Forces and motions
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Forces and Motions. Forces. What is a Force? Anything that changes the state of rest or motion of an object It’s what causes ACCELERATION has magnitude and direction Therefore force is a vector. Force cont.

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What is a Force?

  • Anything that changes

    the state of restor motion

    of an object

  • It’s what causes


  • has magnitude and direction

    • Therefore force is a vector

Force cont
Force cont.

A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

Units = Newtons (N)

Forces can be combined
Forces can be COMBINED:

Net Force: the combination of all of the forces acting on the object

Balanced Forces: When the forces on an object are balanced, the netforce is zeroand there is nochange in the object’s motion.

Warning: Doesn’t always mean

the object is at rest;

Example: Tug of war,

Pushing piano, car

traveling at constant


Unbalanced Forces

  • Results when net force acting on an object is NOTequaltozero

  • When an unbalanced force acts on an object, the object accelerates in the direction of the net force

  • Fig 14, p.332

*(moves in direction of net force)


  • A push or pull on an object

  • Units: Newton (N) = 1 kg x m/s2

  • Net Force (Fnet) : sum of all of the forces that are acting on an object

  • Balanced Forces: Fnet = 0

    • Object is at rest or moving with constant velocity

  • Unbalanced Forces: Fnet is not equal to 0,

    • Object is accelerating or decelerating

Types of forces
Types of Forces

  • Applied – contact force in the direction the object is moving

  • Tension- caused by a rope, cable, etc., directed away from the object

  • Friction- opposes the motion of objects, must have contact

  • Normal- caused by a surface

  • Force of gravity- force at a distance, caused by attraction between two objects

Two types of friction
Two Types of Friction

  • Static

    • Friction of an object at rest

    • Greater than kinetic friction (harder to get an object moving than to keep it moving)

    • Friction is greatest when surfaces are rough

      ex. the force that is keeping this block from

      sliding downhill

  • Kinetic

    • Friction of a moving object

Types of kinetic friction
Types of Kinetic Friction

  • Sliding

    • Two surfaces moving past each other

  • Rolling

    • Movement of a circular object on a surface

    • Ex. Cart on wheels

  • Fluid Friction

    • Movement of an object through a liquid or gas

    • Ex. Air resistance


is defined as

is defined as

is defined as

is defined as

& an example is

& an example is

& an example is

& an example is





The friction between surfaces that are stationary

force that exists when objects slide past each other

force that exists when a round object rolls over a flat surface (usually less than sliding friction)

force that exists when an object moves through a fluid (air, water)

a book sitting on a table

-hockey puck on ice

-child going down a slide

-a sled down hill

-a roller blade on a sidewalk

-bowling ball on bowling alley

-a car driving down the road

-swimmer swimming through pool

How can we decrease friction?

Watch this demo and see…

Can you think of a situation in which you would want to increase friction?

Gravity is a force natural phenomenon in which objects that have mass are attracted to one another
Gravity is a force:natural phenomenon in which objects that have mass are attractedto one another

  • Gravity is an attractive force  pulls objects together

  • Earth’s gravity acts downward toward the center of the Earth.

  • There is an upward force that balances gravity


Falling Objects have two forces

acting on them:

  • Gravity causes objects to accelerate downward

  • airresistance acts in the direction oppositetothemotion & reduces acceleration.

Free fall the motion of a body when only the force of gravity is acting on the body
Free Fall: the motion of a body when only the force of gravity is acting on the body

  • Free fall acceleration of an object is directed toward the center of Earth

  • Because free fall acceleration results from gravity, its symbol is g

  • Acceleration due to gravity on Earth = 9.8 m/s2

Formula for objects in free fall: d = ½ gt2

In a vacuum, two objects would accelerate at the same rate because both are in freefall (Fig 8, p.354)

Question: What other force is not present in vacuum that would affect acceleration?

Answer = air resistance

Drawing free body diagrams
Drawing Free Body Diagrams

  • Identify the types of forces acting on the object

  • Use a dot or a box to represent the object

  • Use arrows to represent the direction and size of the force

  • -change the size of the arrow if more force is

    being applied in one direction versus the other

  • Label the arrows with the type of force

Free body diagrams
Free Body Diagrams

  • How will the object move in the diagram to the right?

  • Answer: Object may be at rest and not move or it could be moving at a constant velocity since all the forces are balanced and the net force is zero

Free body diagram practice
Free body diagram practice

  • Free Body diagram practice

Ii newton s first law of motion
II.) Newton’s First Law of Motion

A) Historical Development

1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving

2) Galileo (1564 – 1642): Concluded that moving objects not subjected to friction or other force, would continue to move indefinitely; Disproved Aristotle

3) Newton (1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion

C newton s 1st law of motion
C) Newton’s 1st Law of Motion

1) According to Newton’s 1st Law, the state of motion of an object does not change as long as the net force is zero.

a) Basically saying that an object at rest will stay at rest unless a force acts on it

1) Example: Soccer ball will remain (atrest) on the grass unless a force is acted on it

2) Sometimes called the “LawofInertia”

a) Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object

1) Car crash: You continue

forward because of inertia

“Science and the Consumer”


How is inertia related to mass p 347
How is inertia related to mass?P 347

  • Mass is a measure of inertia.

    • Who would you rather be tackled by…a toddler or a defensive lineman?

    • What is easier to move? An empty garbage can or a garbage can full of lead? Why?

      • The empty garbage can has less mass= less inertia= less resistance to being accelerated.