Newton’s Laws and Forces

Forces

Force = a push or pull– changes speed of an

object– changes direction of an

object– can be touching or long

distance– always acts in pairs.– Units of force: Newton

(N)– 1 Newton = 1 kg m/s2

Types of forces

Contact Forces

Frictional Force

Tension Force

Normal Force

Air Resistance Force

Applied Force

Spring Force

Action-at-a-Distance Forces

Gravitational Force

Electrical Force

Magnetic Force

Balanced vs. unbalanced forces

Forces are vectors which mean that have a certain strength and point in a certain direction.

Forces are Balanced on one object when the vector sum of the forces acting on that object = zero. There will be no change in the motion of the object.

Balanced vs. unbalanced forces

When the vector sum of the forces on one object do not add up to zero, they are said to be unbalanced and an acceleration occurs.

Inertia

Inertia - the natural tendency of an object to remain at rest or to remain in motion with a constant speed in a straight line.

– The object wants to keep doing what it was doing.

Mass – measure of inertia (how much matter is in an object)

– Units: kg– Mass will stay the same

wherever object is

The driver keeps moving forward,

because that is how he was moving.

Newton’s !st Law ( the law of inertia)

NEWTON’S 1ST LAW – in the absence of an unbalanced force, an object at rest will stay at rest, and an object already in motion will stay in motion with a constant speed in a straight line. – Newton says inertia is the tendency to resist the

change of motion

Newton’s 2nd Law of Motion (“Law of Acceleration”)

The acceleration of an object is directly proportional to the unbalanced force acting on it and inversely proportional to the object’s mass. The direction of the acceleration is in the same direction as the unbalanced force.– Mrs. Nixon’s way – “an uneven force causes a

mass to accelerate in the direction of the total force.”

Force = mass x acceleration

F = ma– Units:

Force = Newton (N)Mass = kilograms (kg) Acceleration = m/s/s

You can have a negative sign for force and acceleration. That sign means that the force and acceleration are OPPOSITE to the motion of the object (the object is slowing down)

Gravity, freefall, and falling objects

All objects (regardless of their mass) free fall with the same acceleration -9.8 m/s/s. This is the acceleration caused by the force of gravity, g.

Objects which are said to be undergoing free fall, are not encountering a significant force of air resistance; they are falling only because of gravity.

As an object falls through air, it usually encounters some degree of air resistance. Air resistance is the result of collisions of the object's leading surface with air molecules

Weight

Weight – the measure of the force of gravity acting on an object.

– Weight is a force– Units are in Newton– Will change from place to place

because gravity changes just a special case of Newton’s

2nd law– F w = m x g

Force of weight = mass x acceleration due to gravity

– On earth, gravity = 9.8 m/s2

Third Law (action- reaction)

Newton’s Third Law – for every applied force, there is an equal and opposite force.

– If you push on something that something will push on you the same amount.

– Two forces, two objects every time – Action reaction forces do not cancel each other out because they

do not come from the same object or act on the same object.– Ex: Foot pushes down on floor, floor pushes up on foot. – Does not describe motion

Just because there is no movement doesn’t mean that there are no forces. No movement just means that all of the forces acting on one of the objects balance each other out.

friction

FRICTION: The force that resists motion between two things in contact w/each other– Friction is parallel to the surface– Friction points opposite to the motion– There are 3 types of friction:

– Sliding, rolling and starting

– Friction is good for walking, writing, standing.– Friction is bad for moving furniture.

COEFFICIENT OF FRICTION μ pronounced (mu)

μ= the force required to slide an object at constant speed The perpendicular force of a surface on object

– μ = Force of pull

Force of Weight– There are no units b/c force divided by force.– Usually a number between 0 and 1– Every different combination of surfaces has their own

coefficient of sliding friction– Depends on type of material, texture, and amount of

lubricant