Forces and Newton’s Laws of Motion

What is a force?

• In common usage, a force is a push or a pull.

• contact forces

• Noncontact forces or action-at-a-distance forces.

• Arrows are used to represent the forces.

• Because a force is a vector quantity and has both a magnitude and a direction.

• Often, several forces act simultaneously on a body, and the net force is the vector sum of all of them.

A spring-scale used to measure forces

Newton’s laws of motion• Isaac Newton developed three important laws that deal with force

and mass.

Newton’s First Law of Motion• An object continues in a state of rest or in a state of motion at a

constant velocity (constant speed in a constant direction), unless compelled to change that state by a net force.

Inertia and Mass• Inertia is the natural tendency of an object to remain at rest or in motion at a constant

velocity.

• The mass of an object is a quantitative measure of inertia.

• The larger the mass, the greater is the inertia.

Check Your Understanding

• 1. Which of the following statements can be explained by Newton’s first law?

• (A): When your car suddenly comes to a halt, you lunge forward. (B): When your car rapidly accelerates, you are pressed backward against the seat.

• (a) Neither A nor B (b) Both A and B (c) A but not B (d) B but not A

Newton’s Second Law of Motion

• Newton’s first law indicates that if no net force acts on an object, then the velocity of the object remains unchanged.

• The second law deals with what happens when a net force does act.

• When an external force F acts on an object of mass m, the acceleration a is directly proportional to the force and has a magnitude that is inversely proportional to the mass.

• The direction of the acceleration is the same as the direction of the net force.

How to apply the second law?

The Vector Nature of Newton’s 2. Law of Motion

Trigonometry

Example

• On a sunny day, a tall building casts a shadow that is 67.2 m long.

• Determine the height of the building.

Vector Addition and Subtraction

Subtraction

Components of vectors