333 PHYSICS - UNIT 2FORCES

Explanations as to why things occur versus just observation.

Copernicus & the moving EarthWhat idea did Copernicus propose that revolutionize science?

What view was held prior to Copernicus proposal?

In the 1500’s, Galileo reasoned…

In the year 1642, Galileo died and Issac Newton was born. He was a genius who essentially wrote the book of physics in 18months.He furthered Galileo’s idea and generated Newton’s Laws of Motion.

NEWTON’S LAWS OF MOTION

Law of INERTIA OR 1ST LAW:

INERTIA

Mass

Mass is not weight.

You can change your weight without changing your mass. How?

• Mass is • Mass is measured

LOCATION MASS WEIGHT

EARTH 50kg 500 Nwtns

MOON ? 83 Nwtns

JUPITER ? 1250 Nwtns

Question: A top view of a circular pie tin with a quarter section cut out. Which path would a marble follow after exiting the circular track?

Explain the purpose of seatbelts using the concept of inertia.

An argument against a moving Earth (Copernicus) was the following:

Consider a bird sitting at rest at the top of a tall tree. If the earth is really moving around the sun at 30km/s and also rotating at 1000mph, then how does a bird dive down to catch a worm?

Why doesn’t classroom wall hit us if we jump up in air?

Why do all objects fall at the same rate? (g=-10m/s2)

Aristotle felt heavy things fell faster than lighter ones. However, without air resistance, a light object falls with the same acceleration as a heavy object… Explain using weight and inertia.

3rd LAW or Action/Reaction

Walking or Jumping Swimming

Use 3rd law to describe the motion of:

A Rocket taking off

A helicopter hovering over the ground

A school bus collides with a bug and kills it.

QUESTION: Which body receives the greater force of impact, bug or bus? WHY?

Who pulls harder on the rope?

Who wins the tug of war?

NET FORCE (FNET )Net =

Net Force =

NET FORCE

Newton’s 2nd Law

maFnet If net force then acc

If net force then acc

If mass then acc

If mass then acc

When FNET = 0, acceleration is 0….,why?

When FNET = 0, velocity is constant…,why?

maFnet

Net Force causes Acceleration

Mass Resists Acceleration

TYPES OF FORCESNormal force = N

Friction force = f

Tension force = T

Weight force = W

1 kg = 2.2lbs

Force diagrams

FBD’s with 2nd law (constant acc) Example 1: A 7.5kg box sits on a table. It is then pulled from rest to the right by rope with a constant force of 50N. There is a resistive force of 13N on the box.

a)Draw a force diagram on the box.

b) Write the 2nd law force equation with specific forces.

c)Determine the acceleration of the box.

d)Determine the speed reached by the box after 3.5sec of being pulled.

Example 2: An empty elevator of mass 2700 kg is pulled upward by a cable. The elevator accelerates up at 1.3m/s2. a) Draw a force diagram for the elevator.

b) Write the 2nd law force equation with specific forces.

c) Determine weight of elevator.

d) Determine the tension in the cable.

e) Determine the tension in the cable if it was moving with a constant velocity.

Draw the forces acting on the skydiver.

When a skydiver reaches terminal speed, they are moving _________________

What is the value of the acceleration of the skydiver when they reach terminal speed? _________________

What is true about the forces acting on the skydiver when they reach terminal speed?___________

maFnet

Which direction does skydiver move (relative to ground) when they open up the parachute?

What does the parachute change about the skydiver?

What happens to FORCE of air resistance due to the parachute? What happens to the skydiver’s weight?

What is the direction of the NET FORCE on the skydiver right after opening parachute?

What happens to speed of skydiver after parachute has been open for awhile? Forces on skydiver?

FRICTION FORCE

Two (2) types of friction:1)Static:

2)Sliding:

Sliding Friction between player and dirt

Static Friction is present when man pushes on box, until box moves. Also present on his feet.

STATIC FRICTION BEHAVIOR

Friction force Normal force

Coefficient of friction

Friction Example: The coefficient of sliding friction is 0.40 for a box being pulled to the right across a table. The mass of the box is 2kg.

a) Draw a force diagram for the box.

d) If the box is being pulled by a force of 12N, determine the net force on the box.

e) Determine the acceleration of the box.

b) Determine the weight of the box.

c) If the normal force equals the weight force on the box, determine the sliding friction between the box and table surface.

Equilibrium

• EQUILIBRIUM IS A CONDITION WHERE FORCES CANCEL OUT IN A PARTICULAR DIRECTION.

• FNET = 0

• BALANCED…ZERO ACCELERATION

Assuming no air resistance, EXPLAIN the following picture.