Chapter 13

The Nature of Forces

What is Force?

A force is a push or pull.• The wind pushes against a sail or a magnet pulls

iron toward it

A force gives energy to an object, sometimes causing it to start moving, stop moving or change direction.

• If force is increased then movement increases

You exert a force on an object & the object exerts a force on you.

Combining Forces• Force involves direction1. Same Direction = when 2 forces act in the

same direction they are added together

2. Opposite Direction = when 2 forces act in the opposite direction they are subtracted

Total Force on an object is in one direction, the force is called Unbalanced

Forces that are in opposite direction and are equal in size are Balanced

+ =

+= 0

+=

Combining Forces cont.

Friction: A Force Opposing Motion

• Friction is a force that exists between 2 objects that are touching.

• It is a force that acts in the opposite direction of a moving object.

• Friction will cause a moving object to slow down and finally stop.

Friction depends on:

1. How hard the surfaces of the touching objects are forced together

The heavier the objects, the more friction there will be & the harder it is to move them.

2. The materials that the surfaces of the objects are made of

You must exert a force that is larger than the force of friction in order to move an object.

Types of Friction

1. Sliding Friction – when solid objects slide over each other

2. Rolling Friction – produced by objects such as wheels & ball bearings

Which has less friction? Why?

3. Fluid Friction – force exerted by a fluid such as water, oil, air

Air Resistance slows down falling objects

Additional Facts ….

• Lubricants = “slippery” substances that help reduce friction

Change from sliding to fluid frictionGrease, oil, wax

• Why is Friction a Good Thing?

Sir Isaac Newton

• Born: 4 Jan 1643 in Lincolnshire, England• Died: 31 March 1727 in London, England• Newton’s Laws describe all states of

motion – at rest, constant motion & accelerated motion

Newton’s Laws of Motion

• First Law = states that an object at rest will remain at rest or an object in motion will remain in motion until acted upon by an unbalanced force

Inertia – matter tends to resist any change in motion; the more mass an object has the more inertia it has

Newton’s Second Law

Force = Mass X Acceleration (F=M*A)

The greater the force, the more the accelerationThe more mass (thus more inertia) an object has

the greater the force required for acceleration of the object

Newton – unit of measurement for ForceThe force required to accelerate 1 kg of mass 1 m/s/s

Newton’s Third Law

• States that for every action, there is an equal and opposite reaction

What’s going to happen to the car? In what direction will it go?

GRAVITY

• In the 1500’s, the Italian scientist Galileo dropped 2 cannonballs of different masses from the Leaning Tower of Pisa.

Both cannonballs landed at the same time.

WHY?

Falling Objects …Galileo’s discovery was important to Newton

1. All falling objects accelerate at the same rate

2. The acceleration of a falling object is due to the force of gravity between the object & the Earth.

3. Acceleration due to gravity (g) near the Earth’s surface is 9.8 m/s/s

Newton’s Law of Universal Gravitation

• States that all objects in the universe attract each other by the force of gravity

• The size of the force depends on:

1. The masses of the 2 objects

2. The distance between the objects

Force of gravity increases as mass of the object increases

Gravitational force decreases as distance between objects increases

Weight and Mass

• Weight is a measure of the force of gravity on an object (measured in Newtons)

• Weight = Mass X Acceleration due to Gravity

Wt = M * g

Gravity = 9.8 m/s/s or 9.8 N