Physics 11

Chapter #4 - Dynamics

Mr. Jean

The plan:

• Video clip of the day

• Gravity

• Introduction to Forces

Dynamics:

• Kinematics - the study of how objects move (velocity and acceleration).

• Dynamics - the study of why objects move.

Sir Isaac Newton:• The connection between

acceleration and its cause can be summarized by three statements known as Newton's laws of motion.– the cause of acceleration is a

force.

• Newton published the three laws of motion in 1687

Forces:

• What is a force?– a force is defined as a push or a pull.

• Some forces occur when one object touches another. Some objects are pulled by a force called gravity. Some forces, like that of gravity, cause an object to accelerate.

Forces:

• Forces are vectors thus they will need a direction to indicate how the force is acting on an object.

• Example: – Buoyant Force (+)– Gravitational Force (-)

Natural forces:

• 1) Gravitational force: an attractive force that exists between all objects. It is the weakest force.

• 2) Electromagnetic force: these forces result from electric charge. Charged particles at rest or in motion exert electric charges on one another and particles in motion produce magnetic forces on each other. It is very large compared to the gravitational force.

3) Strong nuclear force: holds the particles in the nucleus of an atom together. It is the strongest force but only acts over the distance of a nucleus.

4) Weak nuclear force: it is a form of the electromagnetic force. And is involved in the radioactive decay of some nuclei.

Demo #1:

• Mystery Physics Box A

• Mystery Physics Box B

Newton’s Laws of Motion:

• Newton's First Law: Inertia– An object with no net force acting on it remains at

rest or with a constant velocity.

Newton’s Laws of Motion:

• Newton's First Law: Inertia

– an object with no force acting on it moves with constant velocity.

– “Things at rest stay at rest and things in motion stay in motion”

Demonstration 2:

• Rolling a ball across the floor.

• What’s the force that is keeping the ball rolling?

Demonstration 2

• Paul Hewitt conceptual physics quotation on this demonstration.

• “No force, we don’t know why it keeps going. But we call that ignorance Inertia. No body know why it keeps going, we just know that it does.”

• Even today people still believe that a force is required to keep an object moving.

Balanced Forces:

• Inertia - tendency for an object not to change its motion.

• Inertia is the tendency for an object not to change its motion.

Inertia Demonstrations:

• String & weight demonstration

Balanced Forces:

• A marble is accelerated initially by an force in the rightward direction. The marble rolls along a frictionless level surface. – What will happen to the objects velocity?– What role does acceleration play?

Unbalanced Forces: • A marble is accelerated by an force in the

rightward direction. The marble rolls down an inclined plane. – What will happen to the objects velocity?– What role does acceleration play?

Unbalanced Force: • A marble is accelerated by an force in the

rightward direction. The marble begins to roll up the inclined plane. – What will happen to the marbles velocity?– What role does acceleration play?

Newton’s Second Law:• “to move an object with mass you

need a force.” • the acceleration of an object is

directly proportional to the net force on it and inversely proportional to its mass.

• Force = (mass)(acceleration) • F = m a

International Units (SI)

Quantity Symbol SI Unit

Force

Mass

Acceleration

F

m

a

N (Newtons)

Kg (Kilograms)

m/s2 (metres per second squared)

• F = m a

• A net force acting on an object causes it to accelerate.

• The larger the mass of an object, the smaller the acceleration. Thus a massive object has more inertia than a less massive object.

• Unit of force - a force that causes a mass of one kg to accelerate at a rate of one meter per second squared is one newton (N).

• F = ma = (1.00kg)(1.00m/s2 ) = 1.00N

• 1N = 1kgm/s2

Common misconception:

•WEIGHT DOES NOT EQUAL MASS!

Weight: • An objects weight is Fg is the product of its

mass m, and the acceleration due to gravity, g.

• Fg = mg

• Fg = force of gravity in newtons (N)

• m = mass in kilograms (kg)• g = acceleration due to gravity (m/s2)

Weight calculation:

• You have an object which has a mass of 40 kg.

• What is the objects weight?

Weightlessness:

• Demonstration– Does it still have a mass? – Does it still have a weight?

– Why?

To do for tomorrow:

• P. 137

• Questions 1-4