Forces

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# Forces - PowerPoint PPT Presentation

Forces. Chapter 6 Pages: 116-147. Force. A force is a push or pull upon an object resulting from the object\'s interaction with another object. Contact Forces Long-Range Forces. Contact Forces.

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### Forces

Chapter 6

Pages: 116-147

Force

A force is a push or pull upon an object resulting from the object\'s interaction with another object.

• Contact Forces
• Long-Range Forces
Contact Forces

Contact forces are types of forces in which the two interacting objects are physically in contact with each other.

Long-Range Forces

Long-Range Forces are types of forces in which the two interacting objects are not in physical contact with each other, but are able to exert a push or pull despite the physical separation.

Force
• F for use in equations.
• Newton is the unit for Force.
• N abbreviation for Newton.
• Net Force can accelerate.
• N = kg m/s2
Force is a Vector Quantity
• Magnitude
• Direction

4000lb

Homework

Page: 124

Questions: 7-11

Free Body Diagrams

The purpose of a free-body force diagram is to assist you in trying to determine the net force acting on a body.

Net Force

The purpose of a free-body force diagram is to assist you in trying to determine the net force acting on a body.

Free Body Diagrams

The net force is the vector sum of all the individual forces acting on a system.

Fnet = F1± F2± F3 ± F4 …

Constructing “free-body force diagram”
• Identify the object(s) you will draw a diagram for.
Constructing “free-body force diagram”

2. Identify all the forces acting directly on the object and the object exerting them.

• Gravity
• Table
Constructing “free-body force diagram”

3.Draw a dot to represent the object of interest.

Table

Gravity

Constructing “free-body force diagram”

4. Draw a vector to represent each force.

Constructing “free-body force diagram”

5. If the object is stationary or is moving at a constant velocity, the vectors should graphically add up to zero.

Constructing “free-body force diagram”

5.   If the object is accelerating, the sum of the vectors should produce a vector in the same direction as the acceleration.

Floor

Gravity

Constructing “free-body force diagram”

Ffloor=Fgravity

Standing on Floor

Brakes

Motor

Gravity

Types of Motion
• No Motion

FMotor=FBrakes

No Net Force

Friction

Motor

Gravity

Types of Motion
• Constant Velocity

FMotor=FFriction

No Net Force

Friction

Motor

Gravity

Types of Motion
• Speeding Up

FMotor>FFriction

Net Force

Friction

Motor

Gravity

Types of Motion
• Slowing Down

FMotor<FFriction

Net Force

FM=400N

FR=4000N

FF=400N

FG=4000N

Finding Net Force

VerticalFnet = FR - FG

HorizontalFnet = FF - FM

Fnet=4000N–4000N

Fnet=400N–400N

Fnet= 0N

Fnet= 0N

FR=4000N

FF=400N

FG=4000N

Finding Net Force

FM=400N

Not Moving OrConstant Velocity

FM=400N

FR=4000N

FF=100N

FG=4000N

Finding Net Force

VerticalFnet = FR - FG

HorizontalFnet = FF - FM

Fnet=4000N–4000N

Fnet=100N–400N

Fnet= 0N

Fnet= -300N

FR=4000N

FF=100N

FG=4000N

Finding Net Force

FM=400N

Acceleratingto the Left.

Newton’s Second Law of Motion

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

FNet = 1N

Accelerates

Ff = 40N

FP = 41N

Newton’s Second Law of Motion
Inertia

Inertia is the resistance an object has to a change in its state of motion.

Mass

Newton’s First Law of Motion

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Mass vs. Weight

Mass is the amount of stuff you are made up of. (kg or slugs) Does not change!!!!

Mass vs. Weight

Weight depends on how much gravity is acting on you at the moment; you\'d weigh less on the moon than on Earth.

(newtons or pounds)

Weight

To calculate weight use the acceleration due to gravity (9.8m/s2). This will be called g.

F=ma

Fg=mg

Weight Problems

Mr. Clune has a mass of 110kg. How much does he weight?

Given: m=110kg

g=9.8m/s2

Find: Fg=?

Equation: Fg=mg

=(110kg)(9.8m/s2)

Fg=1078N

F=ma Problems

A boy pulls a sled that has a mass of 5kg across the snow. The sled accelerates at a rate of 0.5m/s2. What is the net force of on the sled?

a=0.5m/s2

Fnet

Given: m=5kg

a=0.5m/s2

Find: Fnet=?

Equation: Fnet=mg

=(5kg)(0.5m/s2)

Fnet=2.5N

F=ma Problems

A rock with a mass of 10kg fell off a cliff. At a specific time during its’ fall it had an acceleration of 3m/s2, due to air resistance. What is the force of air on this rock at this time?

+

Fnet=Fair + Fg

Fair

a

Fg

Fnet= ma

Fg= mg

Fnet

Fair=Fnet - Fg

Fair=ma- mg

Fair=m(a– g)

Fair=10kg{(-3m/s2)–(-9.8m/s2)}

Fair=10kg{(-3m/s2)+(9.8m/s2)}

Fair=10kg(6.8m/s2)

Fair=68N

Homework

Page: 147

Questions: 22, 27,29

Due: 10/25/06

Factors that determine Friction

Weight

Moving

Stationary

Surface

Friction Forces
• FN – Normal Force: This force which will affect frictional resistance is the component of applied force which acts perpendicular or "normal" to the surfaces which are in contact and is typically referred to as the normal force.
Friction Forces
• FT – Surface Force: This force opposite the normal force which is equal to this force.
Friction Forces
• FP – Push or Pull Force: This force is pushing or pulling the object.
Friction Forces
• Ff – Friction Force: Frictional resistance to the relative motion of two solid objects.
Friction Forces
• Ffs – Static Friction Force: Static frictional forces are non-moving forces between two surfaces. It will increase to prevent any relative motion up until some limit where motion occurs.
Friction Forces
• Ffk – Kinetic Friction Force: The force between two surfaces that are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds.
Friction Forces
• μ –Coefficient of Friction: The ratio of the force of friction (Ff) between two bodies and the force pressing them together (FN).

Ffk

FN

Friction Problem

A refrigerator of total weight 400N is pushed at a constant speed across a room by pushing horizontally on one side with a force of 160N. What is the coefficient of kinetic friction?

Ffk = 160N

FN = 400N

μk = ?

140N

μk=

400N

Ffk

μk=

FN

μk= 0.35

Ffs

μs=

= μk

Ffs

FN

FN

If the coefficient of static friction between the floor and the refrigerator was 0.6, how much force would be needed to start the refrigerator moving?

μs = 0.6

FN = 400N

Ffs = ?

= (0.6)

= 240N

(400N)

Ffs

Ffs

= μk

Ffs

FN

Homework

Page: 133

Questions: 14,15

Page: 145

Questions: 33-35

Due: 11/2/06

Newton’s Third Law

"For every action, there is an equal and opposite reaction."

While driving, Anna Litical observed a bug striking the windshield of her car. Obviously, a case of Newton\'s third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?

Rockets are unable to accelerate in space because ... There is no air in space for the rockets to push off of. There is no gravity is in space. There is no air resistance in space. ... nonsense! Rockets do accelerate in space.

A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...

• greater than the acceleration of the bullet.
• smaller than the acceleration of the bullet.
• the same size as the acceleration of the bullet.

In the top picture, a physics student is pulling upon a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is held by the Strongman. In each case, the force scale reads 500 Newtons. The physics student is pulling…

with more force when the rope is attached to the wall.

with more force when the rope is attached to the Strongman.

the same force in each case.