Bellringer 10/7

• An astronaut with a mass of 85 kg is outside a space capsule when the tether line breaks. To return to the capsule, the astronaut throws a 2.0kg wrench away from the capsule at a speed of 14m/s. At what speed does the astronaut move toward the capsule?

Chapter 4: Forces

Section 1:Forces & Newton’s 1st / 2nd Laws

• Force – an action (push or pull) exerted on an object which may change the object’s motion. “F”

The SI Unit of force is the Newton (N): 1 N = kg * m/s2

1 Newton of force causes a mass of 1 kg to accelerate at 1 m/s2.

You should recognize those units as “mass” and “acceleration”.

• Forces that do not change an object’s motion are said to be “balanced”.

balanced forces – two forces that are equal in magnitude and opposite in direction.Balanced forces result in zero net force.

net force – the total of ALL forces acting on an object. “Fnet”

Force A Force B Fnet = 0 Na = 0 m/s2

• Forces that DO change an object’s motion are said to be “unalanced”.

unbalanced forces – the net force that causes an object to accelerate.The net (unbalanced) force is a vector quantity.

20 N 35 N Fnet = 15 Na ≠ 0

15 N

Forces can be grouped into two major categories:Field Forces & Contact Forces

• field force – a force that acts on object from a distance (without physical contact). Examples:

GravityMagnetismElectric ForceNuclear Force

Gravity is a field force.

Forces can be grouped into two major categories:Field Forces & Contact Forces

• contact force – a force that acts on objects through physical contact. Examples:

FrictionSpring ForceNormal Force

Friction isn’t always bad.

Types of Forces• Applied – applied by a person/object• Gravity – weight (Fgrav = m x g)• Normal – support force• Friction – exerted by a surface while

moving Sliding or static

• Air Resistance – frictional force by air• Tension – string, rope, cable, wire• Spring – compressed or stretched string

Force Diagrams

• Force is a vector Tail of the vector is attached to the object Points in direction of the force Length is proportional to magnitude

Balanced or unbalanced???

Examples

• A book is being pushed to the right across a table surface with a constant velocity. (Neglect Fair.)

• A ball is dropped from rest from the top of a building. (Neglect Fair.)

• A falling skydiver has reached a terminal velocity

Bellringer 3/7

Cody is dragging his sled across the front lawn. He drags it with a force of 120 N. What is the net force on the sled if the ground produces 17N of frictional force?

Free Body Diagrams

• Show the relative magnitude and direction of all forces acting upon an object

Example:The photograph shows a person pulling a sled. Draw a free body diagram for this sled. The magnitudes of the forces acting on the sled are 60 N by the string, 130 N by the Earth (gravitational force), and 90 N upward by the ground.

Net Forces

• Unbalanced force – force does not become completely balanced

Examples

• Determine the net force for the following:

Examples

• The net force is known for each situation.

Example

• Derek leaves his physics book on top of a drafting table that is inclined at a 35 degree angle. The free body diagram shows the forces acting on the book. Find the net force acting on the book.

Example

• A man is pulling on his dog with a force of 70.0N directed at an angle of 30.0degrees to the horizontal. Find the x and y components of this force.

• Isaac Newton was a mathematician & physicist. also an astronomer and natural philosopher.

In 1687, he published a book named “Philosophiae Naturalis Principia Mathematica”, or “Mathematical Principles of Natural Philosophy”It is often called “The Principia”.

His book is considered one of the most influential works in the history of science.

It contained (among other things), his three laws of motion.

• Building on what Galileo Galilei had suspected in the 1630’s (that objects tend to maintain their state of motion), Newton defined his First Law of Motion.

Newton’s First Law of Motion – an object at rest remains at rest, & an object in motion maintains its velocity (in a straight line) unless the object experiences an unbalanced force.

• What can we infer from Newton’s 1st Law? If the net force on an object is zero, then it’s acceleration

is zero.

Objects moving at a constant velocity have zero net force acting on them.

If acceleration is not equal to zero, then the net force acting on that object is not equal to zero.

The net force on Gertrude’s Harley is greater than zero as she speeds up.

• When an object has zero net force acting on it, it is in equilibrium.

• equilibrium – the state in which the net force acting on an object is zero.

FB

FB

Fg

FgAssume the boat & iceberg are stationary.

• Inertia – the tendency of an object to resist a change in motion. Inertia is determined by mass. Inertia causes:

Objects at rest to NOT want to move.Objects in motion to NOT want to slow down or speed up.

How exactly does an unbalanced for affect an object?

That depends on two things:

• Newton’s 2nd Law of Motion – the unbalanced net force acting on an object equals the objects mass times it’s acceleration.

F = ma From the 2nd Law, we can infer that…

As an object’s mass increases, more force is needed to accelerate it.

As the (unbalanced) force on an object increases, it’s acceleration increases.

Practice

• Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2 kg book toward Roberto. If the net force acting on the book is 1.6 N to the right, what is the book’s acceleration?

Bellringer 10/11

• What acceleration will you give to a 24.3kg box if you push it horizontally with a net force of 85.5 N?

Chapter 4: Forces

Section 2:Newton’s 3rd Law / Everyday Forces

• Newton’s 3rd Law of Motion – For every action force, there is an equal and opposite reaction force.

Forces always exist in pairs.A force cannot exist without a reaction force.

• Misconception: “If all force pairs are equal and opposite, wouldn’t they

all cancel each other out?”

That is a fair argument, but let’s look closer…

• When applying the 3rd Law, you must observe each object individually. The man is applying 350 N of force to the van.

The van accelerates, but the man does not. Why?

Although the force is equal and opposite, 350 N is not enough to move the man because of his MASS and FRICTION.o Do you think the man could push the van if he were on ice?

• Field Forces also exist in pairs. Let’s take gravity, for example… The Earth’s gravity pulls on you, and in turn, you pull on

Earth with your gravity. So…why isn’t Earth orbiting YOU?

The mass of the earth is much greater than your mass. The Earth DOES accelerate towards you, but it’s

acceleration is infinitely small!

• Let’s start with Gravity…• gravity – an attractive force possessed by all objects

in the universe. Gravity is one of the four fundamental forces. Along with electromagnetic force, strong nuclear force,

and weak nuclear force.

Gravity causes objects to accelerate uniformly.

So, two objects of different masses will fall at the same acceleration.

weight – a measure of the gravitational force acting on an object.An object’s weight depends on it’s mass & g.So, if you went to another planet, your weight would change…!

For example:A person with a mass of 75 kg weighs about 165 lbs on Earth.On the moon, this person would weigh 27 lbs on the Moon.

Why? The acceleration due to gravity on the moon is only about 1.63 m/s2.

Weight (w) = mg

• So, how much would you weigh on another planet? Use the following values to find out. Remember that

F=ma gives you units of Newtons. One Pound is equal to 4.45 Newtons.

The Moon: g = 1.6 m/s2

Mercury: 3.59 m/s2

Venus:8.87 m/s2

Mars: 3.77 m/s2

Jupiter: 25.95 m/s2

Saturn: 11.1 m/s2

Uranus: 10.7 m/s2

Neptune: 14.1 m/s2

Pluto: 0.420 m/s2

The Sun: 274. m/s2

Bellringer 3/17

• Draw a free body diagram for a crate being pushed across a flat surface at a constant speed

• When one object rests on another…a force is present that keeps the object from falling through.

• normal force (Fn)– the force that acts on a surface directly perpendicular to that surface. “Normal” is just another way of saying “perpendicular”.

For an object on an incline, the normal force is:Fn = mg*cosΘ

The normal force is a result of Newton’s 3rd law.

• The force of Friction is one of the most common and most important forces.

• friction – a force that causes resistance to motion between two surfaces in contact.

Friction is always opposite to the net applied force on an object.

• Because each object has a different surface texture, friction can only be approximated.

There are two categories of friction, and both oppose motion.

• Consider this: You slide your book across the table. It soon comes to a stop. Friction causes this behavior, of course.

• But what KIND of friction?• kinetic friction – friction existing between surfaces that are

in motion.• Kinetic friction attempts to stop the motion of objects.

• Consider this: You’re trying to push a heavy crate across concrete.

• You push with enough force to match the weight of the box (or Fg), but it doesn’t move.

• Again, friction causes this behavior. An opposite force responds to your push…the force of

static friction.

Static friction increases as the applied force increases, but it has a limit.

Fs

FA

• static friction – friction existing between unmoving objects. In order to cause an object on a surface to move,

enough force must be applied to overcome static friction.

Once motion begins, kinetic friction takes over.

Fk

FA

Wait…WAIT!

Shouldn’t those two forces be equal and opposite??

Misconception AlertIs the force of friction always equal and opposite to the applied force?

• NO. The force of friction is a product of the normal force…

• The (max) value of friction is not affected by the applied force.

• Friction only RESPONDS to the applied force.

• So, how is the force of friction determined? By using the “coefficient of friction”.

• Definition: coefficient of friction – a non-unit, scalar value that represents the amount of friction between two surfaces.

• It is represented with “μ”.

• The higher the coefficient, the larger the force of friction..

• For example, Rubber on Pavement has a higher μ than Ice on Steel.

Pronounced “mew” or “myou”.

• Remember: in order to cause an object to move, enough force must be applied to overcome static friction.

• Fs equals the amount of force needed to set an object in motion.

• Fk equals the applied force when an object is moving at a constant speed. Generally, Fk < Fs. Once an object begins moving, friction becomes weaker

in most cases.

Force of static friction: Fs = μsFn

Force of kinetic friction: Fk = μkFn

Example Problem: Friction• A 24 kg crate is at rest on a flat floor. It requires 74

N of force to set it in motion. What is the μs between the crate and the floor?

• Once the crate is in motion, a force of 53 N keeps it moving at a constant speed. What is the μk between the crate and the floor?