physics lab

Lab 5: Newton’s Second Law *Informal*

Purpose: To experimentally verify Newton’s 2nd Law, that m

F a net



 .

Theory: According to Newton’s 2nd Law, for a cart of mass m1 on a horizontal track with a string attached over a pulley to a hanging mass m2, the net force Fnet on the entire system (cart and hanging mass) is the weight of the hanging mass, Fnet = m2g, (assuming friction is negligible).

Also using Newton’s second law, total

net

m Fa 

 where mtotal is the total mass of the system.

Experimental Diagram:

Equipment: Cart Track; Cart (m=250 g); 2 Mass Bars (m=250 g each); Smart Pulley; Clamp; Thread; Mass Hanger; Slotted Masses

Procedure: 1. Put the track on the table so one end is out over the floor. Clamp the clamp to the end of the track so that the clamp screw points down. Important: Put the cart on the track. If it rolls to one side or the other, prop that side up just a little so it doesn’t roll any more. A good check is if you give the cart just a little push, and it rolls a bit then stops, the track is level. It should roll about the same distance both directions. 2. Place the Smart Pulley in the hole in the clamp, lower it so that the top of the pulley is at about the same height as the top of the cart, and tighten the screw. 3. Cut a length of string about 1.2 m long, tie one end to the cart (there’s a little loop) and one end on the mass hanger. 4. Take a combination of slotted masses that allows you to go from 5g to 30 g in 5 g increments (For example: one 5g, one 10 g, one 20 g) 5. Put the car on the track, place the two 250 g mass bars in the bed, and place all your other masses in the bed. 6. Critical: We’re going to keep the total mass of the system constant by moving masses from the cart, to the hook, and then back again when we change masses on the hook. This keeps the total mass constant.

7. Plug the smart pulley into digital input 1 on the ScienceWorkshop box, and start Datastudio. 8. If you don’t have a blank Datastudio, go to File and choose New Activity. If another dialog box pops up, choose New Experiment. 9. In the window that has a picture of the ScienceWorkshop box and a list of sensors, choose Smart Pulley. 10. Drag a graph window into the middle of the screen, and choose “Velocity (m/s)” for the graph. 11. Set the cart on the track and the mass hanger over the Smart Pulley. Make sure the mass hanger hangs low enough that the hanger can fall freely. 12. Have one partner ready to start the measurement. Hit start, and then release the cart. As soon as the mass hanger hits the floor, hit stop. 13. Look at your data. If it doesn’t fill the graph, resize the graph window to see more of the data. 14. Drag the cursor to select only the data that looks linear, and then go to the menu bar at the top of the graph window, choose Fit, and then choose Linear from the menu. 15. In your lab notebook, make a table that shows the mass on the hanger for each run, and the acceleration for each run. 16. Once you have the data in your notebook, delete all experimental data. 17. Repeat steps 11 – 16, increasing the mass by 5 g for each run. Important: Make sure that any masses that are NOT on the hanger go back in the cart.

Analysis: 1. Write down the total mass of your cart + all masses. This is the total mass. 2. Calculate the force, in Newtons, exerted by the hanging mass on the system for each mass you put on the hanger. Write this in your data table as a new column. 3. Plot, either by hand on graph paper or using Excel, the acceleration on the y-axis vs. the Force on the x-axis. 4. Draw a best-fit line through your data, or if using Excel perform a linear fit on your data. Calculate the slope by hand, or use the slope given by Excel’s best fit.

5. The slope of your line should be totalm

1 . Compare the total mass given by the slope to

the total mass gotten by summing the masses. (Make sure you convert everything to kg.) Calculate the percent error.

Questions: 1. We neglected friction in this problem, but it exists. This should be most noticeable at smaller accelerations, or smaller hanging masses. Using the acceleration you measured for 5g on the mass hanger, calculate what the force of friction must have been for Newton’s 2nd law to give exactly this acceleration.

2. You prop up the end of the track with the pulley by 2˚, and put 100 g on the mass hanger. Find the acceleration of the cart, assuming that the cart has a total mass of 750g.