Force is a any influence that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or a flexible object to deform, or both.

1. Contact Forces

Contact forces are forces which exist between objects that are in contact.

Types:

a. Normal Reaction

The normal reaction by the table surface on the book is perpendicular to the surface.

The push exerted by a surface on an object pressing on it –this push is always perpendicular to the surface.

Types of Forces

b. Friction

The force that opposes or tends to oppose motion between surfaces in contact.

The friction between the soles of our shoes and the floor prevents us from slipping when walking.

c. Tension

The pull exerted by a stretched spring, string or rope on an object attached to it.

The tension in the spring pulls the object downward

2. Non-contact Forces

Non-contact forces are forces which do not require objects to be in contact to exist.

Types:

a. Gravitational Force

The pull exerted by the Earth’s gravity on any object (i.e. weight)

The gravitational force pulls the apple downward from the tree towards the ground.

b. Electric Force

The attractive (i.e. pull) or repulsive (i.e. push) forces between electric charges.

Attractive electric forces between unlike charges.Repulsive electric forces between like charges.

c. Magnetic Force

The attractive (i.e. pull) or repulsive (i.e. push) forces between magnets.

Repulsive magnetic force between like poles.

Attractive magnetic force between unlike poles.

A force is a vector – it has both magnitude and direction. Its SI unit is newton (N). We can use vector diagrams to add up forces with different magnitudes and directions.

In a vector diagram, a vector quantity is represented by an arrow. The direction of the arrow indicates the direction of the vector.

Vector Diagrams

When we add two or more vectors, we cannot add their magnitudes only. We need to find a single vector that produces the same effect as the vectors combined. The single vector is called the resultant vector.

1. Addition of parallel vectors

How do we add vectors?

2. Addition of non-parallel vectors

There are two methods of adding non-parallel:a. Parallelogram methodb. Tip-to-tail method

a. Parallelogram method

b. Tip-to-tail method

1. Balanced forces and Newton’s first law

Newton’s first law of motion states that every object will continue in its state of rest or uniform motion in a straight line unless a resultant force acts on it.

If the resultant force acting on an object is zero, we say the forces acting on the object are balanced.

Newton’s Laws

2. Unbalanced forces and Newton’s second law

Newton’s Second Law of Motion states that when a resultant force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force. The product of the mass and acceleration of the object gives the resultant force

If the resultant force acting on an object is not zero, we say the forces acting on the object are unbalanced.

F = m.a

F = resultant force (in N)m = mass of object (in kg)a = acceleration (in m s-2)

Newton’s Second Law of Motion tells us that:1. A resultant force F on an object produces an

acceleration a;2. Doubling the resultant force F on an object doubles

its acceleration a;3. With the same resultant force F, doubling the mass m

halves the acceleration a.

Examples1. A block of mass 2.0 kg is being pulled on a horizontal bench by a force of 12 N. If the block accelerates at 5.0 ms-2, what is the frictional force f between the block and the bench?

Solution:F = m.a

= 2 kg x 5.0 ms-2

= 10 N

Friction = 12 N – 10 N= 2 N

2. Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

Solution:F = m.a

= 1000 kg x 0.05m/s/s= 50 N

Thank You