Chapter 3 – Forces and Motion

3.1 – Forces, Mass, and Acceleration

Isaac Newton -- physicist & mathematician.

Formulated basic laws of mechanics, discovered the universal law of gravitation, and invented calculus.

A force is a push or pull on an object. It is also any action that has the ability to change the motion of an object.

Units of force: pounds and newtons

1 lb = 4.48 N

A person weights 115 pounds. How much does the person weight in Newtons?

115 lbs x 4.48N = 515.2 N

1 lb

Differences:Force – push or a pull on an objectMass – amount of matter in an object

Weight – changes based on gravityMass – same everywhere in the universe

Newton’s First Law of Motion (Law of Inertia)An object at rest will remain at rest or an

object in motion will remain in motion unless acted on by an unbalanced force.

Inertia ability of an object to resist a change in motion.

Inertia of an object has depends on its mass.

More mass = more inertia

The only way to change the mass of an object is to add or take away weight.

Newton’s Second Law of Motion related the force of an object, the mass of an object, and its acceleration. It says that forces cause acceleration and mass resists acceleration. F = ma

All the forces acting on an object is called the net force.

Equilibrium occurs when the forces on an object are balanced, or the net force is zero. No motion in equilibrium.

3.2 – Weight, Gravity, and Friction

Gravity is the force that pulls every mass toward every other mass. Gravity depends on mass of an object.

Weight = mass and force of gravity.

Fw = mg

Friction describes forces that result from motion between objects. Friction always works against the motion.

Types of Friction:Air FrictionSliding FrictionViscous FrictionRolling Friction

To find out the total force on an object, subtract the friction force from gravity. What is left is called the net force. net = total.

Motion Dot Diagram

Use a dot to represent the moving object.

Speeding up Must

Draw

Slowing Down

Rest

Constant

3.3 -- Equilibrium, Action, and Reaction

Newton’s Third Law of Motion states that for every action, there is a reaction equal in strength and opposite in direction.

Action and reaction forces act on different objects. When you throw a ball you apply the force, creating the acceleration. The reaction is the ball pushing back against your hand.

Momentum is the mass of an object times its velocity. Increase the mass or speed of an object, momentum increases.

p = mv

p = momentum m = mass v = velocity

Law of conservation of momentum says that as long as objects are not acted on by outside forces, the total momentum cannot change. Momentum can be + or -.

Rockets and planes use the law of conservation of momentum to move.

A rocket can accelerate in space without touching anything because it throws mass at high speed out the end of the engine. Forward momentum of a rocket is equal to the momentum of the escaping mass ejected from the end of the engine.

Momentum Calculation

A steel ball whose mass is 2.0 kg is rolling at a rate of 2.8 m/s. What is its momentum?

m=2.0 kg v= 2.8 m/s

p=mv

p=(2.0 kg)(2.8 m/s) = 5.6 kg* m/s