Ballistic Pendulum/Projectile Launcher 012-05375B Ballistic Pendulum - Theory ∆ θ Overview PE = MgRcm (1 – cos ) The ballistic pendulum is a classic method of determining Here Rcm is the distance from the pivot point to the center the velocity of a projectile. It is also a good demonstra- of mass of the pendulum/ballBallistic Pendulum system. This potential tion of some of the basic principles of physics. energy is equal to the kinetic energy of the pendulum Purpose: The ball is fired into the pendulum, which then swings up immediately after the collision: a measured amount. From theTo height experimentally reached by the investigate the laws of conservation of energy and the conservation of linear momentum. KE = 1 Mν2 pendulum, we can calculate its potential energy. This 2 P Introduction: potential energy is equal to the kinetic energy of the The momentum of the pendulum after the collision is just pendulum at the bottom of theIn swing, this experiment just after the we will determine the initial speed of a projectile using two different methods. ν collision with the ball. Pp = M P , Method I: With the first method, the speed of a projectile is found by application of the laws of conser- which we substitute into the previous equation to give: We cannot equate the kineticvation energy ofof linearthe pendulum momentum and of mechanical energy. This application makes use of a ballistic pendulum. An after the collision with the kineticapproximately energy of the spherical ball ball projected from a spring gunP is2 caught and retained in the bob of the pendulum, KE = P before the swing, since the collisionan inelastic between collision. ball and Referring to figure 1, if we let v be2M the velocity of the ball with mass, m, immediately pendulum is inelastic and kinetic energy is not conserved before impact with the pendulumSolving bob this andequationV be for thethe pendulum velocity ofmomentum the ball gives: and pendulum with mass, m + in inelastic collisions. MomentumM, immediately is conserved in after all the collision we can determine an equation for the momentum of the collision. The forms of collision, though; soinitial we know momentum that the momen- at that instant before impactPp =2 is mvMand(KE from) the law of conservation of linear momen- tum of the ball before the collisiontum thisis equal quantity to the mo- must equalThis the momentum final momentum is equal to of the the momentum entire system of the ball immediately after impact. Hence mentum of the pendulum aftermv the= collision. (M + m Once)V and we solvingbefore for v thegives collision: know the momentum of the ball and its mass, we can (M + m) v = (P = mν .)V (1) determine the initial velocity. b m b There are two ways of calculatingAfter the impact velocity the of the ball ball. is raised Setting from these the two initial equations equilibrium equal to each position other ofand the pendulum to a height above The first method (approximatethat method) position. assumes The that kinetic the energyreplacing of KE the with system, our known ball potential and pendulum, energy gives at us: the instant following impact is pendulum and ball together act1 ( Mas a+ pointm)V mass2. When located the at center of gravity reaches its2 highest position h , the potential energy of the system 2 mν =2M gR (1 – cos θ) 2 their combined center of mass.has This been method increased does not by take an amount (M +m)gb∆h and the kineticcm energy has become zero. If the small frictional rotational inertia into account. It is somewhat quicker and Solve this for the ball velocity and simplify to get: 1 2 loss in energy is neglected, the loss in kinetic energy equals the gain in potential energy so 2 (M + m)V = easier than the second method,(M but+ notm) asg∆ accurate.h. Solving for V gives ν M θ b = m 2gRpcm (1 – cos ) The second method (exact method) uses the actual V = 2g∆h (2) rotational inertia of the pendulum in the calculations. The equations are slightly more complicated, and it is neces- sary to take more data in order to find the moment of inertia of the pendulum; but the results obtained are generally better. R Please note that the subscript "cm" used in the following cm equations stands for "center of mass." θ Approximate Method Begin with the potential energy of the pendulum at the cm top of its swing: cm ∆h ∆ ∆ cm PE = Mg hcm cm M m Where M is the combined mass of pendulum and ball, g is the acceleration of gravity, and ∆h is the change in height. Substitute for the height: V Figure 1 ∆h = R(1 – cos θ) Figure 1: Ballistic Pendulum. 4 ® According to figure 1, if we measure the distance, R, from the pivot point to the center of mass of the pendulum and measure the angle, θ, which the pendulum sweeps out from equilibrium, h1, to some height, h2, then we can determine the difference between these two heights, ∆h, by R(1 − cosθ) = ∆h (3) Using equations one through three we can determine the initial velocity of the projectile from measured quantities. You should write out this equation for v in terms of R, θ, M and m. 1 012-5375B Ballistic Pendulum/Projectile Launcher Experiment 5: Conservation of Energy EQUIPMENT NEEDED - Projectile Launcher and plastic ball - plumb bob - measuring tape or meter stick - white paper - carbon paper - (optional) 2 Photogate Heads and Photogate Mounting Bracket Purpose The purpose of this experiment is to show that the kinetic energy of a ball launched straight up is transformed into potential energy. Theory final position The total mechanical energy of a ball is the sum of its potential energy (PE) and its kinetic energy (KE). In the absence of friction, total energy is conserved. When a ball is launched straight up, the initial PE is h defined to be zero and the KE = 1mv2 , where m is the mass of the 2 0 ball and vo is the muzzle speed of the ball. See Figure 5.1. When the ball reaches its maximum height, h, the final KE is zero and the initial position ν0 of Ball Launch Position balls ONLY! PE = mgh , where g is the acceleration due to gravity. Conservation Use 25 mm Yellow Band in Window Yellow Indicates Range. SHORT RANGE SHORT RANGE SHORT CAUTION! DO NOT LOOK CAUTION! DOWN THE BARREL. DO NOT LOOK DOWN BARREL! RANGE of energy gives that the initial KE is equal to the final PE. MEDIUM PROJECTILE LAUNCHER LONG RANGE ME-6800 0 10 20 To calculate the kinetic energy, the initial velocity must be determined. 30 40 50 60 70 90 80 To calculate the initial velocity, vo, for a ball launched horizontally off WEAR SAFETY GLASSES WHEN IN USE. Method II: The second method in this lab involves an independent measurement of the horizontal a table, the horizontal distance travelleddistance by the the projectile ball is given travels by x while= v0t falling, from a certain measured vertical height. We use the fact that where t is the time the ball is in Newton'sthe air. Air second friction law is (F=ma) assumed is to a vectorbe equation. This fact implies the validity of the components equations, negligible. See Figure 5.2. Fx = max and Fy = may, where x and y are the horizontal and vertical directions respectively. If we horizontally fire a ball so the ball is permitted to follow a trajectory, as indicated in figure 2, and if air friction is negligible, there is1 no 2x component of acceleration, so the x component of the velocity remains The vertical distance the ball drops in time t is given by y = gt . Figure 5.1 Conservation constant. On the other hand,2 the y component of velocity of the ball changes precisely as does the velocity of of Energy The initial velocity of the ball canan be object determined that has by only measuring vertical x motion. and y. The y component of acceleration, ay, then equals g, the acceleration m The time of flight of the ball candue be tofound gravity using (9.81 s2 ). 2y t = ν g 0 x and then the initial velocity can be found using v0 = t . y ➤ NOTE: For best results, see the notes on "Repeatable Results" in the Introduction. Setup x ➀ Clamp the Projectile Launcher to a sturdy table near one end of Figure 5.2 Finding the FigureInitial 2: Projectile. Velocity the table with the launcher aimed away from the table. See Figure 5.1. ➁ While following the trajectory, as described above, the ball has a vertical displacement y and a horizontal Point the launcher straight up anddisplacement fire a test shotx as on indicated medium inrange figure to make 2. Since sure the the initial ball doesn’t vertical hit velocity of the ball is zero, its time of flight the ceiling. If it does, use the short range throughout this experiment or put1 the launcher2 closer to is determined by the equation y = vot + 2 ayt , in which vo = 0, ay = g, then, the floor. r2y ® t = (4) 25 g By determining t, the horizontal component of the velocity of the ball is found using x = vxt so, x v = (5) x t Since vx is constant this velocity is the same as the initial velocity, v, with which the ball is projected from the launcher in method I. Laboratory Procedure: Part I - Taking Indirect Measurements with a Ballistic Pendulum 1. Make a table in your notebook of values to be measured.