Children's Museum of Houston

Seat belt safety

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Inertia relates to Newton’s First Law of Motion: “An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an outside force.” Basically, it says that objects don’t like to change their motion.  That’s inertia – the tendency of an object to resist changes in motion.

Now, objects at rest makes sense, but objects in motion doesn’t always make intuitive sense.  After all, if you roll a ball, it comes to stop, right?  So, how is that resisting changes in motion?  Well, that’s what Aristotle originally perceived as well when he described motion.  But, Newton realized that there were forces acting on a rolling ball, like friction, that causes it to slow down and stop.  Without friction, the ball would roll forever.

You may have more direct experience with inertia when running, skating, biking, etc., and trying to come to a sudden stop.  I mean a really, really, really fast stop.  Your body will continue to move forward.  That’s why if you are running at a full sprint and try to stop too fast, you’ll end up moving a leg forward to steady yourself or just flat-out fall down.  And that is also why seatbelts are important!

What You Need:

• A large toy car
• A rubber band
• A ramp
• A stop at the bottom (like a large book or two)
• An action figure or doll that easily fits on top of the car

What to Do:

• Set up your ramp and place the stop at the bottom of the ramp.
• Place the car onto the ramp.
• Place the action figure/doll on top of the car.
• Let go.
• Repeat, but rubber band the action figure/doll to the car. 

What’s Going On?
Inertia – that’s what’s going on!  When the action figure/doll isn’t rubber banded to the car (i.e. NOT wearing a seatbelt), its inertia is separate from the car.  So it builds up the same momentum as the car on the way down, but where the car stops at the bottom (rather suddenly), the inertia of the action figure/doll carries it forward past to where the car stops.  In other words, if you don’t wear your seatbelt, your inertia will keep you moving foward even when the car stops.  So, in the case of a sudden stop or a car crash, you could be quite serious injured.  That’s why you wear your seatbelt – it allows you to be more directly affected by changes to the car’s inertia; when it stops, you are stopped along with it.  It may not be comfortable, but it can save your life.

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