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CORC 1331 MAIN PAGE

PHYSICS TOPICS

Speed
Velocity
Acceleration
Constant Acceleration
Free Fall
Forces and Newton's Law
Kinetic Energy
Potential Energy

LIST OF EXAMPLES

CLASSIC PROBLEMS

Free Fall
Forces
Conservation of Energy
Conservation of Momentum

PHYSICS LINKS

Thinkquest Physics Library
Physics Zone
Multimedia Physics Studio

Forces and Newton's Laws

When physicists talk about force they mean a push or a pull. In many sports, such as football, hockey, basketball, and tennis, in order to launch a shot players push or pull on a ball or similar object. Newton's Laws are concerned with exploring the amount of force that is required to move any object and how the force changes with variation in the mass of the object and acceleration applied.

Newton established the relationship between force and mass. He observed that the amount of force that is needed to move an object is directly proportional to the mass of the object. For example, if we wish to move a pen and a book, greater force would be required to move the book because it is heavier than the pen.

Newton's First Law

In physics the three major laws concerning forces are called Newton's laws because they were established by Isaac Newton during the seventeenth century. The first law is called the law of inertia:

An object continues in its state of rest or motion at constant velocity unless there is a net force (sum of all forces that act on the body) that has been applied to this object.

Books on a table at rest

If a book is simply lying on a table in a state of rest then the sum of all forces that act on that book is equal to zero. When we remove this book from the table we exert a force on the book and change its state of rest to a state of motion. The greater the mass of the book the greater the force that we have to apply to the book in order to change its state from rest to motion. The natural tendency of any object that is not being moved is to be at rest. However it is important to note that the net force that acts on an object when it is at rest (zero) is equivalent to the net force when it is in motion and maintaining a constant speed. The term inertia describes the natural tendency of any object to be in a state of rest or in a state of motion at constant velocity when the net force on the object is equal to zero.

Newton's Second Law

Newton's First Law describes the concept of inertia and the ability of a body to continue to stay in a state of rest or to move with a constant velocity. In both cases the net force exerted on the body is zero. The Second Law deals with situations where the net force is not equal to zero anymore and the velocity of a body starts changing. By performing experiments Newton concluded his Second Law:

The acceleration is directly proportional to the net force and inversely proportional to its mass.

For instance, the same force applied to a child's wagon produces a greater acceleration than when applied to a car.

Comparison of a man pushing a child in a wagon and a man pushing a car

The acceleration always points to the direction of the net force.

 Example 1: How to find the net force of a moving body if you know the mass and acceleration.

Problem:
What is the net force being exerted on a body whose mass is 15 kg and acceleration is 20m/s2?

Solution:

Step 1:

 In this problem we are given the following: m = 15 kg a = 20m/s2

Step 2:

Plug the data into the formula given above and solve the problem:

F = 15 kg 20 m/s2 = 300 N

Answer: The net force being exerted on a body is 300 N.

 Example 2: How to find the acceleration of a moving body if you know the net force and the mass.

Problem:
A hockey puck has a mass of 163 grams and the net force being applied to it is 20 N.  Find its acceleration.

Hockey player hitting a puck

Solution:

Step 1:

In this problem we are given the following:

m = 163 g (.163 kg)
F = 20 N

Step 2:

Plug the data into the formula given above and solve the problem:

20 N = .163 kg x a

In order to find acceleration we need to divide both parts of the equation by .163 kg:

a =123 m/s2

Answer: The acceleration of the hockey puck is 123 m/s2.

 Example 3: How to find the mass of a moving body if you know the net force and acceleration.

Problem:
Find the mass of an object if the net force being exerted on it is 20 N and it accelerates at 5 m/s2.

Solution:

Step 1:

In this problem we are given the following:

F = 20 N
a = 5m/s2

Step 2:

Plug the data into the formula and solve the problem:

20 N = m 5m/s2

In order to find the mass we need to divide both parts of the equation by 5 m/s2.

m = 4 kg

Answer: The mass of the object is 4 kg.