# Introduction

## Net force affects the motion, postion and/or shape of objects (some important and commonly used forces are friction, drag and deformation).

#### Key Points

• Friction is the force that resists relative motion between two surfaces sliding across each other. Friction converts kinetic energy into heat.

• Drag force is the force that resists motion of an object traveling through a fluid such as air or water. Drag force is proportional to the velocity of the object traveling.

• Deformation forces are forces caused by stretching or compressing a material. Some examples would be springs or elastics.

#### Terms

• The energy possessed by an object because of its motion, equal to one half the mass of the body times the square of its velocity.

#### Figures

1. ##### Drag Force on a Barge

(a) A view from above of two tugboats pushing on a barge. (b) The free-body diagram for the ship contains only forces acting in the plane of the water. It omits the two vertical forces—the weight of the barge and the buoyant force of the water supporting it cancel and are not shown. Since the applied forces are perpendicular, the x- and y-axes are in the same direction as Fx and Fy. The problem quickly becomes a one-dimensional problem along the direction of Fapp, since friction is in the direction opposite to Fapp.

We know that a net force affects the motion, position and shape of an object. It is useful at this point to look at some particularly interesting and common forces that will provide further applications of Newton’s laws of motion. Specifically, we will discuss the forces of friction, air or liquid drag, and deformation.

### Friction

Friction is a force that resists movement between two surfaces sliding against each other. When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into heat. This property can have a dramatic effect, as seen in the use of friction created by rubbing pieces of wood together to start a fire. Friction is not itself a fundamental force, but arises from fundamental electromagnetic forces between the charged particles constituting the two contacting surfaces.

### Drag

Another interesting force in everyday life is the force of drag on an object when it is moving in a fluid (either gas or liquid). You feel this drag force when you move your hand through water, or through the wind. Like friction, the force of drag is a force that resists motion. As we will discuss in later units, the drag force is proportional to the velocity of the object moving through it. We see an illustrated example of drag force in Figure 1.

### Deformation

We now move from consideration of forces that affect the motion of an object (such as friction and drag) to those that affect an object’s shape. If a bulldozer pushes a car into a wall, the car will not move but it will noticeably change shape. The change in shape of an object due to the application of a force is a deformation. Even very small forces are known to cause some deformation. For small deformations, two important characteristics are observed. First, the object returns to its original shape when the force is removed (that is, the deformation is elastic for small deformations). Second, the size of the deformation is proportional to the force.

#### Key Term Glossary

application
the act of putting something into operation
##### Appears in these related concepts:
deformation
A transformation; change of shape.
##### Appears in these related concepts:
elastic
Capable of stretching; particularly, capable of stretching so as to return to an original shape or size when force is released.
##### Appears in these related concepts:
electromagnetic force
a long-range fundamental force that acts between charged bodies, mediated by the exchange of photons
##### Appears in these related concepts:
energy
A quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent.
##### Appears in these related concepts:
fluid
Any substance which can flow with relative ease, tends to assume the shape of its container, and obeys Bernoulli's principle; a liquid, gas or plasma.
##### Appears in these related concepts:
force
A physical quantity that denotes ability to push, pull, twist or accelerate a body which is measured in a unit dimensioned in mass × distance/time² (ML/T²): SI: newton (N); CGS: dyne (dyn)
##### Appears in these related concepts:
Force
A force is any influence that causes an object to undergo a certain change, either concerning its movement, direction or geometrical construction.
##### Appears in these related concepts:
friction
A force that resists the relative motion or tendency to such motion of two bodies in contact.
##### Appears in these related concepts:
gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
##### Appears in these related concepts:
heat
energy transferred from one body to another by thermal interactions
##### Appears in these related concepts:
kinetic
Of or relating to motion
##### Appears in these related concepts:
kinetic energy
The energy possessed by an object because of its motion, equal to one half the mass of the body times the square of its velocity.
##### Appears in these related concepts:
Kinetic Energy
The energy associated with a moving particle or object having a certain mass.
##### Appears in these related concepts:
Law
A concise description, usually in the form of a mathematical equation, used to describe a pattern in nature
##### Appears in these related concepts:
liquid
A substance that is flowing, and keeping no shape, such as water; a substance of which the molecules, while not tending to separate from one another like those of a gas, readily change their relative position, and which therefore retains no definite shape, except that determined by the containing receptacle; an inelastic fluid.
##### Appears in these related concepts:
motion
A change of position with respect to time.
##### Appears in these related concepts:
net force
The combination of all the forces that act on an object.
##### Appears in these related concepts:
particle
A very small piece of matter, a fragment; especially, the smallest possible part of something.
##### Appears in these related concepts:
position
A place or location.
##### Appears in these related concepts:
relative
Expressed in relation to another item, rather than in complete form.
##### Appears in these related concepts:
velocity
A vector quantity that denotes the rate of change of position with respect to time, or a speed with a directional component.
##### Appears in these related concepts:
Velocity
The rate of change of displacement with respect to change in time.