Ballistic Pendulum – Measuring a Bullet's Speed
PhysicsEnergy & MomentumAges 15–16
Loading…
Sign in to playA bullet of mass m and speed v embeds in a wooden block of mass M hanging from a light rod of length L. The perfectly inelastic collision conserves momentum but loses most of the kinetic energy; the block then swings up to a height h as mechanical energy is conserved. Bar charts compare momentum and energy before and after the collision, and a measurement mode hides v so students calculate it from h.
Lesson: Perfectly inelastic collisions, conservation of momentum and of mechanical energy – the ballistic pendulum
What it shows
A ballistic pendulum measures the speed of a bullet. The bullet embeds in a hanging block in a perfectly inelastic collision: momentum is conserved, so m·v = (m + M)·V, but kinetic energy is not, and the fraction M/(m + M) becomes heat, sound and deformation. During the swing, mechanical energy is conserved, so ½V² = gh. Combining the two stages gives v = (m + M)/m · √(2gh). The model ignores air resistance and friction at the pivot, treats the rod as massless and assumes the collision ends before the block moves.
How to use
Set Bullet m, Speed v, Block M and Rod L, press Fire and watch the height ruler as the block swings. Compare the momentum bars before and after the collision, then the energy bars. Next tick Measurement mode (hide v): press Fire, read h, calculate the bullet's speed, enter it under Your calculated speed and press Check. New bullet gives another hidden speed.
Parameters you can change
- Bullet mass 2–50 g
- Bullet speed 100–900 m/s
- Block mass 0.5–10 kg
- Rod length 0.5–3 m
- Measurement mode (hide the bullet's speed)
Questions to explore
- Why can't conservation of mechanical energy be applied from the bullet's flight all the way to the block's highest point?
- If the block's mass M is doubled, how do the rise height h and the fraction of energy lost change?
- Into which forms of energy does the kinetic energy lost in the collision go?
