Newton's Laws of Motion

​Newton's Laws of Motion

​Changes in Motion

​A force is an action exerted on an object that may change the object's state of rest or motion.

​

Forces can cause acceleration.​

​The SI unit of force is the Newton (N).

​

1 N = 1 kg x m/s²​

​

Weight is a force.​

Weight = mass x gravity​

Multiple Choice

Forces can cause

• a change in direction

• acceleration

• acceleration and/or a change in direction

​Types of Forces

​

Contact forces result from physical contact between two objects. Examples: friction, a push or pull, an applied force.​

​

Field forces do not involve physical contact between two objects. Examples: gravity, magnetic force, an electric field.​

Multiple Choice

Friction is a type of

• Contact Force

• Field Force

Multiple Choice

Gravity is a type of

• Contact Force

• Field Force

​Force Diagrams

​Force is a vector and has magnitude and direction.

​

A free-body diagram helps analyze a situation​.

Forces on the y-axis are independent of the forces on the x-axis.​

Multiple Choice

Force diagrams allow us to identify

• forces acting on an object

• masses of objects

• acceleration

​Newton's First Law

​Newton's First Law (Inertia)

​

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

​Inertia is the tendency of an object to resist being moved or, if the object is moving , to resist a change in speed or direction.

Multiple Choice

Inertia is the tendency of a motionless object to

• remain motionless

• accelerate

• start moving

Multiple Choice

Inertia is the tendency of an object in motion

• to remain in motion

• to slow down

• come to a stop

• to become motionless

​The sum of forces acting on an object is the net force.

​

Net force is a single force whose external effects on a rigid body are the same as the effects of several actual forces acting on the body.​

Multiple Choice

What is the net force on the airplane?

• 700 N to the right

• 100 N to the left

• 800 N to the right

• 900 N to the right

Multiple Choice

What is the net force on the box?

• 50 N to the right

• 30 N to the right

• 80 N to the right

• 80 N to the left

​Mass is a measure of inertia. The inertia of an object is proportional to the mass of an object. the greater the mass, the greater the inertia.

Multiple Choice

When mass decreases, inertia

• decrease

• increases

• remains the same

​Objects that are either at rest or moving with constant velocity are said to be in equilibrium.

​

Equilibrium is the state in which the net force on an object is zero.​

Multiple Choice

Which object is in equilibrium?

​Newton's Second and Third Laws

​Newton's Second Law -- the acceleration of an object is directly proportional to the net force acting on an object and inversely proportional to the object's mass.

​According to Newton's second law, if equal forces are applied to two objects of different masses, the object with the greater mass will experience a smaller acceleration, and the object with less mass will experience a greater acceleration.

​F = ma

Force is in Newtons (N)

Mass is in kilograms

Acceleration is in m/s²​

Multiple Choice

An object with a mass of 10 kg and acceleration of 5 m/s² has a net force of

• 3 N

• 15 N

• 10 kg

• 15 kg

Multiple Choice

If a force of 20 N is applied to an object with a mass of 5 kg, what will its acceleration be? (Ignore friction)

• 20 m/s²

• 100 m/s²

• 4 m/s²

• 15 m/s²

​Newton's Third Law -- if two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.

​Forces always exist in pairs.

​

The book exerts a force on the table, the table exerts a force on the book.​

​Another way to state it is that for every action force, there is an equal and opposite reaction force.

​Action and reaction forces act on different objects. For an object to move, we must consider only the forces acting on that object.

Multiple Choice

Identify the force action-reaction pairs for a bat and a ball.

• The ball hits the bat, the bat hits the ball.

• The bat hits the ball, the ball flies to the outfield.

• The batter swings the bat, the bat hits the ball.

• The bat hits the ball, the ball changes direction.

Multiple Choice

Identify the action-reaction pairs for the scene pictured.

• The tractor pulls the elephant, the elephant pulls the tractor.

• The tractor pulls the man, the man pulls the elephant.

• The tractor pulls the man, the man pulls the tractor.

• The elephant pulls the man, the man pulls the tractor.