Forces: Understanding Strength and Direction

1. Forces

2. Force • is a push or a pull. • a force is described by its strength and by the direction in which it acts. • The strength of a force is measured in the SI unit called the newton (N).

3. Balanced forces • Balanced Forces-Equal forces acting on one object in opposite directions . • Balanced forces acting on an object do not change the object’s motion . • When equal forces are exerted in opposite directions, the net force is zero. forces cancel out.

4. Unbalanced Forces • Forces that produce a nonzero net force, which changes an object’s motion. • can cause an object to start moving, stop moving, or change direction. • Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.

5. Net Force • When the two people push the box in opposite directions, the net force on the box is the difference between their individual forces. • The overall force on an object when all the individual forces acting on it are added together • The net force determines whether an object moves and also in which direction it moves. Net Force Example

6. Friction • The force that one surface exerts on another when the two surfaces rub against each other. • In general, smooth surfaces produce less friction than rough surfaces. The strength of the force of friction depends on two factors: how hard the surfaces push together and the types of surfaces involved. Friction

7. Types of Friction • Sliding friction- Friction that occurs when one solid surface slides over another. • Rolling friction- Friction that occurs when an object rolls over a surface. • Fluid friction- Friction that occurs as an object moves through a fluid.

8. Newton’s laws • Newton’s first law of motion- states that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force. Inertia-The tendency of an object to resist any change in its motion.( depends on mass) ie. friction

9. Newton’s Laws • Newton’s second law of motion- acceleration depends on the object’s mass and on the net force acting on the object. • Newton’s third law of motion- states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. Action-Reaction forces

10. Gravity • The force that pulls objects toward each other. • Two factors affect the gravitational attraction between objects: mass and distance. • The more mass an object has, the greater its gravitational force. • The farther apart two objects are, the lesser the gravitational force between them.

11. Center of Mass • The center of gravity (mass)- is the point where all the weight of the object can be considered to be concentrated. • To balance an object ( see saw) you would need to find the center of mass

12. Air and Gravity • Free Fall- The motion of a falling object when the only force acting on it is gravity. • Air resistance- The fluid friction experienced by objects falling through the air. • Projectile- An object that is thrown.

13. Weight v. Mass • Weight- The force of gravity on an object at the surface of a planet. • Mass- Amount of matter in an object