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Lab 2

Velocity and acceleration on an incline

Goals: In a series of experiments, we first investigate if acceleration due to gravity isindependent of the mass of the object. We then determine the value of the acceleration due togravity.

Theory: According to Newton's law an unbalanced force results in acceleration. Therefore, anobject kept on a frictionless inclined plane will move down the plane. We experience this firsthand while skiing downhill or bicycling down an incline. We know from experience that steeperthe slope, the faster the acceleration. However, you may wonder if the acceleration is similar ifyou were twice or half as heavy? In this laboratory, we investigate such a situation.

We use a cart rolling on an inclined plane to study if there is any dependence of the mass on the

acceleration due to gravity, and then to calculate the acceleration due to gravity.

Fig. 1Consider the cart shown in the figure above. The unbalanced component of the force along theinclined plane results in the cart accelerating in that direction, i.e.

Mg sin θ = Ma

Where, M is the total mass of the cart, g is the gravitation acceleration, θ is the angle of

inclination of the plane, and a is the resulting acceleration. Therefore the acceleration on aninclined plane is a = g sin θ.

The velocity of an object under constant acceleration will increase linearly and is:v = v0 + a t,

and, the distance s it travels in that time is given bys = v0t + 1/2 a t2.

θMg

Mg cosθ

Mg sinθ

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Apparatus: Cart, masses, meter stick, photogates with stopwatch, and weight scale.

Procedure:1. Configure the Aluminum track as shown in Fig. 2. Set the inclination of the track to

approximately 20, but measure h and d to determine the angle θ accurately. (Note that the

inclination of the track is greatly exaggerated. The experiment will work equally well atlarger inclination angles but you may encounter more difficulties in timing the travel usingthe stop-watch.)

Fig. 22. Place the cart on the track at least 80 cm from the end-stop. Try a few trail runs to see cart

behaves as you allow it to roll on the incline. Do you observe that the cart starts from rest andthe speed increases and appears to have its maximum velocity just as it collides with the endstop?

3. Next use the stop-watch (on the photogate timers) to measure the time the cart takes to travelthe distance s down the incline. Repeat your measurements at least six times (each partner

must do at least two trails.) Note the individual times and the average time along with thedistance s in your notebook. Are all the recorded times the same? How confident are you thatyou started the stopwatch as the cart was released and stopped the timer when the cartcollided with the end-stop?

4. To avoid human errors introduced in measuring time, we will use the photogate timers. Thetimers consist of two photogates. The timer is triggered when the light beam in the first

θ

d

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photogate is interrupted and will stop when the light beam in the second photo diode isinterrupted. Ensure that the two photodiodes are connected to each other, and the modeswtich is set to "pulse" and the memory is turned on. You can test that the timer works bypassing your hand between the U shaped arms and across the two beams. (The red LCDindicator turns on when the beam is interrupted.) Now repeat part 2 by using the photogate

times. You will have to adjust the height and location of the gates. The cart should cut acrossthe first photogate just after you release the cart and then cut across the beam of the secondphotogate just before it hits the end-stop. Check with the instructor if your setup is workingproperly. Do the measurements at least three times and calculate the average value.

5. Next, decrease the distance s in steps of 20 cm and measure the time t for at least fivedistances. (Each trial should be repeated 3 times to minimize error in determining time t.)Plot the distance s versus time t, and the distance s versus the square of the time t. Can youdraw a straight line through any of the graphs? Should this line pass through the origin?Determine the slope and hence the acceleration. According to our discussion earlier, dividingthe acceleration by the sine of the angle of inclination should give us the acceleration due togravity. How do these values compare?

6. Draw a smooth curve that best describes the graph. The curve should pass through the origin.Draw tangents to the curve at four equally spaced time intervals to the graph. The slope ofthe line corresponds to the velocity of the cart at that time. Why? Plot the obtained velocityversus time t. Can you fit a straight line through your data? Does it increase, decrease orremain constant with time? How does this compare with qualitative observation that thevelocity increases as the cart travels further down the track?

7. Next, we investigate if the acceleration depends on the mass of the cart. Use the two suppliedmasses and chose the same distance d as you had in your first trial (step 3). Measure the timet for at least 6 trials as before. Calculate the acceleration directly using the formula a = 2s/t2

(here v0 = 0) which you have already verified earlier. Is the acceleration corresponding to thethree masses of the cart constant?

8. Next, increase the angle of inclination of the track by increasing the height h by 1-2 cm. Youmay use only the cart itself without the additional masses. Measure the time taken to travelthe same distance s and calculate the acceleration. Does the acceleration change? Measurethe acceleration for at least two more angles of inclination (four in all) and plot theacceleration versus sine of the angle of inclination. Does the data fall on a straight line?Determine the slope and compare it to the acceleration due to gravity quoted in your textbook(9.8 m/s2.)