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Chapter 4

- Forces and Newtons Laws of Motion

Isaac Newton (1642-1727)

Isaac Newton (1642-1727)

- IsaacNewton is without a doubt one of the most

influential men in history. - Just a few of his accomplishments
- Built the first practical reflecting telescope
- Developed a theory of color including the idea

that white light is composed of all colors of the

rainbow - Studied the speed of sound
- Developed calculus from scratch!
- Defined the 3 Laws of Motion that govern all

objects - Studied the effects of gravity (story about the

apple)

Force

- A force is a push or a pull.
- Force is not a thing in itself, but rather an

interaction between two objects. - Force is a vector quantity direction matters in

the answer!

Common Forces

Common Forces

Newtons First Law

- The Law of Inertia
- A body remains at rest or moves in a straight

line at a constant speed unless acted upon by a

net force. - Objects do not accelerate unless a net force is

applied.

Newtons First Law

- Inertia is a property of an object most closely

related to its mass (not to be confused with

momentum) that explains why objects with greater

mass resist a change in motion more than those

with a lesser mass.

Net Force

- Net force is the vector sum of ALL forces acting

on an object. - If there is zero net force, then there is zero

acceleration (constant velocity), this is a

special case called equilibrium. - If there is a net force, there will be an

acceleration. That means that the object will be

speeding up, slowing down, or changing direction.

Free Body Diagrams

- A Free Body Diagram is a simple drawing that

shows the magnitude and direction of all of the

force vectors acting on an object. - The length of the arrows in relation to each

other is VERY important - Each arrow must point away from the free body

and be labeled appropriately - The system, the object the force is applied to,

is drawn as a shaded circle

Free Body Diagrams

- Here is an example of a FBD of a book at rest on

a table top. - Fg is acting downward but is balanced by FN

acting upward. - Results in no net force and zero acceleration

FN

The book is drawn as a ball

Fg

Free Body Diagrams

- Here is an example of a FBD of a box being pulled

by a rope at a constant speed on a flat surface. - Fg and FN are still opposite and equal.
- FT and Ff are also opposite and equal.

FN

FT

Ff

Fg

Object is in motion, but not accelerating

Balanced Forces (zero net)

Free Body Diagrams

- Here is an example of a FBD of a ball under free

fall conditions. - Fg is the only force acting on this object.
- The net force is down and the object is

accelerating.

Fg

Object is in motion and accelerating

Unbalanced Forces (non-zero net)

HW Questions P. 89 1-5

- Draw a FBD for the following situations
- A flowerpot falls freely from a windowsill.

(Ignore any forces due to air resistance.) - A sky diver falls downward through the air at

constant velocity. (The air exerts an upward

force on the person.) - A cable pulls a crate at a constant speed across

a horizontal surface. The surface provides a

force that resists the crates motion. - A rope lifts a bucket at a constant speed.

(Ignore air resistance.) - A rope lowers a bucket at a constant speed.

(Ignore air resistance.)

Answers to HW

1

2

Answers to HW

3

4

Answers to HW

5

- Draw a free body diagram for the following

situations - A car accelerates from rest on a flat road (there

is both friction from the air and the ground).

- The space shuttle just after launch is

accelerating upward (include friction from the

air)

Newtons Second Law

- The accel. of an object is directly proportional

to the net force acting on the object, and

inversely proportional to the mass of the object.

Newtons Second law of Motion mathematically

Net Force (mass)(accel) Fnet ma

NEWTON'S 2nd LAW

a

Mass

- Mass is the amount of matter in an object (not to

be confused with weight) - Also considered a measure of the inertia of an

object - measured in SI unit of kilograms (kg) if mass is

given in grams you must convert!

Weight

- Weight is the downward force upon an object due

to acceleration caused by gravity - weight mass (kg) ?accel. due to gravity (m/s2)
- Fg mg
- measured in Newtons (N)

- The weight of a 10 kg brick is...
- A) 98 N
- B) 10 kg
- C) 9.8 kg
- D) 10 N
- E) 98 kg

Newtons 2nd Law Practice

- Two horses are pulling a 100-kg cart in the same

direction, applying a force of 50 N each. What is

the acceleration of the cart? - 2 m/s2
- 1 m/s2
- 0.5 m/s2
- 0 m/s2

- Answer B

Practice 2

- Two friends Mary and Maria are trying to pull a

10-kg chair in opposite directions. If Maria

applied a force of 60 N and Mary applied a force

of 40 N, in which direction will the chair move

and with what acceleration?

- The chair will move towards Mary with an

acceleration of 2 m/s2. - The chair will move towards Mary with an

acceleration of 10 m/s2. - The chair will move towards Maria with an

acceleration of 2 m/s2. - The chair will move towards Maria with an

acceleration of 10 m/s2.

- Answer C
- ReasonSince the force is applied in opposite

direction, if we consider Marias direction of

pull to be positive direction then, net force

60 N 40 N 20 N . Thus, the chair will move

towards Maria with an acceleration.

Weight

Location

Mass

Earth

18.4 kg

180 N

Moon

1/6 of Earths

18.4 kg

30 N

Orbiting Earth

18.4 kg

0 N

Mass and Weight

- On the Moon, the force of gravity is only 1/6 as

strong as on the Earth. (approx. 1.63m/s2) - While orbiting, you are practically weightless

but your mass remains unchanged. - Your mass does not depend on where your are.
- e.g. Earth, Moon, or space

Falling with Air Resistance

- Air resistance (drag force) increases with speed

and increased cross-sectional area and can be

effected by the size and shape of an object.

Terminal Velocity

Net Force

Acceleration g

Velocity 0 but motion is about to begin

mg

F

Acceleration lt g

v increasing downward

mg

F

Acceleration ltlt g

v still increasing downward just not as rapidly

as before

mg

F

Acceleration 0

Terminal velocity

mg

Terminal Velocity

- Terminal velocity occurs when the drag force of

air resistance becomes large enough to balance

the force of gravity. - At this instant in time, there is no net force

the object stops accelerating (see D below)

terminal velocity has been reached.

Friction

- The force that opposes the motion between two

surfaces that are in contact. - Friction is the "evil monster" of all motion.

Regardless of which direction something moves in,

friction pulls it the other way. - Move something left, friction pulls right. Move

something up, friction pulls down. It appears as

if nature has given us friction to stop us from

moving anything. - Friction is actually a force that appears when

there is relative motion between two objects. - Although two objects might look smooth,

microscopically, they're very rough and jagged.

Static (starting) Friction

- The force that opposes the start of the motion.
- Static means stationary ( not moving).

Kinetic (sliding) Friction

- The force between surfaces in relative motion
- For the same object, why is the force of kinetic

friction less than the force of starting friction?

Newtons Third Law

- Action-Reaction Law
- Two forces that make up an interaction pair of

forces are equal in magnitude, but opposite in

direction and act on different objects.

Newtons Third Law

For every action, there is always an equal

(magnitude) and opposite (direction) reaction. By

action or reaction, we mean a

force. Action/reaction forces do not act on the

same object.

Reaction road pushes on tire

Action tire pushes on road

Reaction gases push on rocket

Action rocket pushes on gases

Identify at least six pairs of action-reaction

force pairs in the following diagram

Question 1

- If a stone is hung from a mass-less rope, at

which place on the rope will there be more

tension? - The top of the rope, near the hook.
- The bottom of the rope, near the stone.
- The middle of the rope.
- The tension will be same throughout the rope.

Answer 1

- Answer D
- Reason Because the rope is assumed to be without

mass, the tension everywhere in the rope is equal

to the stones weight .

Question 2

- In a tug-of-war event, both teams A and B exert

an equal tension of 200 N on the rope. What is

the tension in the rope? In which direction will

the rope move? Explain with the help of Newtons

third law.

Answer 2

- Team A exerts a tension of 200 N on the rope.

Thus, FA on rope 200 N. Similarly, FB on rope

200 N. But the two tensions are an interaction

pair, so they are equal and opposite. Thus, the

tension in the rope equals the force with which

each team pulls (i.e. 200 N). According to

Newtons third law, FA on rope ?FB on rope. The

net force is zero, so the rope will stay at rest

as long as the net force is zero.