PHYSICS: Mechanical Equilibrium

What usually comes to mind then you hear the

phrase: “mechanical equilibrium”?

Imperial Walkers?

Mechanical, but probably not…

PHYSICS: Mechanical Equilibrium

What usually comes to mind then you hear the

phrase: “mechanical equilibrium”?

Whiney Jedi?

Again, probably not…

PHYSICS: Mechanical Equilibrium

What usually comes to mind then you hear the

phrase: “mechanical equilibrium”?

Asthmatic super-villains?

Awesome movie effect, but no…

PHYSICS: Mechanical Equilibrium

What usually comes to mind then you hear the

phrase: “mechanical equilibrium”?

Duh, silly…

FORCE!

QUESTIONS AND ANSWERS WITH EINSTEIN

Q: What is a force?

A: A force is any external push or pull exerted on an object.

QUESTIONS AND ANSWERS WITH EINSTEIN

Q: What forces are acting on the book in the picture

below?

A: Weight and Normal

normal force

weight

FORCESFORCE – any external push or pull exerted on an

object

WEIGHT – the effect of gravity acting on the mass of an object

• Weight could also be thought of as the force of gravity

NORMAL – the force of a surface pushing back on an object

• A normal force is also known as a supportive force.

• A normal force is always PERPENDICULAR to the surface.

FORCESPHYSICS A

ccording to G

AB

E

What is a force?

For some reason, I always thought that force was a power that balanced out

good and evil in the universe.

FORCESPHYSICS A

ccording to G

AB

E

What is a force?

Luke…use the force.

May the force be with you…

I guess I gotta quit watching so much T.V.

FORCESNET FORCE – the combination of all forces acting

on an object• Since force is a vector quantity, the direction of the forces

must be considered when figuring the net force.

EQUILIBRIUM – the fancy name for when the net force equals zero

• Static Equilibrium occurs when an object is at rest.

• Dynamic Equilibrium occurs when an object is moving at a constant velocity.

QUESTIONS AND ANSWERS WITH EINSTEIN

Q: What type of motion is occurring when the net force

equals zero?

A: Zero net force means that an object is at rest. Thus, there is no

motion.

A: Zero net force could also mean than an object is moving at a

constant speed.

OR…

FORCESNET FORCE – the combination of all forces acting

on an object• Since force is a vector quantity, the direction of the forces

must be considered when figuring the net force.

EQUILIBRIUM – the fancy name for when the net force equals zero

FREE-BODY DIAGRAM – a sketch used is to illustrate the forces

acting on an object

• Free-body diagrams use arrows to indicate the magnitudes and directions of the forces.

QUESTIONS AND ANSWERS WITH EINSTEIN

Q: What forces are acting on the book in the picture

below?

A: Weight, Normal, and Friction

normal force

weight

friction

QUESTIONS AND ANSWERS WITH EINSTEIN

Q: Is there motion in this situation? Is there a net

force?

A: No motion implies that the net force equals zero.

normal force

weight

friction

FORMULAS FOR-m-U-lasMathematical Version of Net Force

If an object is in a state of mechanical equilibrium then the net force on the

object is zero:

0 FNOTE: The “sigma” () notation means that you need to “sum up” the forces.

FORCES• Forces are vector quantities. This means that:

EXAMPLE: Find the net force on the box. Assume it stays on the ground.

1) The direction is important (FBD helps here).

2) The magnitude may have components in two dimensional situation (FDB helps here too).

3) Different forces acting on the same object can be combined in differently to yield the net force.

20 N10 N 30 N

FORCES

EXAMPLE: Find the net force on the box. Assume it stays on the ground.

20 N10 N 10 N

• Forces are vector quantities. This means that:1) The direction is important (FBD helps here).

2) The magnitude may have components in two dimensional situation (FDB helps here too).

3) Different forces acting on the same object can be combined in differently to yield the net force.

FORCES

EXAMPLE: Find the net force on the box. Assume it stays on the ground.

10 N10 N 0 N

• Forces are vector quantities. This means that:1) The direction is important (FBD helps here).

2) The magnitude may have components in two dimensional situation (FDB helps here too).

3) Different forces acting on the same object can be combined in differently to yield the net force.

FORCES

EXAMPLE: Find the net force on the box. Assume it stays on the ground. 20 N

30°

17.3 N

• Forces are vector quantities. This means that:1) The direction is important (FBD helps here).

2) The magnitude may have components in two dimensional situation (FDB helps here too).

3) Different forces acting on the same object can be combined in differently to yield the net force.

FORCES

EXAMPLE: Find the net force on the box. Assume it stays on the ground. 20 N

30°

27.3 N10 N

• Forces are vector quantities. This means that:1) The direction is important (FBD helps here).

2) The magnitude may have components in two dimensional situation (FDB helps here too).

3) Different forces acting on the same object can be combined in differently to yield the net force.

PHYSICS IS

PHUN!