How to design a simple experiment about force and motion for young kid? I am planning to demonstrate experimental validation F = m*a - but to a 4th stand. student. By now he understands velocity and acceleration well enough. 
I want to design this experiment using household stuff or regular transport related stuff or may be even their toys. I don't want to give a look of 'laboratory' equipment to intimidate kids.   
Any ideas? 
 A: Use a catapult. You can show that if you put it vertically, putting more weight on the elastic stretches it more - so stretching relates to force.
Next use it to "shoot" objects. Pull back a certain distance and the object flies this far; pull back further (more force) and it flies further - because we used more force it went further. Finally double the mass, and pull back the same distance as before. Mass is greater so acceleration will be less and the object won't fly as far.
With some care it will be fun and instructive. And can be semi quantitative.
edit
Apparently British and American words for the same thing are different. This is what I call a catapult source:

A: You can show it using a spring based toy car, The toy car has a spring inside, while pulling it backwards you apply a force which winds up the spring, when released the car is propelled forward by the spring.
The amount of distance the car travels is directly proportional to the acceleration it gains after release, which is again proportional to the mass of the car and with how much force you've pulled it back earlier (ofcourse practically there is a limit to how much you can pull)
