1. Lecture 8 Motion and Forces Ozgur Unal
NIS – PHYSICAL SCIENCE
Lecture 8
Motion and Forces
Ozgur Unal

2. NIS – PHYSICAL SCIENCE Lesson Objectives: Define force.
Explain how forces and motion are related.
Define friction and explain what causes it.

3. What is Force?
A force is a push or pull that one object exerts on another.
Force has a magnitude and a direction: Force is a vector!
Unit of force in SI is Newton (N).
Is Newton a derived unit?
Forces can change the motion
of an object.
What do we mean by
change in motion?

4. Force
When two or more forces are acting on an object, we are interested in the combination of all those forces.
The sum of all forces acting on an object is called net force.
Balanced Forces:
If the net force acting on an object is zero, we say that the forces are balanced.
Balanced forces do not change the motion (velocity) of an object.

5. Force Unbalanced Forces:
If the net force acting on an object is non-zero, we say that the forces are unbalanced.
Unbalanced forces imply a non-zero net force.
Unbalanced forces change the motion (velocity) of an object.

6. Friction
Friction is the force that opposes the sliding motion of two surfaces that are in contact.
The size of the frictional force depends on the following factors:
Materials the surfaces are made from
The roughness of the surfaces
The size of the force pushing the surfaces
What causes friction?

7. What Causes Friction?
Friction is due to the microwelds that form between two surfaces in contact.

8. Lecture 9 Friction and Air Resistance Ozgur Unal
NIS – PHYSICAL SCIENCE
Lecture 9
Friction and Air Resistance
Ozgur Unal

9. Frictional Forces
Static friction is the frictional force that prevent two surfaces in contact from sliding past each other.
Sliding friction is the force that acts in the opposite direction to the motion of a surface sliding another surface.

10. Friction
When an object rolls over a surface, a frictional force due to rolling friction slows the object down.
All three types of frictional forces is due to the microwelds on the surfaces in contact.

11. Air Resistance Consider an object being dropped from an airplane.
Will it keep increasing its speed until it hits the ground?
Another type of frictional force
called air resistance opposes the
motion of objects that move
through the air.
Air resistance is in opposite
direction to the velocity of an
object moving in air.

12. Air Resistance www.youtube.com/watch?v=PE81zGhnb0w
The amount of air resistance on an object depends on the speed, size and shape of the object.
Objects at higher speeds experience more air resistance.
How about shape and size?
In a vacuum all objects fall at the same acceleration.
Check this out!

13. Terminal Velocity As an object falls, the downward
force of gravity causes it to accelerate.
The air resistance opposes the motion.
The speed increases due to gravitational
acceleration, so does the air resistance.
At some point air resistance equals the
gravity and the net force on the object
becomes zero.
The object reaches its terminal velocity.
Terminal velocity depends on the size,
shape and the mass of the falling object.