Chapter 12 Forces

• Force: Is pull of push that acts on an object• A force can cause a resting object to move, or it can accelerate a

moving object by changing the object’s speed of direction.• Combining Forces: Forces in the same direction add together and forces in opposite directions subtract from one another.Net Force: is the overall force acting on an object after all forces are combined.

Forces

• Units of Force: it is measured in newton (N)• Newton: is the force that causes a 1 kilogram mass to accelerate at a

rate of 1 meter per second each second.

• N = 1 kg-m/s/s

• Balanced forces: When the forces on an object are balanced, the net force is zero and there is no change in the object’s motionUnbalanced Forces:When unbalance force acts on an object, the object accelerates.

The Fundamental Forces of NatureThe four fundamental forces of nature:

1.Gravity 2.Electromagnetism3.Weak Nuclear Force4.Strong Nuclear Force

• Gravity is always attractive, and acts between any two objects.• Electromagnetism causes repulsion and attraction between

charged particles, such as the protons and electrons in matter. This gives rise to almost all of the forces .

• Weak and Strong Nuclear forces are important in understanding how atomic nuclei are held together and certain forms of radiation –

Friction

• Friction is a force that opposes the motion of objects that touch as they move past each other.

• There are four types of Friction:• Static friction: is the friction force that acts on objects that are not

moving.• Sliding Friction: force that opposes the direction of motion of an

object as it slides over a surface.• Rolling Friction: friction force that act on rolling objects.• Fluid Friction: opposes the motion of an object through a fluid.

Newton’s First LawObjects at rest tend to stay at rest.Objects in motion tend to stay in motion.

Newton’s First Law

If there is no net force on an object, the object won’t accelerate.

Newton’s First Law

The converse is also true:If an object is not accelerating, then there is no

net force on it.

InertiaNewton’s First Law says that objects do not accelerate

spontaneously.This property of matter, which causes objects to resist

acceleration, has been named “inertia”.Newton’s First Law is often called the Law of Inertia.

Newton's 2nd Law of Motion• If a net force is applied to an object, that object

will accelerate in the direction of the force.

Force = Mass x Acceleration (N) (kg) (m/s/s)

    F = M x A

72 = 6 x 12

     F = M x A

72 = 12 x 6

     F = M x A

72 = 18 x 4

 

Weight is a measure of gravitational pull, which is a force. So if F = M x A, then weight also = M x A

More specifically though, the acceleration for weight is due to gravity, so you can automatically put 9.8 m/s/s in the equation.

So the equation ends up being:

Weight = Mass x 9.8

Projectiles A projectile is any object which has

been shot or thrown through the air, with a 1 time force.

Projectiles follow a perfectly curved path through the air (in a vacuum).

• This perfectly curved path is called a parabola.

• A projectile has both horizontal force and vertical force applied to it. One makes it want to go up and down, the other wants it to go sideways.

The combination of the 2 forces creates the curved path.

Falling Objects• A falling object will accelerate

downward at 9.8 m/s/s (in a vacuum). However, in real life air resistance will gradually slow it down.

• Terminal velocity is the top speed reached by a falling object. At terminal velocity air resistance is equal to gravitational pull.

Circular Motion• According to Newton's

1st Law, an object will move in a straight line unless a net force causes it to do otherwise.

• Therefore, an object moving in a curved or circular path, like a tetherball, must have a force pulling it in a circle.

• A centripetal force is a force which pulls an object toward the center of a curved or circular path.

• For example, the friction between the tires and the pavement will pull the car through the curve.

• What if the road were icy?

Other examples of centripetal force:

• A centrifuge• Washing machine• Fighter jets• Hammer throw

• When an object is pulled in a curved or circular path, this is called centripetal acceleration, since it is changing directions.

Newton's 3rd Law of Motion

• For every force, there is a force equal in size, but opposite in direction.

• Ex.: -hitting a wall

-rocket propulsion

-the kick of a gun

-an arrow being shot

-hard hit in football

-swimming

-pogo stick

Momentum

• Momentum is inertia in motion.

It is determined by how big an object is and how fast it is moving.

p = m x v(kg x m/s)

A stationary object has zero momentum.

Law of conservation of momentum, if not net force acts on a system, then the total momentum of the system does not change