MOTION

Prepared by: Linh Pham

Prepared for: 2nd-year science students

Online source: http://www.physics4kids.com/files/motion_intro.html

Today you will learn:

• Introduction of motion
• Forces
• Laws of motion
• Gravity
• Velocity
• Acceleration

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Introduction of motion

• Everything in the universe moves.
• Question: if you are sitting still at your desk, are you moving?
• Answer: Yes. You are on Earth. The Earth is moving around the Sun. The Sun is moving around our galaxy.

Forces

• Forces need to act upon an object to get it moving or to change its motion.

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• Forces could be big or small.
• Big forces: the pull of a star on a planet
• Small forces: the pull of a nucleus on an electron.

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More about forces

Scientists measure forces in units called Newtons (N).

Example: you kick a soccer ball with a force equivalent to 12 N. The force of gravity causes the ball to return to the surface.

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What is the force of gravity?

• Gravity or gravitational forces are forces of attraction.
• Every object in the universe that has mass exerts a gravitational pull, or force, on every other mass.

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Do you know this famous scientist?

• Sir Isaac Newton came up with the three basic ideas that are applied to the physics of most motion.
• Scientists now call them Newton's Three Laws of Motion. �

Newton´s First Law on Motion

• An object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted upon by an unbalanced force.
• That is, objects tends to keep on doing what they're doing!!!

Newton´s Second Law of Motion

• F = force
• M = mass
• A = acceleration

Question: how does this picture demonstrate Newton´s 2nd Law? (Remember that F = ma)

Newton´s Third Law of Motion

• The third law says that for every action (force) there is an equal and opposite reaction (force). Forces are found in pairs!

Velocity

• Velocity is word used describe if a motion is fast or slow. That is, velocity is the rate of motion in a specific direction.
• Question: if you are walking 5 km/hour. Would you say that you know your velocity?
• Answer: No. You only know your walking speed. Your velocity would be if you are walking 5 km/hour north (or S/W/E).

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Velocity…continued

• Question: A car travels 420 km in 4 hours from Alicante to Madrid. What is its velocity?
• Velocity = 105 km/h Alicante-Madrid.

Acceleration

• When velocity changes, the word “acceleration” is used.
• Acceleration is also a vector. That is, the direction needs to be specified.

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Constant acceleration

• There are a few special situations where acceleration may be constant. This type of acceleration happens when there is a constant net force applied. The best example is gravity.

Let´s see if you know…

• The acceleration of an object due to gravity is constant near the Earth´s surface. Do you know its general value?
• G = 9.82 m/s2

Just one more question….

• In a vacuum (el vacío), which would fall faster: a feather or a ball of the same mass?
• Answer: they would fall at the same rate in a vacuum.
• Note: in real life, there is air all around us. When a feather falls, it falls slowly because the air is in its way. There is a lot of air resistance and that resistance makes the feather move slower.

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Ok, last question…for real!

• Do you think that gravities (or gravitational forces) on other planets are the same on Earth?
• The gravities of other planets are different from Earth's gravity because they may have different masses and/or sizes. Even though the gravity may be smaller or larger, it will still create a constant acceleration near the surface of each planet.�

Today you have learned:

• Introduction of motion
• Forces
• Laws of motion
• Gravity
• Velocity
• Acceleration

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