Water Roket
Build Water Rockets
Explain how they work and ask the students which of Newton’s laws apply to them. You should talk about what forces
are acting on the water rocket to make it move (talk conceptually about the idea of a
free body diagram). You should have the force from the ejected propellant acting
upwards, and the weight and air resistance acting downwards.
Talk about how each law applies to the physics of the water rocket. Try to have the
students identify which of Newton’s laws applies to the following descriptions of the
rocket’s physics. The rocket moves because the ejected combusting propellant exerts and
opposite and equal force on the bottle which pushes the bottle up (3rd law), and the larger
that force is, the faster it will accelerate and move (2nd law). Eventually the rocket
reaches a peak height because external forces (air friction/drag and gravity) act on it (1st
law).
Now talk about what happens if we add too much water. The mass of the bottle will
increase, so it takes a larger force to make it accelerate and hence it won’t go as high as
when we give it the same force as a rocket with less water. But if there is too little water,
there isn’t as much force exerted from the propellant because the propellant mass is small
and as in the case of too much water, the rocket does not go as high as when it has an
optimal amount of water.
If there is time, talk about how the water rockets are simple models of actual rockets that
travel up into space. Each group will design its own water rocket, and decide how much
water to put in. For the design, you can suggest to the students ideas on how to reduce
drag/air friction (i.e., a cone top), or that they might want to add fins. We will not have
the students add a parachute to their rockets. In addition, do not allow the students to
cut into the bottle since this prevents the bottle from being
Explain how they work and ask the students which of Newton’s laws apply to them. You should talk about what forces
are acting on the water rocket to make it move (talk conceptually about the idea of a
free body diagram). You should have the force from the ejected propellant acting
upwards, and the weight and air resistance acting downwards.
Talk about how each law applies to the physics of the water rocket. Try to have the
students identify which of Newton’s laws applies to the following descriptions of the
rocket’s physics. The rocket moves because the ejected combusting propellant exerts and
opposite and equal force on the bottle which pushes the bottle up (3rd law), and the larger
that force is, the faster it will accelerate and move (2nd law). Eventually the rocket
reaches a peak height because external forces (air friction/drag and gravity) act on it (1st
law).
Now talk about what happens if we add too much water. The mass of the bottle will
increase, so it takes a larger force to make it accelerate and hence it won’t go as high as
when we give it the same force as a rocket with less water. But if there is too little water,
there isn’t as much force exerted from the propellant because the propellant mass is small
and as in the case of too much water, the rocket does not go as high as when it has an
optimal amount of water.
If there is time, talk about how the water rockets are simple models of actual rockets that
travel up into space. Each group will design its own water rocket, and decide how much
water to put in. For the design, you can suggest to the students ideas on how to reduce
drag/air friction (i.e., a cone top), or that they might want to add fins. We will not have
the students add a parachute to their rockets. In addition, do not allow the students to
cut into the bottle since this prevents the bottle from being