1. Chapter 13
The Nature of Forces

2. 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.

3. Combining Forces Force involves direction
Same Direction = when 2 forces act in the same direction they are added together
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

4. Combining Forces cont. + = = + = +

5. 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.

6. Friction depends on:
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.
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.

7. Types of Friction
Sliding Friction – when solid objects slide over each other
Rolling Friction – produced by objects such as wheels & ball bearings
Which has less friction? Why?
Fluid Friction – force exerted by a fluid such as water, oil, air
Air Resistance slows down falling objects

8. Additional Facts ….
Lubricants = “slippery” substances that help reduce friction
Change from sliding to fluid friction
Grease, oil, wax
Why is Friction a Good Thing?

9. 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

10. 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

11. Newton’s Second Law Force = Mass X Acceleration (F=M*A)
The greater the force, the more the acceleration
The more mass (thus more inertia) an object has the greater the force required for acceleration of the object
Newton – unit of measurement for Force
The force required to accelerate 1 kg of mass 1 m/s/s

12. 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?

13. 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?

14. Falling Objects … Galileo’s discovery was important to Newton
All falling objects accelerate at the same rate
The acceleration of a falling object is due to
the force of gravity between the object & the
Earth.
Acceleration due to gravity (g) near the Earth’s
surface is 9.8 m/s/s

15. 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:
The masses of the 2 objects
The distance between the objects
Force of gravity increases as mass of the object increases
Gravitational force decreases as distance between objects increases

16. 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