An Introduction to Forces

Definition: A force is a push or a pull in a particular direction.

How does it balance?Textbook p. 53

You probably would be uneasy standing under Balanced Rock near Buhl, Idaho. Yet this unusual rock stays in place year after year. The rock has forces acting on it. So, why doesn’t it fall over?

Answer: The forces acting on the rock combine, and the rock does not move.

Now, let’s read page 54.

EXAMPLE of a PUSH:When you ride a bike, your foot pushes against the pedal. The push makes the wheels of the bike move.

EXAMPLE of a PULL:When you drop something, it is pulled to the ground by gravity.

Remember… A force is a push or pull in a particular direction.

Forces Packet p. 1

There are contact forces. Example: Air Resistance, pushing a shopping

cart, pulling on a rope in Tug’o’War

There are non-contact forces. Example: Gravity, magnetism, static

electricity

Remember…

Textbook p. 54, Review from Ch. 2- Lesson 1

Forces can make objects:start moving.move faster.move slower.stop moving.change direction.change shape.

The Big Ideas…Forces Packet p. 1

Velocity is the speed and direction of a moving object.

Acceleration is the measure of the change in velocity during a period of time.

Since forces cause a change in the speed or direction of an object, we can say that forces cause changes in velocity. SO… FORCES CAUSE ACCELERATION!

REMEMBER…Forces Packet p. 1

Forces are measured in Newtons (N).

Forces come in pairs.Forces act in a particular

direction.Forces usually cannot be seen,

but their effects can be seen.

FYI- A Newton is equal to the amount of force required to accelerate a mass of

one kilogram at a rate of one meter per

Force Facts…Forces Packet p. 1

For the next few slides:Label each picture as a push

or pull. Then describe whether the force is causing

a change in speed or direction or both.

Forces Packet p. 2

Lift Off!

Push Change in

speed

Parachute

Pull (gravity) Change in

speed and direction

Rolling a Rock Uphill

Push Change in speed

Teeter Totter

Push AND Pull Change in

speed and direction

Hitting a Golf Ball

Push Change in

speed

Net Force: The combination of all the forces acting on a object.

Forces may work together. This means the forces act in the same direction.

Forces may be opposite forces. This means the forces act in opposite directions.

Combining ForcesTextbook p. 55-56, Forces Packet p. 3

Balanced and Unbalanced ForcesTextbook p. 56, Forces Packet p. 3

Balanced Forces:Forces that combine and

form a net force of zero.

Balanced forces do not cause a change in motion.

Unbalanced Forces: Forces

acting on an object that combine and form a net force that is not zero.

Unbalanced forces cause a change in motion.

Balanced Forces

Unbalanced Forces

Video Clip- Balanced and Unbalanced Forces

2:53

Circle the best answer.1. The forces above are showing PUSHING/PULLING

forces.2. The forces above are WORKING

TOGETHER/OPPOSITE forces.3. The forces are EQUAL/NOT EQUAL.4. The forces DO/DO NOT balance each other.5. The resultant force is 1000N TO THE LEFT, 1000 N TO

THE RIGHT, or ZERO.6. There IS/IS NO motion.

Forces Packet p. 4

Answers:1. The forces above are showing PUSHING/PULLING

forces.2. The forces above are WORKING

TOGETHER/OPPOSITE forces.3. The forces are EQUAL/NOT EQUAL.4. The forces DO/DO NOT balance each other.5. The resultant force is 1000N TO THE LEFT, 1000 N TO

THE RIGHT, or ZERO.6. There IS/IS NO motion.

Forces Packet p. 4

Circle the best answer.7. The forces shown above are PUSHING/PULLING forces.8. The forces shown above are WORKING TOGETHER/OPPOSITE forces.9. The forces are EQUAL/NOT EQUAL.10. The forces DO/DO NOT balance each other.11. The stronger force is pulling to the RIGHT/LEFT.12. The weaker force is pulling to the RIGHT/LEFT.13. The motion is to the RIGHT/LEFT. Forces Packet p. 5

Answers:7. The forces shown above are PUSHING/PULLING forces.8. The forces shown above are WORKING TOGETHER/OPPOSITE forces.9. The forces are EQUAL/ NOT EQUAL.10. The forces DO/ DO NOT balance each other.11. The stronger force is pulling to the LEFT/RIGHT.12. The weaker force is pulling to the LEFT/RIGHT.13. The motion is to the LEFT/RIGHT. Forces Packet p. 5

Newton’s 1st Law(also known as The Law of Inertia)

Textbook p. 57

A moving object moves in a straight line with constant speed unless a force acts on it.

The tendency of an object at rest to remain at rest and an object in motion to remain in motion unless acted upon by an unbalanced force.

Objects do not change their motion unless a force acts on them

Newton’s First Law of MotionTextbook p. 57

Objects at Rest

Objects at rest tend to stay at rest unless acted upon by a force. [push or pull]

Newton described this tendency as inertia.

Inertia can be described as the tendency of an object to keep doing whatever’s it’s doing.

What about objects that are already in motion?

Newton stated that objects in motion tend to stay in motion until acted upon by a force (or it hits an outside force.)

The truck is in motion. What is the force that causes it to stop?

The push of the stopped car.

The car is at rest. What is the force that causes it to move?

The push of the truck.

Mass & Inertia

Mass is the amount of matter in an object.

The more MASS an object has, the more INERTIA the object has.

Bigger objects are harder to start & stopWhich vehicle has more

inertia?

Video Clip: The Science of Football- Newton’s First Law of Motion

3:51

LINK