Forces and Motion

2-2 Newton’s Laws of Motion

Newton’s First Law of MotionAn object at rest remains at rest, an

object in motion remains in motion at a constant speed and in a straight line unless acted on by an unbalanced force.

A golf ball will remain at rest on a tee until it is acted on by the unbalanced force of a moving club.

An unbalanced force from

another car acts on your car and changes your car’s motion.

The collision changes your

car’s motion, not your motion. Your motion

continues with the same velocity.

Another unbalanced

force, from your seatbelt,

changes your motion.

Newton’s first law

Friction and Newton’s First LawAn object in motion remains in motion—So, why

would your desk stop if you gave it a shove?

An UNBALANCED force!

In this example, that would be friction

Newton’s first law has to do with inertia.

Inertia is the resistance to a change in motion

Mass is a measure of inertia.

An object with a small mass has less inertia than an object with a larger mass.

Inertia and Newton’s First Law

Newton’s Second Law of MotionThe acce le ra t i on o f an ob jec t depends on the mass o f t he ob jec t and the amount o f force app l i ed

Part

1

Acceleration depends on mass. The acceleration of an object decreases as the mass increases and the acceleration increases as the mass decreases.

Part

2

Acceleration depends on the force applied. Acceleration increases as the force on the object increases.

If the force applied to the carts is the same, the acceleration of the empty cart is greater than the acceleration of the loaded cart.

MASS, FORCE AND ACCELERATION

Acce lerat ion w i l l increase when a larger force i s exer ted.

The relationship of acceleration (a) to mass (m) and force (F) can be expressed by:

F= m × a

Expressing Newton’s Second Law Mathematically.

The apple has less mass than the watermelon does. So, less force is needed to give the apple the same acceleration that the watermelon has.

What is the acceleration of a 3 kg mass if a force of 14.4 N is used to move the mass? (Note: 1 N is equal to 1 kg • m/s2)

Second Law Problems

1. What is the acceleration of a 7 kg mass if a force of 68.6 N is used to move it toward Earth?

2. What force is necessary to accelerate a 1,250 kg car at a

rate of 40 m/s2?

MORE PRACTICE

3. Zookeepers carry a stretcher that holds a sleeping lion. The total mass of the lion and the stretcher is 175 kg. The lion’s

forward acceleration is 2 m/s2. What is the force necessary to

produce this acceleration?

1. What is the acceleration of a 7 kg mass if a force of 68.6 N is used to move it

toward Earth?

2. What force is necessary to accelerate a 1,250 kg car at a rate of

40 m/s2?

3. Zookeepers carry a stretcher that holds a sleeping lion. The total mass of the lion and the stretcher is 175 kg. The lion’s forward acceleration is 2 m/s2. What is the force necessary to produce this acceleration?

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Newton’s Third Law of Motion

Forces act in pairs.

All Forces Act in Pairs Action and Reaction

Gravity is the force of attraction between two objects due to mass.

In the picture, the ball pulls on the Earth with a force equal to the Earth’s pull on the ball.

The Earth is just so much more massive!!

The effect of a Reaction can be diffi cult to see.

Homework

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