Create a FBD that represents the situation below

2 ropes are holding the object up and the object is motionless

Fg (E, B)

Ft1 (R, B)Ft2 (R, B)

Should the forces be balanced?• Yes, because the object is motionless (constant

velocity)• Therefore, net force = 0

But this is different than any of the other problems we have done because for the first time, we’ve encountered a scenario where it’s not completely obvious to the eye exactly how the forces balance

Balanced (obvious to the eye)

Balanced (not so obvious to the eye)

= +

2-D force vector (up & to the right)

Any vector directed in 2 dimensions can be thought of as having an influence in 2 different directions • Made up of 2 partsEach part of a 2-D vector is known as a component vector• The combined influence

of the 2 components is equal to the influence of the single 2-D vector

• Therefore, we can replace the 2-D vector with the component vectors

Component vectors2 horizontal component vectors cancel (balanced)

2 vertical component vectors cancel downward vector (balanced) Much more obvious to

the eye how the forces balance

Now try to create the FBD of this situation

Object is motionless on an incline

Fg (E, B)

FN (R, B)Ff (R, B)

Remember, normal force is perpendicular to surface

Remember, friction force is parallel to surface

We know that the force should balance since the object is motionless (constant velocity)• Net force = 0

Fg (E, B)

FN (R, B)

Ff (R, B)

Fg perp.

Fg parallel

Object slides without frictionWe know that the forces must be unbalanced because the problem does not state that the object is going at a constant velocity• We know from past

experience that objects going down ramp accelerate

• Net force not equal to 0

FN (T, B)

Fg (E, B)

Fg

Fg

F net = 0Fg

By displaying this, we are 100% sure the force that is responsible for this accelerated motion and in what direction• VERY IMPORTANT FOR OUR

UNDERSTANDING OF MOTION!