Projectile Motion. y - y o = v oy t - (1/2)gt 2 [2]

Transcription

1 Projectile Motion In this experiment we will study motion in two-dimensions. An object which has motion in both the X and Y direction has a two dimensional motion. We will first determine at what velocity the ball is being fired from the firing mechanism, and then with this knowledge and some calculations. Determine how far the ball will travel when it is fired at an angle other than the horizontal. Theory: In introductory physics courses, a projectile is an object which is given some initial velocity, v 0, and thereafter, subjected only to gravity. This definition of a projectile assumes that no force due to air resistance is acting on the projectile. This assumption is approximately valid if the velocity of the projectile is relatively small (less than 10 meters/sec) and the cross-sectional area of the object is small, which will be the case in this experiment. Since gravity is the only force assumed to act on the object after it is given its initial velocity, the object will be in free-fall in the vertical direction, and will move with a constant velocity in the horizontal direction. Consider an object projected horizontally with a velocity, v ox, from some initial height, H, above the floor, as sketched below. The object will travel a horizontal distance, R, during the time it falls a vertical distance, H. Since the velocity in the horizontal direction is constant, R = v 0X t [1] v ox Where t is the time that the object is in flight (which is also the time it takes the object to fall a distance H). H In free fall, the vertical distance moved during a time interval, t, is given by the equation, R y - y o = v oy t - (1/2)gt 2 [2] where y o is the initial position of the object, g is the acceleration due to gravity (about 9.8 m/sec 2 ), and v oy is the initial velocity of the object in the vertical (y) direction. In equation [2], up is taken as the positive direction, and down is

2 the negative direction. For the case of an object propelled horizontally, v oy is zero (no component of initial velocity up or down). If the object is initially propelled from a height H above the floor, (y o = H) then at a later time it hits the floor, and y = 0. Thus, from equation [2], and the time of flight is H 1 2 t 2 / gt [3] 2 H/ g [4] The initial velocity of the projectile can then be calculated from equation [1]. Projectile Fired at Angle above the Horizontal: Consider a projectile projected with an initial velocity, v o, at angle above the horizontal at height, H, above the floor, as sketched. v o v ox Figure 5-3 H The range, R, the projectile travels can be found using kinematics equations. R First, the initial velocity v o is broken down to its initial horizontal and vertical velocities, v ox = v o cos, v oy = v o sin. By rewriting equation [2] for the figure aboveit yields equation [5]. The term y o is replaced by the term H, which is the height from the floor to the bottom of the projectile, and is shown in figure 5-3. The term v oy is the initial vertical velocity of the projectile. Using the floor as the reference point the term y can be given a value of zero. -1/2 gt 2 + v oy t + H = 0 [5]

3 Equation [5] is a second order polynomial and time, t, can be found using the quadratic equation. 0 = at 2 + bt + c t b b 2a 2 4ac Once a time is found the range, R, is the initial horizontal velocity, v ox, multiplied by the time. See equation [1]. : Initial Velocity from the Range of Projectile Fired Horizontally. Setting up the apparatus 1. Using the two thumb screws mount the ME-6800 projectile launcher near the bottom of the ME-6831 ballistic pendulum so that it seats within the two parallel horizontal grooves. Tighten the screws enough that it doesn t wobble within the grooves. Please do not over tighten these screws. Initial measurements. 2. Move the launcher to the end of the table so that the side of the launcher is parallel to the edge of the table. Examine the launcher and notice that at the end of the launcher is a circle which is used to represent the projectile. This is the point at which the projectile leaves the launcher and where all the following length measurements will be made. Use the meter stick and measure the height, H, from the floor to the bottom of the circle shown on the launcher. Think about it, does the center of the ball hit the floor or the bottom of the ball. Finding landing location 3 Assign a lab partner as a spotter his/her duties is to mark where the ball lands. Assign another lab partner as a chaser he/she will track down the ball once it strikes the floor. Rotate the launcher so that it faces into the room, and the front edge of the base plate is at the edge of the table. Use the plunge to load the ball into the mechanism to the desired range. Ensure that the ball remains seated in the mechanism and does not roll into the barrel and if necessary reseat the ball. When all is ready release the projectile by pulling up on the tie wrap attached to the trigger. The spotter should mark the location with a coin or tape. The chaser should retrieve the ball.

4 Preparing the target area. 4 From the center table obtain a sheet of the computer paper, only one sheet is needed. Tape this sheet at the corners to the floor at the location where the ball landed. This will be your target. Assure that the center of the target is where the ball landed. Repeat step 3. When the ball strikes this sheet, it will leave an imprint. Verify that the ball strikes the target. If it doesn t reposition the target. For this method the Range is vital. The better the measurement of the range the more the calculated velocity can be trusted. Our method outlined in the following steps divides the range into 3 parts that 2 can be easily measured and 1 part will be given. You are free to ignore them and follow whatever method you prefer to determine the range, however be for warned while you are not graded on what method you use your grade is determined by how you score on the grading target. More measurements 5. Measure the distance from the edge of the paper to the edge of the target Record this value as d 2. Mark on the target which edge you made the measurement to. More launches 6. Repeat step 3 to obtain 5 data positions, imprints, on the paper. There is no need to make any measurements at this time. One more set of measurements 7. Carefully remove the paper from the floor. Please also remove any remaining tape scraps. Take the paper to the table and measure from the marked edge to each imprint. Record each distance into the Data Table 2. Find the average value for d 3. Finding the Range, time and velocity 8. Assuming that the launcher was position as directed in step 3. Add the values d 1, d 2 and d 3 this is the range of the projectile, R. Use equation [11] and find time t. Velocity is range divided by time.

5 Data Distance, d 1, distance from where the ball leaves the launcher to the edge of the base plate = m Shot 1 Shot 2 Shot 3 Shot 4 Shot 5 Ave = d 2. d 2 = d 3 = 4) The range the ball travel horizontally R = d 1 + d 2 + d 3. and the height the ball fell vertically is H = R = 5) Use the equation the air. t 2H where g = 9.8 m/s g 2. to find the total time the ball was in Use t, to find the velocity from the equation R = v * t. t = v =

6 Finding a Range for an angle other than zero degrees. Setting up the Apparatus 1. Ask you instructor for an angle. Remove the projectile launcher from the ballistic pendulum. Examine the picture to the right. The launcher will be mounted on the opposite side of the apparatus as shown in the picture. Secure the projectile launcher near the top of the ballistic pendulum; the front will be secured using the single hole near the top of the apparatus. The rear of the projectile launcher will be secured using the curved slot. Set the angle you were given by loosening the rear screw of projectile launcher and lower it until the string with the plumb indicates the desired angle. From the previously determined velocity find the x and y components v x and v y. = v x = v cos = m/s v y = v sin = m/s 2. Measure in meters the height, H, from the floor to the bottom of the depiction of the ball launching position. (Remember the bottom of the ball hits the ground first). H = m 3. To determine the time of flight for the projectile use the equation y = H + v y sin t (g/2)t 2 0 = c + b(t) + (-a)(t) 2 If we set the point of impact (the floor) as zero then y = 0 in the equation above. The quadratic equation can be used to determined t. b b 2 4ac t = t 2a v y sin (v y sin ) 2( g / 2) 2 4( g / 2)H a = b = c = t = s

7 4. Once the time of flight is determine, calculate the range, R, of the projectile. R = v x * t = m 5. Measure off your predicted range. Mark this point with tape or a coin. Notify your instructor that you are ready to take your shot. Once given the scoring target place the 100 at where you have marked your range. Load the projectile. Do not forget to use the same setting. SCORE! 6. Fire the projectile as the instructor looks on. Where the projectile strikes is your grade for the lab.