Newton’s Laws of Motion
From ZuluNotes - Free Leaving Cert Notes
| Previous: | Vectors and Scalars | Next: | Force, Mass and Momentum | Up: | Physics |
| Newton’s Laws of Motion | |
|---|---|
| |
| Subject: | Physics |
| Paper | 1 |
| Section | mechanics |
| Question | |
| Level | H&O |
| Note | |
| |
Contents |
Laws
Newton’s First Law of Motion
States that every object will remain in a state of rest or travelling with a constant velocity unless an external force acts on it.
Simply stated, an object moving at a constant velocity will continue to move at a constant velocity (moving at a constant speed and in a straight line) unless a nonzero net (or total) force acts upon the object. Recall that constant velocity means that the object is moving at a constant speed and in a constant direction. You might think about something you saw in the world that contradicts this law. For instance, think of a car rolling in a straight line while in neutral. If Newton's first law is true, then the car should be able to roll in a straight line at a constant speed forever. However, there is a nonzero net force acting on the car, causing it to slow down. The force responsible for slowing down the car is friction. Therefore, the above observation about a car slowing down while in neutral is not inconsistent with Newton's first law of motion.
Newton’s Second Law of Motion
States that the rate of change of an object’s momentum is directly proportional to the force which caused it, and takes place in the direction of the force.
Whenever you see an object accelerating, there must be an external force acting on the object because, as stated in Newton's first law, objects move at a constant velocity unless acted upon by an outside force.
Mathematically, Newton's second law of motion can be expressed by the following formula: a = F/m where a = acceleration, F = force, and m = mass.
What this formula tells us is that force causes an object to accelerate. However, it also tells us that the acceleration an object feels, in response to an applied force, does not solely depend on the amount of force applied. It also depends on the mass or inertia of that object. It also tells us that the more mass an object has, the less it accelerates in response to an applied force. This makes intuitive sense. For instance, if I apply the same force to a cotton ball and an elephant, the cotton ball would experience a greater acceleration than the elephant because the elephant has much more mass or inertia. Therefore, an object with a greater mass has a better tendency to resist a change in its motion when an external force is applied to that object. In other words, we say that the elephant has more inertia than the cotton ball.
Newton’s Third Law of Motion
When body A exerts a force on body B, B exerts a force equal in magnitude (but) opposite in direction (on A). Whenever a force is exerted, an equal and opposite force arises in reaction to this force. In other words, every force has an equal and opposite reaction force.
For example, when you push on a wall, the wall will also push back on you with an equal and opposite force. The reason why you don't move backwards when you push against a wall is because static friction is pushing you back with an equal amount of force, so you experience zero total (or net) force and don't move anywhere. If you go skating, ice skating for example, you are reducing the friction between you and the floor because ice is very slippery. As a result, there isn't enough friction to compensate for the wall pushing you back. If you push against the wall while ice skating, you will move backwards as a result of the reaction force to you pushing against the wall.
Applications
- Seat belts
- Rocket travel
- Ball games
