Physics Day 1 of Force

LO: Difference between Weight and Mass

LO: Explain Newton's first law of MotionAgendaDo Now NotesWorksheetHW # 10 Read page 120, page 125-129

Do Pr. 1, 2and 5 on page 129

Newton’s First Law of Motion

Think about:

Is A force required to keep an object in motion?

Would it require a force to set an object in motion?

Force A force is a push or pull A force can:

Start an object moving Stop an object Speed up an object Slow down an object Exerted 施加 on an object can change the velocity.

Change the shape of an object

Force

A force is a vector, therefore it has a magnitude and direction

A force follows the rules of vector addition

Force is measured in Newtons (N)

Why do objects move?

Is a force required to keep an object in motion? Only if there is another outside force

acting on the object such as friction. Galileo found this out

An object moving horizontal moves with a constant velocity on a smooth surface.

A ball has this quality

Net Force When forces are applied to an

object, you take the sum of the force vectors to find the total force on the object

This total force is called the net force.

When the net force equals zero then all the forces are said to be balanced.

Force are balanced

Net Force

Weight

Normal force

The net force equals zero because the weight equals the normal force.

Normal Force

The normal Force is the force placed on an object by the surface it is on.

The normal Force is always perpendicular to the surface.

Weight

The weight of an object is determined by two things Gravity Mass

Formula:

W = mg(g= 9.81 m/s2)

Net Force

When a force is applied to an object it has an effect on the object.

What happens when two forces of equal magnitude are applied on an object, but are in opposite directions?

10 Newtons -10 Newtons

(The net force equals zero)

平衡

Newton’s First Law

1. An object at rest stays at rest2. An object in motion stays in

motion and continues in a straight line.

Condition 1 and 2 are true if there is no outside force (unbalanced) acting on the object.

Write in you own words

An object at rest remains at rest, and an object in motion continue in motion with constant velocity( that is constant speed in a straight line) unless the object experience a net external force.

Forces Don't Keep Objects Moving

Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and it slide to a rest position.

Newton’s First Law

Also called the law of inertia. Inertia:

The more mass an object has the more force it takes to change the motion of the object.

The faster the object moves the more force it takes to change the motion of the object.A train has a lot of inertia

Mass

Mass is essentially the measure of the inertia 惯性 an object has.

The more mass the more inertia.

Does an object have same mass on the Earth and Jupiter?

W= mg ( g earth = 9.81m/s/s) W= mg ( g Jupiter =26m/s/s) W= mg ( g moon =1.6m/s/s) http://phet.colorado.edu/sims/

mass-spring-lab/mass-spring-lab_en.html

Inertia Inertia: the resistance an object

has to a change in its state of motion.

Mass as a Measure of the Amount of Inertia

Newton's first law of motion - sometimes referred to as the law of inertia.

Question?

Have you ever experienced inertia (resisting changes in your state of motion) in an automobile while it is braking to a stop?

Inertia

Inertia: tendency 趋势 of an object to resist 抵制 changes in its velocity.

Inertia: tendency of an object to resist accelerations.

Why does he continue moving?

Which one has more inertia? Truck or car ?What happen with ladder?

Does he have a seat belt?

Can you explain Why it happens?

Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.

The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.

A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer. (CAUTION: do not attempt this at home!)

Think about:

Will an object change its state of motion with balanced force on it?

Will an object resist changes in its velocity with balanced force? Why?

Applications 1. Imagine a place in the cosmos

far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will

a. gradually 逐步 stop. b. continue in motion in the same

direction at constant speed.

Applications

2. A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

What is its mass in space?

3. Supposing you were in space in a weightless environment, would it require a force to set an object in motion?

Inertia depend upon…?

4. Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why.

Check Your Understanding 1. A group of physics teachers is

taking some time off for a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim that putters must use to guide their ball towards the hole. Mr. S guides a golf ball around the metal rim When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?