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