Ch. 2 The Laws of Motion

Lesson 1 Gravity and Friction

Force - A push or pull

we pull on a locker handle push a soccer ball or on the computer keys

Contact force - push or pull on one object by another that is touching it.

Noncontact force - a force that one object can apply to another object without touching it

Some contact sports?

Noncontact sports?

Forces have both strength and direction. can be shown by arrows

The SI unit of force is the Newton (N)

Gravity - an attractive force that exists between all objects that have mass.

Mass - the amount of matter in an object

The law of universal gravitation states that1. All objects are attracted to each other by

a gravitational force.2. The strength of the force depends on the

mass of each object and the distance between them.

As mass increases, gravitational forces increase

As distance increases, gravitational forces decrease

Weight - the gravitational force exerted on an object

Friction - a force that resists the motion of two surfaces that are touching

there are several types:

Static Friction - friction between two objects that are not movingSliding Friction - friction between two objects that are sliding past each otherFluid Friction - friction between a surface and a fluid (liquid or gas) in air - air resistance

Causes of friction - - roughness of the surfaces - weak electrical charges between atoms and molecules

How can friction be reduced? - reduce the contact area between the microscopic bumps on the surface - reducing the surface area (air resistance)

Lesson 2 Newton’s First Law

Review Forces - a push or pull

Net Force - the combination of all the forces acting on an object need reference direction

Balanced Forces - forces acting on an object that combine and form a net force of zero

Unbalanced Forces - forces acting on an object that combine and form a net force that is not zero

Newton’s First Law of Motion Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force.

If the net force on an object is zero, the motion of the object does not change.

Inertia - the tendency of an object to resist a change in its motion

Lesson 3 Newton’s Second Law

Forces change an object’s motion Only unbalanced forces change an object’s velocity

Change in velocity means a change in speed, or direction or both.

Velocity (from last chapter) is speed and direction of an object.

Acceleration is the change in velocity over time.

Unbalanced forces can make an object accelerate by changing its speed, its direction, or both.

Newton’s Second Law

The acceleration of an object is equal to the net force acting on the object divided by the object’s mass.

a = F/m

Circular Motion - any motion in which an object is moving along a curved path

Centripetal Force - in circular motion, a force that acts perpendicular to the direction of motion, toward the center of the curve

Lesson 4 Newton’s 3rd Law

The 3rd law states: when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.

Force Pair - the forces two objects apply to each other

Momentum is the measure of how hard it is to stop a moving object.

The Law of Conservation of Momentum The total momentum of a group of objects stays the same unless outside forces act on the objects.

Momentum Equation momentum = mass x velocity p = m x v

or v = p/m or m = p/v

momentum before collision = momentum after collision

momentum of 2 cars momentum of bothbefore collision = cars after collision

Elastic Collision - objects bounce off each other

Inelastic Collision - objects stick together

A ball with a momentum of 16 kg m/s strikes a ball at rest. What is the total momentum of both balls after the collision?

first ball p = 16 kg m/ssecond ball at rest p = 0

total of both before impact p = 16 kg m/s

since momentum is conserved - same after collision as before, after collision has to be 16 kg m/s also.

The force of a bat on a ball and the force of a ball on a bat are _____ _____.

A 2.0 kg ball rolls to the right at 3.0 m/s.A 4.0 kg ball rolls to the left at 2.0 m/s. What is the momentum of the system after a head on collision of the two balls?

Reference direction - to the right

Total momentum before collision:(2.0 kg)(3.0 m/s) + (4.0 kg)(-2.0m/s) 6.0 + -8.0 = -2 kg m/s

What is Newton’s 1st Law of Motion?

An object at rest remains at rest, or an object in motion remains in motion, in a straight line, unless an unbalanced force acts on it.

What is inertia?

The tendency of an object to resist a change in motion.

What force resists motion?

Friction

The gravitational force exerted on an object is its weight or mass?

Weight.

What is mass?

The amount of matter in an object

The strength of gravitational force depends on ________ and ___________

mass of the objects and distance between them

What is the direction of the force of air resistance?

UP

What is Newton’s 2nd Law of Motion?

The acceleration of an object is equal to the net force acting on the object divided by the mass

a = F/m or F = m x a or m = F/a

A cat pushes a 0.25 kg toy with a net force of 8 N. According to Newton’s 2nd Law, what is the acceleration of the toy?

a = F/m = 8 N / 0.25 kg = 8/.25 = 32

What is the difference between a balanced and unbalanced forces?

Forces acting on an object and combine and form a net force of 0 are balanced forces.

Forces that combine and add to a value that is not 0 are unbalanced forces.

What is Newton’s 3rd Law of Motion?

When one object exerts a force on a secondobject, the second object exerts an equal andopposite force on the first object.

How was Newton’s 3rd law demonstrated inthe balloon lab?