Forces

Newton’s 1st Law of Motion An object at rest will stay at rest, and an object in motion will stay in motion UNLESS acted on by an unbalanced force In other words: An object will keep doing what it’s doing until a force changes its motion

Forces A force is a push or a pull on an object All changing motion is caused by ONE or MORE forces. The SI unit for force is Newton (N).

Kinds of forces we will talk about Frictional Force Air Resistance Gravitational Force Tensional Force Applied Force Electrical Force Normal Force Spring Force Magnetic Force

Newton’s Second Law of Motion An object will accelerate in the same direction as the net force on an object! Force = mass x acceleration F = m x a

Net Force Net force = F1 + F2 + F3 + … The net force on an object is the sum of all the forces on the object in each direction. Net force = F1 + F2 + F3 + …

Practice Problem Bob the squirrel is mowing his lawn. A lawn mower is pushed across the ground with a force of 25 N. The ground provides 14 N of resistance. What is the net force on the lawn mower in the x-direction (along the ground)? Net Force = F1 + F2 + F3…… F1 = + 25 N F2 = - 14 N (negative because it is resisting) Net Force = (+25N) + (-14N) = 25N – 14N = 11N

TRY IT! If the mass of a helicopter is 4,500 kg. and the net force on it is 18,000 N, what is the helicopter’s acceleration? Given: Finding? m = 4,500 kg a = ? (acceleration) F = 18,000 N F = m x a a = F_ m = 18,000 N 4,500 kg a = 4 m/s2 F a m

Gravity Gravity is the attractive force between two objects Gravity’s strength depends on the masses of the objects and the distance between them

Force of Earth’s Gravity F = mg m is the mass of the object gravity is acting on g is the gravitational acceleration constant: g = 9.8m/s2 The “F” on Earth is weight.

Let’s try one! On earth, what is the weight of a television having a mass of 75 kg? F = mg where ‘F’ is weight m= 75 kg g = 9.8 m/s2 weight =? weight = (75 kg) (9.8 m/s2)= 735 kg x m/s2 weight = 735 N

Try one by yourself….. Find the acceleration due to gravity on Mars if a person with a mass of 60.0 kg weighs 22.2 N on Mars?

Normal Force Fnormal + Fgravity = 0 N When gravity acts on an object, but the object does not fall, there is a normal force pushing up on the object Fnormal + Fgravity = 0 N

Balanced Forces If an object is not accelerating, either: 1. no forces are acting on it, or 2. all the forces balance out Two forces are balanced if they have BOTH: equal sizes opposite directions

Unbalanced Forces Every force has a magnitude and a direction If the net force (sum of the magnitude in each direction) is not zero, the object accelerates Objects can have the same direction, but different magnitudes.

Force Diagrams Force diagrams use different sized arrows to represent the Forces acting on an object Including: sizes, directions, and type of force The object is usually represented by a box

EXAMPLE 1 Draw the forces affecting a textbook on the table BOOK BOOK

EXAMPLE 2 Draw the forces affecting a textbook being pushed across the table at a constant velocity BOOK BOOK

Example 3 Draw the force affecting a magnet being held to close to another magnet. N S S N S S N N S

Special Forces

Newton’s Third Law For every action force, there is an equal and opposite reaction force Action and Reaction forces do not cancel -they can act on different objects or spread in different directions

Air Resistance Air resistance is a force that opposes the motion of objects that move through air Air resistance depends on an object’s size, shape, and speed Ignore air resistance in this class unless you are told to include it

Friction Friction is the force that opposes motion. Sliding friction – opposes the motion of 2 surfaces as they slide against each other

Static Friction Static friction – prevents 2 surfaces from sliding against each other at all Rolling friction – slows down the motion of an object rolling on a surface Static friction keeps a wheel from spinning in place and makes it turn instead

Projectile Motion

The “nitty-gritty” of projectiles The only forces that acts on a projectile is gravity. So, the only acceleration is in the downward direction.

Types of Projectiles Drops- no initial velocity in the horizontal or vertical direction Throw vertically- initial velocity in the vertical, but not in the horizontal Launch- initial horizontal and vertical velocity