Chapter 4Forces

Force-•An action exerted on an object which may change the objects state of rest or motion. It is a Vector!

How Can We Measure Force?System Mass Acceleration Force

SI kg m/s^2 Newton

Cgs g cm/s^2 Dyne

Avoirdupois Slug ft/s^2 Pound

The Newton• The Newton is how we measure Force in

Physics.• Abbreviation = N• It can be broken up into- (kg*m/s^2)• 1N = 0.225lbs• 1lb = 4.448N

Newton’s Laws• 1st: Objects at rest stay at rest, and objects in

motion stay in motion.• 2nd: F = ma• 3rd: When an object exerts a force on another

object, the second object exerts a force on the first object equal in magnitude and opposite in direction.

Types of Forces•Contact–Applied–Friction–Tension–Normal

•Field–Gravity–Electro-

Magnetic–Nuclear (strong

and Weak)

Free Body Diagrams• Since Force is a Vector, we can

represent it as a Vector Arrow. • When there are more than one Forces

acting on an object we represent all of them by different Arrows on the object.

• We call this a Free Body Diagram

Free Body Diagrams

• When Drawing free body Diagrams. (remember)– Each Vector originates

from the center of mass of the object.

– The Length of the Arrow = Magnitude of the Force.

– Direction of the Force = Direction of the Arrow.

• A toddler drags a box of toys Horizontally across the floor.

Normal

Applied

Gravity

Exit Ticket• A truck pulls a trailer on a flat road.

Gravity pulls down on the trailer 250,000N, the road pushes back up with 250,000N. The force exerted on the trailer by the cable is 20,000N to the right.

• Draw a Free Body Diagram of the Trailer?

Do Now

• Draw a Free Body Diagram on a Car that is not moving but is being pulled down with a Force of 25,000N and is supported by the ground with 25,000N of Force.

•What is the Difference between Force, and Net Force

Chapter 4Newton’s First Law

Newton’s First Law• An object at rest remains at rest; and an

object in straight line constant speed motion stays in straight line constant speed motion, unless acted on by an outside Force.

Inertia:• The tendency for an object to sty at rest, or

stay in straight line constant speed motion.• The Tendency for an object to stay in

Equilibrium.• The tendency for an object NOT to accelerate.• The Laziness of an object.• Amount of mass an object has.

Stations• At each Station there is a “puzzle”

set up. As a group you must try to figure it out and answer the Question. The rules and explanation are at the stations.

Net Force• The addition of all the Forces

acting on an object.• Later on we will use NSEW to

solve for this

Equilibrium• The state in which the Net Force

is equal to Zero• Or, a state at which an object is

not accelerating.• What does that say about the

objects motion?

Yes!• For an object to be in

equilibrium, it does not have to be at rest. • It can also be moving in straight

line constant speed motion.

A look back at Newton’s 1st Law• An object at rest remains at rest; and an

object in straight line constant speed motion stays in straight line constant speed motion, unless acted on by an outside Force.

• OR,• An object in equilibrium stays in equilibrium,

unless acted on by an outside Force!

Calculating Net Force• Follow the same rules from before

when dealing with Vectors.1 Break all Forces into Components.2 Put all Components into NSEW3 Find a Net Force in the x and y direction.4 Find the Resultant Force from the x and the y.

Page 128

• # 1: A man is pulling on his dog with a Force of 70.0N directed at an angle of 30degrees above the horizontal. Find the x and y components.

Page 128

• # 2: A gust of wind blows an apple from a tree. As the apple falls, the gravitational force on the apple is 2.25N downward, and the force of the wind on the apple is 1.05N to the right. Find the magnitude and direction of the Net Force on the apple?

Page 128

• # 3: The wind exerts a force of 452N North on a sailboat, while the water exerts a force of 325N west on the sailboat. Find the magnitude and direction of the Net force on the sailboat?

DO NOW• Draw a Free Body Diagram of a

Car that is Driving to the right in a straight line at a constant velocity, and weighs 26,000N.

•What Is different about Static Equilibrium and Dynamic Equilibrium?

•What Do They Have In Common?

Chapter 4Newton’s 2nd and 3rd Laws

Newton’s 2nd Law

• The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the objects mass.

• Or,

•F = ma

Let’s try our New Equation Out

• The Net Force on the propeller of a 3.2kg model airplane is 7.0N forward. What is the acceleration of the airplane?

• Draw the FBD first then Solve!!!

Newton’s 3rd Law• If an object exerts a force on a second

object, then that second object exerts a force on the first object that is equal in magnitude but opposite in direction.

• Forces always exist in Pairs!

With That Being Said• A little sports car collides head on

with a big Ford Pickup truck. Which experiences the most damage?• Which experiences the most Force?• How can that Be?

Field Forces also work in Pairs.

• If the Earth pulls on you, you also pull on the Earth. • The Earth just has a little

bit more mass so it doesn’t quite feel it.

Types of Forces

• Weight: a measure of Gravitational Force exerted on an object. W = mg

• Normal Force: A force that supports an object and acts perpendicular to the contact surface.

• Tension: The pulling Force in a rope.• Friction: The force that resists the Motion of

an object.

FBD• Draw a free body diagram on a

Text book that is sitting on an incline and is not moving.

Friction• Static Friction: The

Frictional Force that resists an initial sliding motion of an object at rest.

• Kinetic Friction: The Frictional Force that opposes the motion of an object that is moving.

Ticket To Exit• Draw a Venn Diagram of Static and Kinetic Friction.

• Find the Net Force!

• If the object has a mass of 60kg, what acceleration will it have?

Fn=50N

Ɵ=30°Ff=45NFa=100N

Fg=100N

• Find the Net Force!

• 500kg car, how much acceleration do the tow ropes apply?

Ff=2000N

Ɵ=25°

Ff=1500N Ff=1500N

Ɵ=25°

400N

150N

200N

400N 30 degrees

500N

150N

???300N 60 degrees

• Find the Net Force and acceleration of the 10kg object

300N

300N 500N

500N

282N 45°

• Find the Net Force and acceleration of the 100kg object.

100N

200N 100N55°

400N

260 57°

• Find the Normal Force

50N

???N

400N 10 degrees

• A 40kg object is being pulled at an angle of 40° with a Force of 100N. What is the Normal Force? And Frictional Force?

20°

100N

400N

???

???

Fn 366 Ff 94

• Draw a free body Diagram of an wagon being pulled to the right with a Force of 40N at an angle of 30degrees above the horizontal.

•Draw a Free Body Diagram of a Book on a 30 degree inclined plane that is not moving.

•What is Newton’s First Law?•What is Inertia?

•What is Newton’s 2nd Law?

•What is Newton’s 3rd Law?

Friction Continued…• Coefficient of Friction (µ)• The ratio of the magnitude of the Frictional Force

and the Normal Force.

• Two Types– Static Coefficient of Friction– Kinetic Coefficient of Friction

Neither of which have any Units

Coefficient of Friction•Coefficient of Friction•µ = Frictional Force/Normal Force

Calculating Coefficients• A 30N box sits on the floor, and it takes a

25N applied horizontal force to get it to move. What is the Coefficient of Static Friction?

• Then once the 30N box is moving it requires a 18N applied Horizontal Force to keep it moving. What is the Coefficient of Kinetic Friction?

Bringing It All Together!!!

• A student attaches a rope to a 20.0kg box of books. He pulls with a Force of 90.0N at an angle of 30 degrees. The coefficient of kinetic friction between the box and the sidewalk is 0.500. What is the acceleration of the box?

• 0.12m/s2 to the right

Still Bringing It Together

• A lawn care professional pushes his lawn mower with a Force of 485N at an angle of 35degrees below the horizontal. If the mower has a mass of 31.9kg answer the following question.

• A. If the µk between the box and the floor is 0.57 how long does it take to move the box 4m?

• B. If the µk between the box and the floor is 0.75 how long does it take to move the box 4m?

• A. 2s• B. Never

Home Work!!!• Page number: 145- 148• #’s 12, 21, 22, 45, 46

Another Day, Another Problem

• What is the acceleration of the 45kg object, if the coefficient of friction is 0.37?

300N25 degrees