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Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.
Albert Einstein
A thought experiment is a term used to describe an experiment that is not actually performed, but instead it is carried out in the mind of the experimenter using his or her knowledge of science and logical reasoning. One very famous thought experiment was carried out by Galileo and it went something like this:
When a ball is rolled down a frictionless slope, the velocity of the ball increases.
When a ball is rolled up a frictionless slope, the velocity of the ball decreases and the ball comes to rest.
The fact that the velocity of a ball increases as it rolls down a slope and decreases as it rolls up a slope led Galileo to the logical conclusion that the velocity of the ball must remain the same as it rolls along a horizontal, frictionless surface. Simply put, the ball would have a tendency to keep moving at a constant velocity in a straight line forever.
This thought experiment by Galileo laid the groundwork for Newton and his three Laws of Motion.
Newton’s First Law of Motion: The Principle of Inertia
All objects possess inertia, which is the property of an object to resist changes in its motion. Basically, inertia says that objects are lazy because they want to keep doing whatever it is they are already doing. If they are at rest, they want to remain at rest; if they are moving, they want to continue moving at the same speed.
Newton’s First Law of Motion states that:
Every object will continue in a state of rest, or moving at a constant speed in a straight line, unless acted upon by an external, unbalanced force.
Several examples of Newton’s First Law of Motion were presented in the interactive investigation you have just completed.
We can simplify this law by saying: if the forces acting on an object are balanced, there is no change in velocity.
This does not mean that objects need a constant overall force to keep them moving. A constant overall force, also called a net force, will cause an object to accelerate. To understand how this works, you need to consider Newton’s Second Law of Motion.
Newton’s Second Law of Motion
Newton’s First Law tells us what happens to the motion of an object if the forces are balanced – when there is no net force. Newton’s Second Law tells us what happens to the objects motion when the forces are not balanced; that is, when there is a net force.
Newton’s Second Law of Motion states that:
When a net force acts on an object, the object accelerates in the direction of the net force according to the equation: 
. Where,
= the net force measured in Newtons (N)
m = the mass of the object in kilograms (kg)
= the acceleration of the object in the direction of the net force measured in m/s2
Both the acceleration and the net force are vectors. They will always be in the same direction.
This law provides a way for us to link force problems with motion problems.
Example 1
Fred rolls a 7.0 kg bowling ball giving it an acceleration of 4.0 m/s2 [forward]. What is the net force Fred exerts on the ball?
Given:
m = 7.0 kg
= 4.0 m/s2 [forward]
Required:
= ?
Solution:
= (7.0 kg)(4.0 m/s2 [forward])
= 28 N [forward]
Fred exerts a net force of 28 N [forward] on the bowling ball.
Example 2
Jennifer and her motorcycle have a combined mass of 280 kg. On her way to school, Jennifer suddenly realizes that she is late for her homeroom class. To get there on time she accelerates from 7.0 m/s [E] to 34 m/s [E] in 4.2 s. What is the net force on Jen and her motorcycle?
Given:
m = 280 kg
7.0 m/s [E]
34 m/s [E]
?t = 4.2 s
Required:
= ?
= ?
Solution:
= 6.43 m/s2 [E]
= (280 kg)(6.43 m/s2 [E])
= 1800 N [E]
The net force on Jen and her motorcycle is 1800 N [E].
Example 3
Kelly has just returned home from a night of trick-or-treating and has collected 15 kg of candy in a large bag. She pulls the bag across the floor to the right with a force of 40 N. If the force of friction between the bag and the floor is 10 N, what is the acceleration of the bag?
Given:
m = 15 kg
= 40 N [R]
= 10 N [L]
Let right be positive and left be negative.
Required:
= ?
= ?
Solution:
= 40 N -10 N
= 30 N[R]
= 2 m/s2[R]
The acceleration of the bag is 2 m/s2 to the right.
Example 4
A car is accelerated by a net force of 1 000 N [R]. If the force of friction (Ff) between the car and the road is 2 000 N, what is the total force applied on the car by the engine?
Given:
Let right be positive and left be negative.
=1 000 N [R]
= 2 000 N [L] = – 2 000 N [R]
Required:
= ?
Solution:
We know that the car is accelerating to the right because the net force is acting to the right.
=
+
=–
=1 000 N [R] – (-2 000 N [R])
= 3 000 N [R]
The engine applies a total force of 3 000 N [R].
Newton’s Third Law of Motion
According to Newton’s Third Law of Motion, forces always occur in pairs.
Newton’s Third Law of Motion can be written in two ways:
Whenever one object exerts a force on a second object, the second object exerts a force that is equal in magnitude and opposite in direction back on the first object.
or
For every action force there is an equal and opposite reaction force.
Watch the video below to see how Newton’s three laws can be explained using a bicycle!
Example 5
1. When a hockey player applies a force and pushes against the boards, the boards apply a reaction force back on the hockey player in the opposite direction. The harder the hockey player pushes the boards, the harder the boards push back on the hockey player.
2. When a soldier fires a rifle, the rifle applies a large force onto the bullet. The bullet applies the same amount of force back onto the rifle that is then driven backwards into the soldier’s shoulder; this is referred to as recoil.
3. When we run, we apply a force backwards onto the ground. The ground applies a reaction force forwards onto us, causing us to move forwards.