Gravity & Newton's Laws of Motions

Gravity & Newton's Laws of Motions

September 22, 2020

Open Ended

Would you rather live in a treehouse or on a boat? Why?

Multiple Choice

What is gravity?

• gravity is an attractive, vertical, noncontact force that acts at a distance

• gravity is a repulsive, noncontact force

• gravity is an attractive, contact force

• gravity is a horizontal force

Gravity

• Measured in Newtons (N)

• Pull of gravity on a mass gives an object weight

• To find weight, w=m*g

Multiple Choice

What is the weight, on Earth, of a book with a mass of 1.5 kg?

• 1.5 N

• 6.5 N

• 11.3 N

• 14.7 N

Universal Law of Gravitational

• Any two objects in the universe are attracted to each other by a force called gravity

• Gravitational force is affect by two factors: (1) mass & (2) distance

• Mass is measured in kilograms and grams

• Distance is measured in kilometers

• Weight is measured in Newtons.

Effect of Mass & Distance on Objects on Gravity

• Mass of an object increases ↑, gravitational force of an object increases ↑

• If distance between objects decreases ↓, gravitational force between objects increase ↑

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Multiple Choice

Increasing which factor will cause the gravitational force between two objects to decrease?

• weights of the object

• distance between the objects

• acceleration of the objects

• masses of the objects

Forces that Can Affect Something Falling

• Two forces that affect something falling: gravity & air resistance

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Terminal Velocity

• Force of gravity and force of air resistance are equal to zero

• Acceleration is also zero.

Free Fall

• When the only force acting on a falling body is gravity, it is called free fall.

• Air Resistance → Objects with less surface area will fall faster

• No Air Resistance → in a vacuum (objects will fall at the same rate & gravity only)

Projectile Motion

• Gravity can cause a thrown object to move in a curved, downward path called projectile motion

• Inertia causes ball to move horizontally, gravity causes the ball to move vertically

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Newton's Laws of Motion

Lesson Question: How do Newton's laws describe the motion of an object?

Newton's First Law (Law of Inertia)

• An object at rest stays at rest

• Object in motion stays in motion, unless acted on by an unbalanced force

• This is why rollercoasters & cars have seatbelts. If the car or roller coaster makes a sudden stop, your body will continue moving unless another force stops it. Seatbelts provide that force.

Newton's Second Law

• Total net force acting on an object is equal to mass times acceleration

• F=m*a

• Force measured in Newtons or kg x m/s², mass is measured in kg, acceleration is measured in m/s²

• To convert g → kg = divide g by 1000

Three alternative ways to use the formula:

• Finding force:

   $F\ =\ m\cdot a$  

• Finding mass:  $m\ =\ \frac{F}{a}$  

• Finding acceleration:  $a\ =\ \frac{F}{m}$  

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Multiple Choice

A force of 5 N accelerates an object. The object’s mass is 50 g (make sure to convert to kg - divide by 1000). What is the acceleration of the object? (Formula:

 $a\ =\ \frac{F}{m}$  )

• 0.01 m/s²

• 0.1 m/s²

• 10 m/s²

• 100 m/s²

Newton's Third Law of Motion

• States that for every action, there is an equal and opposite reaction

• Action and reaction forces

• Example: Rocket's engines push on those gases (action force), and the gases push back (reaction force). The reaction pushes the rocket off the ground.

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Multiple Choice

Juan inflates a balloon and then releases its end to let the balloon go free as air comes out. The balloon then flies around the room. Which statement describes how this example is an application of Newton’s laws of motion?

• According to Newton’s first law of motion, the balloon continues moving until the forces on it are unbalanced.

• According to Newton’s third law of motion, the balloon is pushed forward as the air is forced out.

• According to Newton’s third law of motion, the balloon resists any change of motion unless an unbalanced force acts upon it.

• According to Newton’s second law of motion, the balloon exerts a force on the air, which exerts a force back on the balloon.

What's Due Today and Tomorrow

• Today: Gravity Quiz & start Newton's Laws of Motion: Warm-up, Instruction, Summary, and Assignment 1

• Tomorrow: Newton's Laws of Motion: Assignment 2 & Quiz