# Impulse – Rockets Lecture (video)

1. Ballistic Pendulum:

A massive pendulum absorbs a bullet and the bullet’s momentum. The kinetic energy which is left over after this completely inelastic collision is converted to potential energy of the pendulum. The relationship between horizontal displacement of the pendulum and bullet velocity is derived and empirically observed. The initial kinetic energy in the bullet is almost totally converted into heat.

2. Impulse and Impact Time:

Impulse is the product of a force (acting on an object) and the brief time that it acts. This results in an abrupt change of momentum. For a ball bouncing off the floor, the impact time is typically milliseconds. A movie is shown to demonstrate this. Courtesy of Dr. Peter Dourmashkin, MIT.

3. Surprising Bounce Demo:

A tennis ball on top of a much heavier basketball is dropped from a height of about 3 m. The tennis ball bounces way higher than 3 m. Try calculate how high it bounced by assuming the basketball bounces off the floor elastically and then collides elastically with the tennis ball.

4. Thrust of a Rocket:

An analogy is drawn between the force felt by the target of a tomato thrower, the reaction force felt by the thrower, and the propulsion (thrust) of a rocket. The Saturn rockets spewed out about 15 tons/sec at a speed of 2.5 km/sec relative to the rocket to provide a thrust of about 34 million Newton. The mass of the rocket decreases substantially with time as it burns its fuel, so the rocket’s acceleration increases.

5. Fuel Consumption and Rocket Velocity:

Consuming a given amount of fuel translates into a fixed change of the rocket’s momentum, not into a fixed change of the rocket’s kinetic energy.

Watch the video by clicking on the image bellow:

Lecture 17: Impulse – Rockets

Walter H. G. Lewin