GP Pre-Finals

Force - is operationally defined based on observed effects. This means that a force can be described in terms of what it does. However, forces do not always cause motion.

TYPES OF FORCES

Contact forces happen when objects touch each other. For example, contact forces happen when a person kicks a ball or pulls a wagon.

• Friction, spring, tension, applied, buoyant, normal

Field forces happen when two objects interact without touching each other. Field forces can create a push or a pull even at a distance. Gravity is a field force.

• Magnetic, electric, gravity

Net force (Fnet) - which is the vector sum of all the forces acting on a body, causes an object at rest to start moving. Once the object is set into motion, force can also speed it up or slow it down. An example is when you throw a ball.

FREE-BODY DIAGRAM (FBD) - You can find the net force by first constructing an idealized model of the system. To do this, you draw a free-body diagram, which is a technique used to illustrate all the external forces acting on a body or system. To construct a free-body diagram, include only the forces that act on a body, not the forces exerted by the body on other bodies.

NEWTON’S LAW OF MOTION

1st Law - 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 external/unbalanced force.

2nd Law - The acceleration of an object is dependent upon two variables. - the net force acting upon the object and the mass of the object.

Fnet = mass x acceleration

MASS VS WEIGHT (CONSEQUENCE OF GRAVITY)

• Mass is the quantity or amount of matter.

• SI unit of mass is kilogram (kg).

• Mass is constant for a body.

• Weight is the force exerted on a mass by gravity.

• SI unit of weight is Newton (N).

• Weight is not constant for a body.

3rd Law - Law of Interaction: For every action, there is an equal and opposite reaction.

EQUILIBRIUM - A body is said to be in equilibrium if no net

force acts on it. Newton’s first law of motion tells us that a body continues its state of rest or of uniform motion in a straight line if no resultant or net force acts on it.

THE FIRST CONDITION FOR EQUILIBRIUM

Forces with lines of action through the same point are called concurrent forces, and when the vector sum of these forces is equal to zero, the body is in translational equilibrium. A body in translational equilibrium has zero net force and zero acceleration.

THE CONCEPT OF TORQUE

Torque is the quantity that measures how effectively a force (F) causes a body to rotate. A torque is produced when the force is applied with leverage. It is defined as the product of the force and the lever arm. The lever arm is the perpendicular distance (l) from

the axis of rotation to the line along which the force acts.

SECOND CONDITION OF EQUILIBRIUM

The condition for an object to be in rotational equilibrium is that the sum of the torques acting on the object about any point must be equal to zero.