Picture this scene: England, mid 1600's, a young scientist meanders through a garden where he notices an apple fall to the ground. He takes notice.
What is special about this apple? I am sure he had noticed apples falling before in his lifetime. But this particular time, with this particular apple,
he started thinking. Since Earth's gravity causes this apple to fall to the ground, how far does the Earth's gravity actually extend? Just above the
trees? To the moon? He knew that gravity was a force caused objects to fall down. He also knew that objects projected through the air also come down.
After a little time, Sir Isaac Newton proposed that there were 3 laws that governed
all moving objects, even the objects in space. We know them today as Newton's 3 Laws of Motion.
Newton's 1st law of Motion
Newton's First Law
states that an object in motion will stay in motion and an object at rest will stay at rest until an unbalanced force is applied.
In other words, boxes resting on a wagon will stay at rest on the wagon until you give them a big push. On the flip side, if the wagon is moving and hits
a rock, the wagon stops, but the boxes continue moving, until they hit the ground. Of course friction
from the air also affects the boxes.
is a property of matter that resists a change in motion when a force is applied.
In order to change an object's motion, we need to apply a force bigger than what is already acting on it.
If a box it at rest on the floor, we say the forces acting on the box are
. The net force
is zero. In order for this box to move, we need to apply a force
larger than the force of gravity holding the box down. The forces would then be
Newton's 2nd Law of Motion
Newton's 2nd Law
states that the force applied to an object to change its motion is equal to the object's mass times the
acceleration of the object. The formula to calculate this force is F=ma. The unit for force is Newton's, or N. Suppose we want to push a 20 kg box across the floor. How much force is
is required to make this box accelerate 2 m/s2
First, plug the values into the formula:
F = (20kg)x (2 m/s2
F = 40 kg * m/s2
. The expression "kg * m/s2"
can be replaced by "N"
F = 40N
It takes 40N of force to accelerate a 20kg box.
Newton's 3rd Law of Motion
Newton's 3rd Law
states that every action has an equal and opposite reaction. If Box A exerts a force on Box B,
then Box B exerts an equal and opposite force on Box A.
When does this happen in the real world? Let's take rocket liftoff, for instance. When a rocket is launched, it pushes on the ground and the
ground pushes back.
It is true that all you need to do in order to make an object move is apply enough force to it. If it moves, you have just done
. Work is described
as the force required to move an object multiplied by the distance the object travelled, work = force x distance. The ability to do that work is called
energy. There are 2 types of energy related to motion: potential and kinetic.
is stored energy. That stored energy can be released at
a later time in the form of kinetic energy
. Take for example a roller coaster. As the coaster is pulled up the first high hill, it stores more and
more potential energy. As it goes down the hill, the potential energy is released as kinetic energy. If you raise a box into the air, it gains
potential energy as you lift it higher. That energy is converted to kinetic energy when you drop the box.