FORCES and Newton’s 1 st Law

A force is a push or pull on an object which can cause the motion of the object to change. Forces cause accelerations! If an object at rest is subjected to an unbalanced force, it will start to move. If an object that is moving is subjected to an unbalanced force, its movement will change. What is a force?

Newton’s 1 st 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.

Unbalanced force? Consider a physics book at rest on a tabletop. There are two forces acting on the book. One force - the Earth's gravitational pull - exerts a downward force. The other force - the push of the table on the book - pushes upward on the book.

Unbalanced forces: Now consider a physics book sliding to the right. Sometime in the prior history of the book, it may have been given a shove and set in motion from a rest position. However, our focus is not upon the history of the book but rather upon the current situation of a book sliding to the right across a tabletop. The book is in motion and at the moment there is no one pushing it to the right.

Unbalanced forces Unbalanced forces cause accelerations! What will happen to the motion of this book as it slides to the right?

Contact Forces: forces that arise from direct physical contact of two objects (examples: you push on a door, or kick a ball) Field Forces: forces that exist between two objects even in the absence of physical contact between the objects (examples: gravity, electrostatic attraction between two charges, magnetism) Two categories of forces:

Examples of Forces

An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person. Applied force (a direct force)

Forces: Gravity (a field force) Weight is a force caused (on Earth) by the gravitational attraction of a mass to the Earth’s center. The weight of a body, of mass m, is defined to be the force, W, with which it is attracted to the Earth. On Earth, W = mg, where g is the acceleration due to gravity. (g ≈ 9.81 m/ s 2 on Earth). Weight depends on what planet you’re on. It doesn’t stay the same, like mass does.

Forces: Tension (a contact force) Many physics problems involve objects being pulled or suspended from a string, spring, or something similar. The force that the string (or similar) exerts on the object in these types of problems is called tension.

Friction (a contact force) The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. Friction usually opposes the motion of an object. For example, if the boy pushes on the table, it would continue moving across the room without slowing down or stopping if it weren’t for friction.

Forces: Air Resistance A ir resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speed.

Normal force A mass lies on a horizontal surface. The weight of the mass pulls it downwards. The reason it does not fall to the ground is because the horizontal surface exerts an equal and opposite force on the mass called the normal force. The normal force always acts perpendicularly to the surface that is causing it.

The Normal Force Remember, the normal force is perpendicular to the surface that causes it. It’s not always perpendicular to the ground!

Force Diagrams: A force diagram shows all the forces acting on an object, including the force's direction and its magnitude. Example: If a box were dragged along the floor, these would be the forces acting on it:

Force Diagrams: An egg is free-falling from a nest in a tree. Neglect air resistance.

Force Diagrams A tire is suspended motionless from a tree branch by two ropes.

Now you try some: A rightward force is applied to a book in order to move it across a desk with a rightward acceleration.

Now you try some: A rightward force is applied to a book in order to move it across a desk with a rightward acceleration.

How would this one be different? A rightward force is applied to a book in order to move it across a desk at constant velocity.

How would this one be different? A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. If the book is moving with a constant velocity, it isn’t accelerating. Therefore, the forces must be balanced.

How about this one? A football is moving upwards towards its peak after having been kicked by the punter. (Unless specifically mentioned, neglect air resistance.)

How about this one? A football is moving upwards towards its peak after having been kicked by the punter. (Unless specifically mentioned, neglect air resistance.)

Answer: A football is moving upwards towards its peak after having been booted by the punter. (Unless specifically mentioned, neglect air resistance.)

One more… A car is coasting to the right and slowing down.

One more… A car is coasting to the right and slowing down. Brakes work by applying friction to the wheels of a car!