Chapter 4: Forces and the Laws of Motion

• Section 4-1: Changes in motion

lab

10/31 do now• T or F? Statement1. A force is required to keep an object moving in a given direction.2. An upward moving object must be experiencing (or at least

usually does experience) an upward force.3. A rightward moving object must be experiencing (or at least

usually does experience) a rightward force.4. A ball is thrown into the air and is moving upwards and

rightwards towards its peak. The ball experiences a force which is directed upwards and rightwards.

5. If a person throws a ball with his hand, then the force of the hand upon the ball is experienced by the ball for at least a little while after the ball leaves the hand.

6. A cannonball is shot from a cannon at a very high speed. The force of the explosion will be experienced by the cannonball for several seconds (or a least a little while).

7. If an object is at rest, then there are no forces acting upon the object.

Objectives

1. Explain how force affects the motion of an object.

2. Distinguish between contact forces and field forces.

3. Interpret and construct free-body diagrams.

• Homework: Castle Learning

What is a force?• A force is a push or pull upon an object resulting from

the object's interaction with another object. • Force is the cause of an acceleration, or the change in

an object’s velocity.• Figure 4-1 on page 124 – The ball is experiencing force in all three pictures.

• Question: How can you tell that the ball experience at least one force in each picture?

• By changing in the ball’s speed or direction.

• What is the cause of acceleration?

• What is the definition of acceleration?

• Why is a projectile has no horizontal acceleration?

questions

The SI unit of force is Newton

• 1 Newton is defined as the force needed to accelerate 1 kg mass at 1 m/s/s.

1 Newton = 1 kg 1 m/s∙ 2

Contact forces vs. field forces• Demo• What made the ball change its motion?• What made the ball change direction at the floor and accelerated upward?

• Contact forces – arises from the physical contact of two objects.

• Field forces – exists between the two interacting objects are not in physical contact with each other.

A force is a vector quantity• Force is a vector quantity• Force has a magnitude and a direction• Demo: which way is easier to pull?

15 N, E

A B

FAx FBx

FA FB

Smaller horizontal force Bigger horizontal force

• A force diagram is a diagram of the objects involved in a situation and the forces exerted on the objects.

• Free-body diagrams isolate an object and the forces acting on it.

Drawing Free-Body Diagrams• Free-body diagrams are used to show the relative magnitude and

direction of all forces acting upon an object in a given situation. • The size of the arrow reflects the magnitude of the force. The arrow

shows the direction that the force is acting. • It is generally customary to draw the force arrow from the center of the

object outward in the direction that the force is acting.

• Each force arrow in the diagram is labeled to indicate the exact type of force.

Lab - Force and Changes in Motion.

• Page 126• Objective: observe the motion of a toy car

before and after it strikes a book and diagram the forces involved in the collision.

• Material: 1 toy car, 1 book

Class work

• Page 128 – section review #1-6• Section 4-1: Diagram Skills