Newton’s Laws of Motion

Housekeeping

① Who is this Newton anyway?② Revision Newton’s First Law of Motion③ Revision Newton’s Second Law of Motion④ Introduction: Newton’s Third Law of Motion

Glossary Inertia is the property of objects that makes them resist changes in their motion. So, the greater the mass of an

object, the more inertia it has. For example, it takes a much larger force to change the motion of a heavy train than it does to change the motion of a smaller car.

A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction. Forces are measured in newtons (N).

The net force is the vector sum of all the forces that act upon an object.

Resistance force is the force which an effort force must overcome in order to do work on an object.

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.

Weight is the Earth’s gravitational force of attraction on a body and is directed towards the centre of the Earth.

Mass refers to the quantity of matter in a body and is measured in kilograms (kg).

Thrust is the force applied on a surface in a direction perpendicular or normal to the surface.

Who is this Newton anyway?

English physicist, astronomer, philosopher, scientist and mathematician Sir Isaac Newton, most famous for his law of gravitation, was instrumental in the scientific revolution of the 17th century. In 1687, he published his most acclaimed work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), which has been called the single most influential book on physics.

Revision:

Newton’s First Law of Motion. Also known as the law of inertia.

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Remember Mr. van der Kwast’s demonstration using paper and weights? The weights were not moving because of the small unbalanced force acting due to friction. However, if the paper is pulled away quickly enough, this force does not act for long enough to make the weights move. If the paper is pulled to slowly, the force of friction on the weights will pull them off the table as well.

Another example:

Let us have a look at one last example:

A bus stops suddenly when the brakes are applied. The resistance forces are large and there is no thrust. Your seat is rigidly attached to the bus, so it also stops suddenly. However, the resistance forces are not acting on you. You continue to move forward at the speed that you were travelling at before the brakes were applied until there is a force to stop you.

Q: And that force could be provided by?

?

What would be your immediate resulting motion as a passenger on the bus if the bus performed the following manoeuvres…a) A very quick start from rest?b) A very sharp right-hand turn?c) An emergency stop from a speed of 60 km/h?

Revision:

Newton’s Second Law of Motion

..describes how the mass of an object affects the way that it moves when acted upon by one or more forces.

In symbols: (The acceleration is the net force on an object divided by the mass of the object.)

This formula describes the observation that larger masses accelerate less rapidly than smaller masses acted on by the same total force. This formula also describes how a particular object accelerates more rapidly when a larger total force is applied. When all of the forces on an object are balanced, the total force is zero.

Remember:

When using this formula in calculations you need to put emphasis on the preparation, and than you put the data into the formula:

Examples

The launching of a space shuttle at Cape Canaveral in Florida. At launch, a space shuttle has a mass of about 2.2 million kilograms. There are 2 forces acting on a space shuttle as it blasts off:1. The downward pull of gravity (weight) in this case 22 million newtons and2. The upward thrust resulting from the burning of fuel, which is about 29 million

newtons.The total force on the space shuttle is 7 million newtons upwards. Newton’s Second Law of Motion can be used to estimate the acceleration of the space shuttle at blast-off.

Practice question:

A 10 kg sled is pulled across the snow so that the total force acting on it is 12N. What is the average acceleration of the sled?

Newton’s Third Law of Motion

For every action, there is an equal and opposite reaction.

The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

A force is a push or a pull that acts upon an object as a results of its interaction with another object. Forces result from interactions! Some forces result from contact interactions (normal, frictional, tensional, and applied forces are examples of contact forces) and other forces are the result of action-at-a-distance interactions (gravitational, electrical, and magnetic forces). According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion.

Action Reaction

Examples of action-reaction force pairs:

• Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

• Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.

• Consider the motion of a car on the way to school. A car is equipped with wheels that spin. As the wheels

spin, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

Check Your Understanding

While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious

mess in front of the face of the driver. The firefly hits the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly

or the force on the bus?

Trick Question!

Each force is the same size. For every action, there is an equal reaction. The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.

Besides, fireflies have guts and bug guts have a tendency to be splatterable. Windshields don't have guts. There you have it.

Experiment:

Pair up, blow up a balloon and let it fly away!

Instructions:1. Inflate a balloon and hold the opening closed2. Release the balloon and observe its motion through the air3. Discuss in your group

Q: What happens to the air inside the balloon when you release the balloon?Q: Which way does the balloon move as the air is pushed out?Q: What provides the force that pushes the balloon through the air?

Summary of the laws

Newton’s Third Law of Motion: For every action, there is an equal and opposite reaction.

Newton’s First Law of Motion: An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless

acted upon by an unbalanced force.

Newton’s Second Law of Motion describes how the mass of an object affects the way that it moves when acted upon by one or more forces.

Thank you!