Motion is a major topic in mechanics.
There are different laws that explain motion and the causes of changes in motion. The most famous of these laws of motions were proposed by Sir Issac Newton. He compiled the three laws of motion in Mathematical Principles of Natural Philosophy (published in 1687).
Before we start discussing Newton’s Laws of Motion, let us look at certain basic terms and concepts that are used to describe motion.
Force is a push or a pull that acts on an object to move it or to change its motion.
Velocity is also known as speed. The velocity of an object is influenced by forces.
Acceleration is a measure of how much velocity of an object changes in a certain time (one second).
Mass is the amount of something present and is measured in grams or kilograms.
Momentum is the total amount of motion present in a body.
Newton’s First Law of Motion
A body continues to be in its state of rest or in uniform motion along a straight line unless an external force is applied to it. Whether we push on the bike pedals to get up the hill, push on the ground to walk to the park, or pull on a stuck drawer to make it open, the force we exert makes things move. Newton’s first law tells us that when zero net force acts, the object’s velocity must remain constant. If the object is standing still, it continues to stand still. If it is moving initially, it continues to move in a straight line at a constant speed.
Newton’s first law defines inertia and is rightly called the Law of Inertia. To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrust downward at high speed and then abruptly halted.
Some applications of Newton's first law of motion are as below:
- In order to catch a moving bus safely, we must run forward in the direction of the motion of the bus.
- Whenever it is required to jump off a moving bus, we must always run for a short distance after jumping on the road to prevent us from falling in the forward direction.
- Blood rushes from your head to feet while quickly stopping when riding on a descending elevator.
- The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.
- Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
- While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or another object that abruptly halts the motion of the skateboard.
Newton’s Second Law of Motion
According to Newton’s second law of motion, the rate of change of momentum is directly proportional to the applied force and this change always takes place in the direction of the applied force. The net force acting on an object is equal to the product of the object’s mass and its acceleration.
Net force = mass * acceleration or F = ma
The more mass the object has the more net force has to be used to move it.
Some applications of Newton's second law of motion are as below:
- If you use the same force to push a truck and push a car, the car will have more acceleration than the truck, because the truck has less mass.
- It is easier to push an empty shopping cart than a full one because the full shopping cart has more mass than the empty one. This means that more force is required to push the full shopping cart.
- A cricket player lowers his hands while catching the ball. If a player does not lower his hands while catching the ball, the time to stop the ball is very small. So, a large force has to be applied to reduce the velocity of the ball to zero or to change the momentum of the ball. When a player lowers his hands, the time taken to stop the ball is increased and hence, less force has to be applied to cause the same change in the momentum of the ball. Therefore, the hands of the player are not injured.
- A karate player breaks the piles of tiles or bricks with a single blow. When a karate player strikes the piles of tiles with his hands, he does so as fast as possible, In other words, the time taken to strike the piles of tiles is very small. As the momentum of the hand of a karate player reduces to zero when his hands strike the piles of tiles in a very- very small interval of time, therefore, a very large force is exerted on the pile of tiles. This force is enough to break the pile of tiles.
Newton’s Third Law of Motion
The third law of motion states that for every action there is an equal and opposite reaction that acts with the same momentum and the opposite velocity. 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.
Some applications of Newton's third law of motion are as below:
- When air rushes out of a balloon, the opposite reaction is that the balloon flies up.
- When you dive off a diving board, you push down on the springboard. The board springs back and forces you into the air.
- Think about how fish swims through the water. A fish uses its fins to push water backward. The water also pushes the fish forward thus 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 (backward) 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). These action-reaction force pairs make it possible for birds to fly.
