Motion Analysis

• By Leung Ho Fai 6B(35)

• Siu Long Kan 6B(25), group2

• In this motion, we are going to analysis the motion of “flying disc” in a projectile path.

How can we analysis it?

• We throw a disc and use a digital camera to record the motion of the “flying disc” , then make a v-t graph to find the velocity change of it.

Velocity-time graph of the motion of the “flying disc”

Analysis

• For the first 0.9 second, we are getting ready to throw the disc.

• Before we throw the “flying disc” out. We have to grab the disc towards our body so that we can throw it away. This can explain why the velocity became negative around the first 0.6 second as we have taken the right hand side to be positive.

• From 0.6 to 0.9 second, the disc has just be thrown above the horizontal level, so there exists a positive sign on that instant as the upward motion is taken to be positive.

Analysis II

• From 0.9 to 1.25 second, the disc keeps rotating itself below the horizontal level about a fixed position in a very short period, so the velocity decreases.

• From 1.25 to 1.5 second, the disc moves forward in a steady slope of path with a uniform acceleration.

Conclusion

• This disc-throwing is a counter example since it is both a projectile and a rotated motion.

• In this case, we consider the projectile motion only.• In projectile motion, the horizontal velocity is being

unchanged at a fixed value throughout the whole journey and the vertical velocity is increasing with required time. At first, the initial vertical velocity is zero.Applying the equation of motion, s=usin0+1/2at2, the vertical velocity will start to increase when the motion is keeping on.

Conclusion II(vertical v)

• However, this example exists the presence of air resistance. The vertical velocity starts to decrease rapidly since s=usin0-1/2gt2. As we have taken downward motion as negative sign, thus it decrease negatively. Finally, when the air resistance force reaches its limits and is balanced by the downward force, net force=mg=ma, free fall of gravity occurs prove that the v-t graph during the period of 1.25 to 1.5 second.

Conclusion III(horizontal v)

• It is no longer constant due to air resistance. This air resistance which is acting upward increase the speed of the disc, causing the above graph results.