# Homework Help: Confusing free-body diagrams help

1. Jan 6, 2012

### Kinermatics

1. The problem statement, all variables and given/known data

1. A truck is pulling a car on a horizontal highway using a horizontal rope. The car is in neutral gear, so we can assume that there is no appreciable friction between its tires and the highway. As the truck is accelerating to highway speeds, draw a free-body diagram of the truck. (Assume that the truck is accelerating to the right.)

2. A large box containing your new computer sits on the bed of your pickup truck. You are stopped at a red light. The light turns green and you stomp on the gas and the truck accelerates. To your horror, the box starts to slide toward the back of the truck. (Assume that the truck is accelerating to the right.) Draw clearly labeled free-body diagram for the truck. (The bed of the truck is not frictionless.)

2. Relevant equations
Just drawing

3. The attempt at a solution
1. I know i have the weight and the normal force correct (weight pointing down and normal force pointing opposite) however I can't seem to figure out where to put: the Tension force and the Friction force. My first instinct was to put the friction force left (isn't the direction of friction opposite that of motion) and to put the Tension force left (the tension is always pointing towards the rope). Idk somewhere i did something wrong.

2. This part is very confusing. Like the previous problem i know i got the weight and normal force part correct. But for this question they want to know the direction and orientation of: 1. the box's friction force on the truck bed 2. reaction force to tires' backward push 3. reaction to the normal force on the box from the truck.

Here are better illustrations of my attempt:
1.http://img685.imageshack.us/img685/7545/95398466.jpg [Broken]

2.http://img804.imageshack.us/img804/7533/51523982.jpg [Broken]

Last edited by a moderator: May 5, 2017
2. Jan 6, 2012

### Kinermatics

labels for 1:
nT = Normal Force
wT = weight of truck
f = friction force
T = tension force

labels for 2:
nground = Normal Force
nB = Reaction to the normal force on the box from the truck
wT = weight of truck
fk = the box's friction force on the truck bed
fread = reaction force to tires' backward push

3. Jan 6, 2012

### PhanthomJay

If both the friction force and tension force acted left, the net force would be left, but the truck is accelerating right. You are correct about the direction of the tension force. Remember that friction force is always opposite to the direction of the relative motion or pending motion beween the surfaces, and that the wheels of the truck are turning clockwise as you view them from the right side (that's a backward push of the tire on the road, as you noted in question 2 below).
Your free body diagram looks good. F_road is the static friction force between the road and the tires.

4. Jan 6, 2012

### SammyS

Staff Emeritus
For #2) You show the box (via its friction force) exerting a forward force on the truck. How does it do that?

5. Jan 6, 2012

### Kinermatics

for #1 should the frictional force be the same length as the tension force? Cause when i put it in the direction towards the right (and your explanation made a lot of sense, thank you!) it counted it as wrong.
and for #2 the mp counted the diagram wrong, so i guess there is something we missed?

6. Jan 7, 2012

### PhanthomJay

If they were the same length, there would be no net force, but the truck is accelerating right.
Yes, we missed SammyS' comment in Post #4

7. Jan 7, 2012

### Kinermatics

Hmm it must be something I'm doing then because here are my attempts so far on #1:

my mind is out of ideas of what the answer could be lol. need some help :"(
and for #2 (taking into consideration SammyS' comment) i put fk (the box's friction force on the truck bed) toward the right because the friction force would have to be in that direction for the box to not move. Or should fk = 0 since when your drawing free body diagram you only include forces acting and that body?

8. Jan 7, 2012

### SammyS

Staff Emeritus
The truck exerts a force on the box which is to the right.

Therefore, the box exerts a force on the truck which is to the   ?   .

You're drawing free body diagrams for the truck, not the box. What forces are exerted on the truck ?

9. Jan 7, 2012

### Kinermatics

I figured out #2! Thank you good sir!. You where right, I just forgot that the net force for y = 0 which made normal force greater :).

I'll get back to you guys on #1 lol