Chapter 4

NEWTONS LAWS

Newton’s 3 Laws

Force Diagrams

Balanced Forces

Unbalanced Forces

Force: a push or a pull• Measured in Newton

• Vector Quantity

Contact Force:

• applied by direct contact

Field Force:

• can be applied over a distance

Newtons 1st Law: Law of Inertia

A body at rest remains at rest,

A body in motion remains in motion, at a constant velocity, in a straight line

Until an Unbalanced force acts on it.

INERTIA: Tendency to resist changes in motion.

Inertia is a property of matter

Examples:

• Basics

• Table Cloth Trick

• Crashes

• Seat Belts

Newtons 2nd LawAn unbalanced force causes acceleration

Acceleration is:

• directly related to force.

• inversely related to mass.

𝑎 = 𝐹𝑚

1 kg * 1m/s2 = 1 Newton

Newtons 2nd Law:

Example: An 1800 kg bus accelerates from 0 to 25 m/s in 5 seconds. What is the force required?

Newtons 2nd Law:

Example: A 50 kg kid runs into a tree with and initial speed of 6 m/s. The tree exerts a force of 350 N. In what distance does the boy come to rest?

Weight: Force exerted by gravity

𝐹𝑔 = 𝑚𝑔 = 𝑚𝑎𝑠𝑠 ∗ 9.8earth

10 kg = how many Newtons?

How much ya bench? (175 lbs) (1kg = 2.2 lbs)

Convert your weight to Newtons.

What do you weigh on the moon? (gm = 1.6 m/s2)

Newtons 3rd Law

• When one object exerts a force on a second object, the 2nd object exerts an equal Force in the opposite direction on the first object.

• For every action there is an equal and opposite reaction.

• All Forces come in pairs, equal in magnitude, opposite in direction.

Newtons 3rd law Examples• Kicking Soccer ball:

Action: you apply force to ball via the kick.

Reaction: Ball exerts Force on Your Foot

• Air Resistance:

Action: Ball pushes air out of the way.

Reaction: Air pushes on ball to slow it down.

• Earth-Moon System:

Action: Earth pulls on moon causing the moon to orbit the earth.

Reaction: Moon pulls on earth, causing the tides.

Force Diagrams (Free Body Diagrams)

• Identify the object you will draw a diagram for.

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

• Draw a dot or line to represent the object of interest.

• Draw a vector from the dot to represent each force. Draw it in the direction the force is being exerted.

• Label it by (a) the type of force, and/or(b) the object exerting the force

Force Diagrams (Free Body Diagrams)

• A ball is dropped (no air resistance)

• A ball is dropped (with air resistance)

• Coffee mug sits on table.

• Car accelerates forward

• Car slows down

• Crate pushed along ground with friction.

Force Diagrams (Free Body Diagrams)

• Draw Force diagram of the people• Draw Force diagram of the beam

Force Diagrams (Free Body Diagrams)

• Draw Free body diagram of the person.

• Draw free body diagram of the rope.

Force Diagrams (Free Body Diagrams)Determine the net Force on each object below,

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

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

Balanced Forces• Equilibrium :

• Net Force = Zero• Object is at rest or moving at a constant velocity

Tacos(10 kg) PIZZA (5kg)

40o 40o

Find the Tension on each chain

Balanced Forces The ninja has a mass of 50 kg, ϴ =30o

Find the Tension in Each Rope

Balanced Forces• A 20 kg crate of mangos hang from two cables. The cables make an

angle of 90o with each other. One cable has 3 times the tension of the other cable. What is the tension on each cable?

Unbalanced ForcesWorker pushes a 100 kg box with a Force of 85N. Friction exerts a Force of 65 N. What is the acceleration of the box?1) Draw Force diagram

2) Determine Net Force

3) Solve the Problem

Unbalanced ForcesA Rescue Helicopter lifts 2 people with a total mass of 200kg with an acceleration of 3.0m/s2.

What is the Tension on the Rope holding the people?

1) Draw Force diagram2) Determine Net Force3) Solve the Problem

Riding in an elevator

A 50 kg woman is standing on a scale in an elevator. The scale reads 55kg. What is the acceleration of the elevator?

A 50 kg woman is standing on a scale in an elevator. The scale reads 45kg. What is the acceleration of the elevator?

A 50 kg woman is standing on a scale in an elevator. The scale reads 0 kg. What is happening to the elevator?

1) Draw Force diagram2) Determine Net Force3) Solve the Problem

Unbalanced Forces: If the boat below as a mass of 500kg. What is the acceleration?

1) Draw Force diagram2) Determine Net Force3) Solve the Problem

Unbalanced Forces: A pair of fuzzy dice hang from the rear view mirror o a car. The car begins to accelerate forward. The dice now make an angle of ϴ =30o

with the vertical. What is the acceleration of the dice?

1) Draw Force diagram2) Determine Net Force3) Solve the Problem