Forces and The Laws of Motion

Newton’s Second and Third Laws

Chapter 4: Section 3

Learning Targets• Describe an object’s

acceleration in terms of its mass and the net force acting on it

• Predict the direction and magnitude of the acceleration caused by a known net force

• Identify action-reaction pairs

P3.3A, P3.4A, P3.4 B, P3.4C

• While Newton’s First Law predicts the behavior of objects with balanced forces, Newton’s Second Law applies to the behavior of objects for which all existing forces are not balanced

• Newton’s Second Law relates force, mass, and acceleration

Newton’s Second Law

• According to this law, the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass– As the force acting upon an object is

increased, the acceleration of the object is increased.

– As the mass of an object is increased, the acceleration of the object is decreased.

Newton’s First Law Newton’s Second Law

Second Law Calculations

ΣF = ma Net force = mass x

acceleration

• Sigma (Σ) represents the vector sum of all external forces acting on the object, or the net force

In the following situations, which direction is the net force acting in?

Units of Force• Remember that the unit used to

measure force is the Newton (N).– 1 N = 1 kg * m/s2

• This unit conversion can be explained by Newton’s Second Law

The Big MisconceptionIt is often believed that

sustaining motion requires a continued force

However, according to Newton’s Second Law…

“Forces do not cause motion; forces cause acceleration”

Practice Calculations• The net force on the propeller of a 3.2

kg model airplane is 7.0 N forward. What is the acceleration of the airplane?

• The net force of a golf cart is 390 N north. The cart has a total mass of 270 kg, what are the magnitude and direction of the cart’s acceleration?

• A soccer ball kicked with a force of 13.5 N accelerates at 6.5 m/s2 to the right. What is the mass of the ball?

Reviewing Force• A force is a push or a pull upon

an object that results from its interaction with another object.– Forces result from interactions

Newton’s Third Law• Forces always exist in pairs

– For example, when a car exerts a force on a concrete barrier, the barrier also exerts a force on the car

• 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– These two forces are in opposite

directions

• Newton’s Third Law states that for every action, there is an equal and opposite reaction– These forces are called action-

reaction pairs

• Examples: – The wings of a bird push air

downwards while the air pushes the bird upward

– Fish fins push water backwards while the water pushes the fish forwards

Action-Reaction Forces• The action and reaction forces occur at

exactly the same time and because they coexist, either force can be called the action or the reaction

• Action and reaction forces each act on different objects

– Although the action-reaction forces are equal and opposite, either object may still have a net force acting on it.

• This makes a change in the state of motion possible

Net Force and Motion• So how can an object with equal

and opposite forces be in motion?– If the mass of one object is much larger

than the mass of another, the force exerted by the larger object would cause the smaller object to accelerate away• The force of the smaller object wouldn’t

cause much acceleration at all

• Force and acceleration are related but they aren’t identical

Field Forces• Remember that field forces

include gravity and attraction/repulsion forces– These forces do not involve physical

contact between two objects

• Field forces also exist in pairs– As a falling object accelerates

toward Earth due to the force of gravity, Earth also accelerates toward the object

Examples• Consider the interaction between

a baseball bat and a baseball

• Baseball pushes glove leftwards.

• Bowling ball pushes pin leftwards.

• Enclosed air particles push balloon wall outwards.