- Good morning.
Let's measure how helmets affect impulse and impact force.
♫ Flipping Physics ♫
- Nice 70s clothes everybody.
- [Students] 70s day!
- Sadly, I'm actually not willing
to slam my head into a force sensor.
(students voice agreement)
Therefore, modeling a human head today
is this medicine ball, which has a mass of 2.733 kilograms.
Class, is the medicine ball wearing a helmet?
- Uh, no.
- No helmet on that medicine ball.
- I drop the medicine ball,
which is currently not wearing a helmet,
from a height of 39.5 centimeters,
and it lands on a force platform.
The force as a function of time looks like this.
The maximum force of impact was 1310 Newtons.
The average force of impact was 618 Newtons.
The time during collision is 0.015 seconds
and the impulse, or the area under the curve,
is 9.84 Newton seconds.
Again, this is all information while not wearing a helmet.
Bo, remind me, what is the equation for impulse?
- Net force equals change in momentum over change in time
where both force and momentum are vectors.
- That's Newton's Second Law.
- Yeah.
Okay, multiply both sides by change in time
and you get impulse equals change in momentum,
which equals net force, which is the force
of impact during the collision times change in time.
- Remember, impulse is a vector,
and impulse is the area under the curve.
- And we often replace the net force,
or the force of impact, with the average force of impact.
- Yeah, I remember all that.
- Billy, how does adding a helmet
to somebody's head during a collision
affect all of the variables in this equation?
- Adding a helmet increases the distance a human head has
to slow down and, therefore, it increases
the time during the collision.
However, it does not change the initial velocity,
or final velocity, or mass of your head,
therefore, the change in momentum,
or impulse, should stay the same.
Therefore, the net force,
or force of impact during the collision,
is decreased, because the change in time is increased.
Yeah, that's how it works.
- Thank you, Billy.
Now, let's give the medicine ball a helmet.
- [Bo] Wait a second.
I think that's a cloth diaper, not a helmet.
- That's true.
I actually could not find a helmet
which fit this medicine ball, and putting a cloth diaper
on the force platform serves the same purpose.
It increases the distance during the collision
and, therefore, increases the change
in time during the collision.
Now, when I drop the same medicine ball
from the same height, only this time with the helmet,
this is the graph of impact force as a function
of time acting on the helmeted medicine ball.
The maximum force of impact was 971 Newtons,
the average force of impact was 544 Newtons,
the time during the collision is 0.018 seconds,
and the impulse, or the area under the curve,
is 10.3 Newton seconds.
Notice how the change in time
during the collision is increased.
The average force of impact is decreased,
and the maximum force of impact is decreased.
However, the impulse was actually
slightly increased because of the diaper.
If you watch the video closely,
you can see that the rebound height of the helmet
is actually a little bit higher.
This means the velocity after the collision
is a bit larger, which confirms the fact
that the impulse was actually increased slightly.
Now, this did surprise me a bit,
but the helmet still increased the change in time
during the collision and decreased both the maximum force
and the average force experienced by your head.
Thank you very much for learning with me today.
I enjoyed learning with you.
- Bo, is that a dress?
- It is a dress, and I look good in it.