physics lab

Lab 4 – Projectile Motion *Informal*

Purpose: To examine the kinematics of projectile motion. To compare calculated and measured initial speeds for a projectile.

Apparatus: Projectile Launcher, two photogates, Time of Flight accessory, meter stick, computer interface.

Theory: Using the equation 2

2 1 gttvyy iyif  you can find the initial speed

of a projectile.

Procedure: 1. Set up the equipment so that the photogate nearest the launcher is plugged into digital input 1, the other photogate is plugged into digital input 2 on the ScienceWorkshop interface, and so that the Time of Flight accessory is plugged into digital input 3. 2. Start DataStudio, from File select Open Activity, and then in the DataStudio folder on the Mac choose Exp 37 – Time of Flight. Ignore the instructions – we’re doing a slightly different lab. Dr. Kelly will make sure the timers are set up right. 3. Double click on the table in the lower left pane – it should pop up over everything else. 4. Using the SHORT range only, test fire the launcher. Hit the START button just before the launch. If you do NOT see a speed in your table, ask Dr. Kelly 5. The basic idea in this lab is you want to find the time of flight (the time in the air) of the ball. The timer pad works properly when the ball hits in the white area of the pad. 6. It’s just trial and error to place the pad so that the ball hits the right spot. Try not to move the launcher at all as you pull the firing cord. When you get the right spot, don’t move anything and do your measurements. 7. You’ll do 5 time trials at 3 different angles. Try to space the angles out so there’s at least 20˚ between each angle. Use the SHORT range every time. 8. For each ANGLE, write down the launch angle, the launch height, and the landing height. Measure the landing height to the TOP of the pad. 9. For each trial, (5 for each angle) write down the time of flight and the initial speed.

Analysis: 1. For each angle, find an average time of flight and an average speed.

2. Using the equation in the theory section and your knowledge of projectile motion, solve for the initial speed (i.e. vi – not viy)

3. Calculate the initial speed for each angle

4. Compute the % Error between the calculated initial speed (what you get from equation and time of flight) and measured initial speed (what you measured using photogates.)

Questions:

1. Discuss sources of error in the experiment. For each source of error, try to estimate whether it made the calculated speed larger or smaller.

2. A snowball is launched off a roof that is 5.0 m high. Its initial velocity is 10.0 m/s at an angle of 30˚ above the horizontal. Neglect air resistance.

a) Find the time of flight of the snowball. (You’ll need to use the quadratic equation. Remember than negative times don’t make any sense.)

b) Find the horizontal distance the snowball travels.