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ENDURING UNDERSTANDINGS

* A force is a push or pull in a given direction due to applied energy.

* Inertia is the tendency of an object to resist change in its motion.

* Gravity is the force that pulls an object toward a rotating body.

* Friction is the force that one surface exerts on another when the two rub against each other.

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WHAT IS FORCE?

* Force is a push or pull, described by its strength the direction in which it acts.

* Force is measured in Newtons

Newton = mass x acceleration

1 N = 1 kg . m/s2

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* Net force is the combination of all forces acting on an object.

NET FORCE

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BALANCED vs UNBALANCED FORCES

* Unbalanced forces acting on an object result in a net force, causing a change in an object's motion.

* Equal forces acting on an object in opposite directions are called balanced forces, and do not change the object's motion.

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What forces do you see in this picture?

Are the forces balanced or unbalanced?

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FRICTION

* The strength of the force of friction depends on the types of surfaces involved and how hard the surfaces push together.

1. Static friction is the force acting on objects that are not moving. Static friction must be overcome to begin moving an object.

2. Sliding friction occurs when solid surfaces slide over each other.

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4. Rolling friction occurs when an object rolls over another.

3. Fluid friction occurs when an object moves through a fluid.

* The force required to overcome fluid or rolling friction is much less than that required to overcome sliding friction.

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TYPES OF FRICTION

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GRAVITY

* Gravity is the force that pulls an object toward a rotating body.

* Newton’s Law of Universal Gravitation states: "The force of gravity acts between all objects in the universe."

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MASS VS DISTANCE

* Gravity between 2 objects depends on mass and distance.

* The greater the mass of a body, the stronger its force due to gravity.

* The equation for the force of gravity between 2 objects is:

g = G

where G is Einstein’s Universal Constant equaling 6.67 x 10 ­11 N . kg2/m2

mass1 x mass2r2

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Try it . . .

A moon with a mass of 9 x 1015N orbits a planetwith a mass of 6 x 1027N at a distance of 100,000 km. What is the force gravity between the 2 bodies?

g = 6.67 x 10­11N x m2 9 x 1015kg x 6 x 1027kg

kg2 (100,000km)2

g = 6.67 x 10­11N x m2 54 x 1042kg2

kg2 (100,000,000m)2

1.

2.

g = 6.67 x 10­11N xm2 5.4 x 1043kg2

kg2 (1 x 108m)2 g = 6.67 x 10­11N x 5.4 x 1043

1 x 10164.3.

g = 6.67 x 10­11N x 5.4 x 1027 g = 36.018 x 1016N5. 6.

g = 3.6018 x 1017N7.

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FREE FALL

* When the only force acting on a falling object is gravity, it is said to be in free fall.

* In free fall, the force of gravity is an unbalanced force that causes objects to accelerate.

* The force of acceleration due to gravity on Earth is 9.8 m/s2

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GRAPHING GRAVITY

Acceleration due to gravity is plottedon a speed versus time graph

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AIR RESISTANCE

* Air resistance is a type of fluid friction that falling objects experience.

* Air resistance is an upward force, opposite of gravity.

* Air resistance increases as velocity increases.

* Terminal velocity is the greatest velocity that an object reaches; such as when air resistance equals the force of gravity on a falling object, so although the object continues to fall, it is no longer accelerating.

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FALLING MOTION

A projectile falls at the samerate as an object in free fall

In the absence of friction,all objects free fall at the

same rate regardless of mass

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NEWTON’S LAWS

* Newton’s First Law: Inertia is the tendency of an object to resist change in motion. Also stated as, "An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an unbalanced force."

* Newton’s Second Law: Force equals mass times acceleration.

F = ma

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NEWTON’S THIRD LAW

* If an object exerts a force on another object, the second object exerts a force of equal strength in the opposite direction.

In other words, for every action there is an equal and opposite reaction.

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MOMENTUM

* Momentum deals with an object's inertia. The higher the velocity and mass, the more momentum an object has.

Momentum = mass X velocity measured in kg * m/s

* The Law of Conservation of Momentum states: “The total momentum of any group of objects remains the same unless outside forces act on the objects.” When 2 objects collide in the absence of friction, momentum is not lost.

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Momentum of 2 Moving Objects

* When a fast­moving object hits a slower moving object traveling in the same direction, the slower moving object will accelerate to the speed of the fast­moving object, and the fast­ moving object will slow to the speed of the slower moving object. No momentum is lost, except to friction.

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Momentum of 1 Moving Object

* When a moving object hits a stopped object its motion transfers to the stopped object, and the first moving object will stop moving.

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Momentum of 2 Connected Objects

* When a moving object hits and connects with a stopped object, half of its momentum transfers to the other object and they travel together with twice the mass but half the velocity, so total momentum is conserved.

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CENTRIPETAL FORCE

* Centripetal Force is a balance between the force of gravity of one object and the inertia of the other; or a force that causes an object to move in a circle, as one body orbiting another.

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Click on the yellow boxes to reveal the resulting speeds of the objects.

Connecting objects. One moving one stopped object. One fast one slow object.

Click on the red lines to reveal the resulting momentum of the objects.

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