# EK 30 min exam question

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### TommyTman

##### Full Member
7+ Year Member
Hey guys I attached a pic of question 20 in the 30 minute exams. Was hoping someone could clarify why the net force is in the given direction. Thanks.

#### Attachments

• Screen Shot 2018-04-12 at 4.41.30 PM.png
468.3 KB · Views: 30

#### sapientnarwhal

##### Full Member
5+ Year Member
@TommyTman

Question stem indicates a component of it's acceleration is in the x-direction following the application of a 3rd force. The sum of the forces along the x-axis is negative in this case. Recall that Ftot = Fa + ... + Fn = ma.

While the RBC may still be traveling in the +x direction, it is accelerating in the opposite direction (i.e. decelerating in the +x direction).

In response to @NITRAS , The question stem does not indicate anything as to the nature of the y-component of acceleration after the 3rd force has been applied. The y-direction force components do not necessarily cancel out, but we do know that the net force could be in quadrants 2,3, or along the x-axis because each would have a component of force in the -x direction.

Last edited:

#### NITRAS

##### Hospitalist
5+ Year Member
The cell is accelerating in the x plane, therefore the y components of the two forces are a net zero. The net force is +x.

The addition of a force in the -x direction should accelerate the cell in the -x direction.

#### TommyTman

##### Full Member
7+ Year Member
@TommyTman

Question stem indicates a component of it's acceleration is in the x-direction following the application of a 3rd force. The sum of the forces along the x-axis is negative in this case. Recall that Ftot = Fa + ... + Fn = ma.

While the RBC may still be traveling in the +x direction, it is accelerating in the opposite direction (i.e. decelerating in the +x direction).

In response to @NITRAS , The question stem does not indicate anything as to the nature of the y-component of acceleration after the 3rd force has been applied. The y-direction force components do not necessarily cancel out, but we do know that the net force could be in quadrants 2,3, or along the x-axis because each would have a component of force in the -x direction.

Super helpful explanation. Thanks for breaking it down.