider to transfer weight to the rear axle as the front axle goes over the edge, so there will be less rotation of the board. The scenario results will then be similar.

If it goes wrong, the survivability will be greater with injury reduced in scenario 1, since the rider will be better able to land and roll like a parachutist. That will transfer vertical momentum into horizontal momentum more efficiently than in scenario 2, where energy must be dissipated in a small area.

 

[kevshabar Reference]

@Baluncore we haven't even started debating about survivability yet.. I updated the original post with some more assumptions

 

[berkeman]

Welcome to PF.

It's actually a debate with my brothers, thanks for your concern though.

We need to know if it's schoolwork so we can be sure to keep such posts in the dedicated schoolwork forums. Hence the question. Thanks for your reply.

@Baluncore we haven't even started debating about survivability yet.. I updated the original post with some more assumptions

I hope you and your freinds are not contemplating trying this on a dare or bet or something. My teenage son did something almost similar from about 7 feet, and ended up with casts on both arms for a while (not a good scenario, for multiple reasons that was a very bad result).

I've flat-landed my MX dirtbikes from maybe 7-8 feet high a few times, and flat-landers are always uncomfortable. Much better to land on a down-slope, from no matter how high.

BTW, please try not to update your previous posts with new information -- that can be very confusing for otheres who are responding. Please instead just post a new reply. Thanks.

 

[Baluncore]

There can be no difference between the two scenarios if the goalposts are assumed to be in the same hole.

 

[annamal]

They should be the same because the x and y components are independent of each other and the force of the feet on the ground is in the y direction. The x component of the skateboarder riding 15 mph (x component) would not affect the y component.

 

[PeroK]

They should be the same because the x and y components are independent of each other and the force of the feet on the ground is in the y direction. The x component of the skateboarder riding 15 mph (x component) would not affect the y component.

The force is almost certainly greater in the second case, as you have horizontal as well as vertical deceleration on landing.

 

[Baluncore]

The force is almost certainly greater in the second case, as you have horizontal as well as vertical deceleration on landing.

But there is no friction, so cannot decelerate horizontally.

 

[jbriggs444]

But there is no friction, so cannot decelerate horizontally.

There will be, once the skateboard shoots out from under and the posterior hits the sidewalk. [But OP specifically mentions "skateboarder's feet"]

 

[Baluncore]

There will be, once the skateboard shoots out from under and the posterior hits the sidewalk. [But OP specifically mentions "skateboarder's feet"]

The OP post #1 assumptions were extended.
Assumptions:
-- no friction
-- maximum vertical fall distance is the same in both scenarios
-- roof and ground are level in both scenarios
(more assumptions)
-- board and rider remain in a horizontal (board) and upright (rider) position
-- rider lands with both feet on the board
-- board lands with all wheels coming in contact with the ground at the same time
-- rider's shock absorbing body movements are the same in both scenarios