Forces in One DimensionChapter 4

Forces and Motion

A force is a push or pull on an object that causes it to speed up, slow down or change direction.

Force is a vector and is usually written F or .

It is important to distinguish between the object to which the force is applied, which is called the system, and the rest of the world.

Forces are usually classified into two types: Contact forces, where an object from the outside world touches a system

and exerts a force on it (e.g., friction).

Field forces, which exert a force on a system without contacting it (e.g., the gravitational force, the electromagnetic force).

A free-body diagram is a physical model that represents the forces acting on a system.

Newton’s First Law of Motion

Newton’s First Law (aka Law of Inertia): Every object continues in a state of rest or of motion in a

straight line at a constant speed, unless it is compelled to change that state by forces exerted on it.

Inertia is the resistance an object has to a change in its state of motion.

Units of force are newtons (N);

Intro to Forces Newton’s Laws of Motion Forces in Football

What is Mass?

Mass is a measure of inertia—the greater the mass of an object, the harder it is to get it to move. Mass is not volume. For example, your weight on the Moon is different from your weight on

Earth, but your mass is the same in both places,

Mass is not weight. Mass is a fundamental property, weight is not.

Mass is the amount of material in an object.

The SI unit of mass is the kilogram (km).

Net Force and Equilibrium

We may have more than one force acting on an object.

The net force is the sum of all the forces acting on an object But remember that forces are vectors, so we have to add up the

forces vectorially.

When all the forces acting on an object add to zero (i.e., the net force is zero) then the object is said to be in equilibrium.

Forces and Motion Basics

Adding Forces

Force Tutorial

Free Body Diagrams

Common Types of ForcesForce Definition Direction

Friction A contact force that acts to oppose sliding motion between surfaces

Opposite to the direction of sliding

Normal A contact force exerted by a surface on an object

Perpendicular to and away from the surface

Spring A restoring force; the push or pull a spring exerts on an object

Opposite to the displacement of the object attached to the spring

Tension The pull exerted by a string or rope when attached to a body and pulled taut

Away from the object and parallel to the string or rope at the point of attachment

Weight A field force due to the gravitational attraction between two objects, generally Earth and the object

Straight down towards the center of the Earth

Electrical A field force due to the electrical charges on two or more objects

Attractive (unlike charges) or repulsive (like charges) along the line between the objects

Newton’s Second Law

Mass and Acceleration In Chapter 2 we learned that

Force causes acceleration; Newton observed that: The acceleration produced is directly proportional to the net force.

For a given force the acceleration produced is inversely proportional to the mass.

Combining those observations gives us

or Newton’s second law:

The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of an object.

Free Fall and Air Resistance

We’ve seen that objects in free fall all fall with an acceleration equal to g. But that’s true only if we can ignore air resistance.

When the air resistance on a falling object equals its weight, the net force is zero and no further acceleration occurs. At that point the object has reached its terminal velocity.

Felix Baumgartner skydive Altitude 39 km (24 mi) Reached an estimated terminal velocity of 1,357.64

km/h (843.6 mph), breaking the sound barrier

Free-Body Diagrams

A free-body diagram is a diagram showing all the forces acting on an object.

Free-Body Diagram Interactive

Force Exercises

Force Interactive

Example Problem 1

Unfortunately for Vanessa, the wheels on her suitcase are not working. She pulls on the strap in an effort to budge it from rest and drag it to the curbside check-in desk. The free body diagram at the right depicts the forces acting upon the suitcase. Use force values to determine the net force, the mass and the acceleration of the suitcase. The values of the individual forces are:

Fgrav = Fnorm = 207 NFtens = 182 NFfrict = 166 N

Example Problem 2

It's Friday night and Skyler has been assigned the noble task of baby-sitting Casey, his 2-year old brother. He puts a crash helmet on Casey, places him in the red wagon and takes him on a stroll through the neighborhood. As Skyler starts across the street, he exerts a 52 N forward force on the wagon. There is a 24 N resistance force and the wagon and Casey have a combined weight of 304 N. Construct a free body diagram depicting the types of forces acting upon the wagon. Then determine the net force, mass and acceleration of the wagon.

Example Problem 3

Moments after making the dreaded decision to jump out the door of the airplane, Darin's 82.5-kg body experiences 118 N of air resistance. Determine Darin's acceleration at this instant in time. 

Newton’s Third LawAction and Reaction

Forces and Interactions Forces involve interactions between one thing and

another. So forces always come in pairs—the action force and the

reaction force. Newton’s third law:

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Writing this as an equation:

where is the force object 1 exerts on object 2 and is the force object 2 exerts on object 1.

Newton’s Third Law in Action

EVA video

Third Law Video