# Newton’s Three Laws of Motion

Learn.

Newton’s first law of motion, also known as the law of inertia, states an object will remain at rest or moving with constant motion unless an unbalanced net force acts on it. This law at first seems counter intuitive since you see moving objects come to rest. For example, a tennis ball will eventually stop rolling when it travels along a surface. However, this is due to an unbalanced force due to friction, causing it to stop.

When we approach problems, we can draw a force diagram to determine whether the forces acting on an object are uniform.

The second law of motion relates the Force of an object to be directly proportional to the mass of the object and the velocity of the object. This can be written as F = ma where m is the mass of the object in kg, a is the acceleration (or deceleration) of an object in m/s/s and is F is the force of the object in newtons (N).

The third law of motion states that every action has an equal and opposite reaction. This means that the force one object applies to another will result in an equal and opposite force to the other object.

Memorize.

1. An object will remain at rest or moving with constant motion unless an unbalanced net force acts on it.
2. F = ma
3. Every action has an equal and opposite reaction.

Master.

Question 1. [1 mark]

If an object of mass 3 kg has an acceleration of 36 m/s/s, what is the net force acting on the object to cause this acceleration.

Question 2. [2 marks]

If an object with mass 10 kg has a force of 4 N to the left and 6 N to the right, what is the net acceleration and direction the object will move in.

Question 3. [3 marks]

Explain the forces which are acting on a table on earth which is holding a book.

Question 4.

Explain how a magician is able to pull a table cloth off a table without disturbing anything on top of the table cloth.

Extension.

Question 5. [Thought experiment]

If an object of mass m and velocity v is moving along a frictionless rail and begins to loss water from the bottom, explain what will happen to the acceleration of the object.

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