Section 2 Newton’s First and Second Laws of Motion.

Section 2

Newton’s First and Second

Laws of Motion

Key Concepts

• How does Newton’s first law relate change in motion to a zero net force?

• How does Newton’s second law relate force, mass, and acceleration?

• How are weight and mass related?

Aristotle, Galileo, and Newton

• Aristotle incorrectly proposed that force is required to keep an object moving at constant speed.

• Galileo concluded that moving objects not subjected to friction or any other force would continue to move indefinitely.

Galileo’s workhelped correctmisconceptionsabout force andmotion that hadbeen widely heldsince Aristotle’stime.

Isaac Newton

• 1665

• Trinity College in Cambridge, England

• He published a book entitled Principia.

Isaac Newtonpublished his work on forceand motion in the bookentitled Principia.

Newton’s First Law of Motion

• According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.

• Unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction.

• Newton’s first law of motion is sometimes called the law of inertia.

• Inertia is the tendency of an object to resist a change in its motion.

Newton’s First Law of Motion

• an object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed.

This crash sequence illustrates inertia—the tendencyof an object in motion to remain in motion.

At impact, the air bag deploys. Note that the test dummy continues its forward motion as the collision begins to slow the car.

WHY?

Newton’s Second Law of Motion

• How do unbalanced forces affect the motion of an object?

• An unbalanced force causes an object’s velocity to change (accelerate).

• The acceleration is directly proportional to the force acting on it.

• Newton also learned that the acceleration of an object depends upon its mass

• Mass is a measure of the inertia of an object and depends on the amount of matter the object contains.

Newton’s Second Law of Motion• According to Newton’s second law of

motion, the acceleration of an object is equal to the force acting on it divided by the object’s mass.

Formula:Acceleration = Force

Massor a = f

m

• A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

• What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?

• An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve Newton’s Second Law formula for force.)

• A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair? (Hint: Solve Newton’s second law formula for mass.)

Weight and Mass

• Weight is the force of gravity acting on an object.

• Formula:• Weight = Mass x Acceleration due to gravity

or

W = mg

Weight and Mass

• Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.

Astronaut on EarthMass = 88.0 kg; Weight = 863 N

Astronaut on MoonMass = 88.0 kg; Weight = 141 N

Reviewing Concepts

• 1. State Newton’s first law of motion in your own words.

• 2. What equation states Newton’s second law of motion?

• 3. How is mass different from weight?