FORCES

ISP 209 - 2B 29/7/2006

Which vehicle exerts a greater force ― the tow truck or the car?

What is a force?

• A push or a pull

• When one object pushes or pulls another object, you say that the first object exerts a force on the second object.

• Let’s practice. What forces are at play around you?

What forces are exerted?

What forces are exerted?Playing catch with a medicine ball

ISP 209 - 2B 69/7/2006

Playing catch with a medicine ball

A throws the ball and B catches it. four forces

When A throws the ball he exerts a force on the ball (toward the right) and the ball exerts a force on him so he recoils (toward the left).

A B

When B catches the ball he exerts a force on the ball (toward the left to stop it) and the ball exerts a force on him so he is knocked back (toward the right).

Scalars and Vectors

• Are both quantities

• BUT…..• Scalars are fully described by

magnitude (magnitude= number)

• Vectors are fully described by both magnitude(number) and direction

Let’s take a minute to discuss….

QUANTITY SCALAR or VECTOR?

5 m

30 m/sec, East

5 mi., North

20 Degrees Celsius

256 bytes

4000 calories

QUANTITY SCALAR or VECTOR?

5 m Scalar

30 m/sec, East Vector

5 mi., North Vector

20 Degrees Celsius Scalar

256 bytes Scalar

4000 calories Scalar

What is force categorized as?

• Force is described by the strength and direction in which it acts.

• Therefore,• Force is a VECTOR

The strength of force is measured in…

• The SI unit called the Newton (N)• One Newton is the amount of force required

to give a 1-kg mass an acceleration of 1 m/s/s. • Thus, the following unit equivalency can be

stated:

Combining Forces

• Often, more than one force acts on an object at a time.

• Net force is the combination of all forces acting on the object

• What does the net force determine?– Motion– direction

Net force Practice

• For each diagram, write the net force and direction.

HINT: The net forcealways acts in the directionof the greater force.

Balanced forces

• Equal forces acting on one object in opposite directions.

Balanced Forces

• Balanced forces result in equilibrium• How can I make this box in equilibrium?

Unbalanced Forces

• Unbalanced forces result when the forces exerted on an object are not equal.

• Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.

Free-Body Diagrams

• Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

Draw a free body diagram

• A physics book is sitting on a table and is pushed 10 N to the right. The opposing frictional force is 7 N.

ISP 209 - 2B 199/7/2006

Resolution: Consider each part separately, and don’t forget that there are other forces acting.

So how can they start moving, or accelerate?

A puzzleHorse and Cart

The horse pulls the cart with a force A (to the right).

According to Newton, the cart pulls the horse with a force –A (to the left).

Homework

• Read Introduction of Forces notes• Give 3 examples of force for each of the

following: (YOUR OWN EXAMPLES!)– Gravity– Tension– Friction– Drag/Air resistance