Kinematics – the study of how things move

Dynamics – the study of why things move

Forces (the push or pull on an object) cause things to move

Newton proposed three Laws of Motion that allow us to understand how forces cause things to move

Four Basic Forces

Applied force (Fa) – a force that is done by an external cause (agent). It can be any direction.

Gravitational force (Fg or W) -- the force caused by gravity (weight). It always acts downward.

Frictional force (Ff) -- a force that opposes motion and slows down objects. It is always parallel to the surface.

Normal force (Fn) – the force exerted by a surface on which an object is resting. It is always perpendicular to the surface.

A free body diagram (FBD) shows all of the forces that are present on an object both in the horizontal and vertical direction.

Net force (ΣF) – the sum of all forces that act on an object.

Example: book being pushed on

table

Σ Fx = Fa - Ff

Σ Fy = Fn - Fg

FaFf

Fn

Fg

book

book

FBD Net Force Equations

Mass

•A measurement of an object’s quantity of matter

•A measurement of an object’s inertia

Inertia – the tendency of objects to maintain their state of rest or to maintain constant velocity

Example: car slams on brakes and items on seat fall to floor

Objects with a larger mass have a greater inertia. Therefore, they are harder to accelerate (speed up or slow down)

Inertia can give the impression that forces are being applied.

Force is not being applied to rider. Rider is moving at constant velocity because of inertia.

Newton’s First Law – an object at rest tends to stay at rest and an object in uniform motion tends to stay in uniform motion (constant velocity) unless acted upon by a net external force

Newton’s First Law is also known as the Law of Inertia.

Newton’s Second Law – the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass

Newton observed some things about accelerating objects:

•The bigger the force, the greater the acceleration

•The larger the mass, the smaller the acceleration

Σ F = ma

F = force (Newtons)

m = mass (kg)

a = acceleration (m/s2)

What net force is required to bring a 1500-kg car to rest from a speed of 28 m/s within a distance of 55 m?

A 70-kg person traveling at 100 km/hr strikes a parked car. At the instant of impact, the seat belt restrains the person with a force of 21,000 N bring them to rest in the car. How far does the person travel before coming to rest?

When a force is applied to an object, it is always exerted by another object.Examples: a hammer hits a nail

a child pulls a sled an apple is pulled to the Earth

Newton believed that the “force-providers” also are “force-receptors.” Examples: the nail pushes back on the hammer

the sled pulls back on the child the Earth is pulled to the apple

If every force has an equal and opposite force, why dos objects ever move?

The forces are NOT exerted on the same object.

Newton’s Third Law – Whenever one object exerts a force on an second object, the second exerts an equal force in the opposite direction on the first

Evidence: Hammer causes the nail to accelerate (+ force) while the nail causes the hammer to decelerate (- force)

Example: If a hammer exerts a 50-N force on a nail, the nail exerts a 50-N force on the hammer in the other direction.

Weight and Normal Force

• A measure of the gravitational force on an object

• Always directed downward (toward the center of the Earth

Fg = mgA person’s mass does not change, but his weight does depending on the magnitude of gravitation force.

An average man has a weight of 686 N on the Earth.

1. What is the man’s mass?

1. What would his weight be if he was standing on the moon (ag = 1.6 m/s2)

A person pulls upward on string attached to a box with a force of 150 N. The box has a mass of 12 kg. Does the box move upward and if so, with what acceleration does it move?

• A contact force that is perpendicular to the surface

• The force that pushes up on the object resting on the surface

Since the statue is at rest FN is equal and opposite to FG. FN has another equal and opposite force (F’N is reaction force on table)

Friction

Caused by a rough surface.

Object on a rough surface actually has to move up and down because the two rough surfaces catch on each other.

Because energy is used to move the moving object up and down, less energy is used to move the moving object forward.

Friction is the resistance that an object experiences when moving.

The force of friction is influenced by two factors – the surface on which an object is moving and the weight (gravitational force) of the object.

Ff = μFn

Coefficient of friction (μ) – indicates the “roughness” of the surface. Unique to each surface.

Typically, the normal force of the object is just equal and opposite to the gravitational force (but not always).

Two Types of Friction

Static (Stationary Object)If an object is at rest, an applied force has to exceed the maximum static frictional force of the object for it to move

Ffs = μsFn

Kinetic (Moving Object)If an object is moving, there is a kinetic frictional force that opposes motion (always less than static frictional force)

Ffk = μkFn

applied static friction

box at rest

applied kinetic frictionbox in

motion

A 10.0-kg box rests on a horizontal floor. The coefficient of static friction is 0.40 and the coefficient of kinetic friction is 0.30. Determine the maximum static frictional force and the kinetic frictional force. Would the box move if a 10 N force was applied? If so, what would be its acceleration?

Would the box move if a 40 N force was applied? If so, what would be its acceleration?