Monthly Archives: May 2016

Week 3 ET 521

Create a blog with one posting reflecting an on-going conversation you would like learners to have in your classroom.
Here is my “conversation” I would like learners to have…
Student one “Joe”
This week, we worked on Newton’s Laws problems and I think I have a really good handle on how to solve them. I understand that the steps to solve any force problem are:
1. Draw a free body diagram and identify all forces
2. Based on the free body diagram and the problem, determine for what I am trying to solve, 3. identify the appropriate equations to use.
I can solve problems when an object is moving horizontally, but I cannot figure out what to do when an object is falling through the air! These problems should be easier because all object fall at the acceleration due gravity (9.81 m/s/s downward), right? I am hoping that one of my classmates can help me figure out where I am going wrong!
Here is the problem that I am stumped on… It is from the worksheet we did in class today, #4).
Determine the weight, the net force, and the air resistance acting on a skydiver that has a mass of 76.0 kg if the acceleration is 2.00 m/s/s upward?
I can figure out the weight, but not the net force, isn’t it the same as the weight since it is free falling? (because it falling through air?) Here is a picture of my FBD

FBD_free_fall

Thanks! Joe

Student two response

Happy to help Joe! First you are absolutely right in terms of the steps to take to solve the problem, but you are forgetting one important piece when it comes to falling object. Objects only accelerate at “g” in the ABSENCE of air resistance. This object clearly has air resistance acting on it since the acceleration is 2 m/s/s upward.

See my FBD below.

fbd_w_air_resistance

We know the mass & the acceleration, so we can find the Net Force by using Newton’s 2nd law Fnet = ma.

This next idea is really important…acceleration is upward, so the net force is also upward (because the the direction of acceleration always matches the direction of the net force).

Then we use the idea that the Fnet =  ΣF = Fair + Fgrav

Look at my picture for how I solved it.

I hope this helps! 🙂

Tanya