# Unit 01 “Forces and the Laws of Motion” Introduction to Forces

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Unit 01 “Forces and the Laws of Motion” Introduction to Forces

Force A force is a Push or a Pull Net force: All forces on an object.
A NET force can change the motion of an object… Net force: All forces on an object. Unit: Netwons (N)  Kgm/s2

There are 3 ways a Net Force can affect the motion of an object.
A Net Force ACCELERATES an object… which means it can make an object… Speed Up Slow Down Change Direction

A force can Speed Up an object.
The man pushed the boy, the boy speeds up.

A force can Slow Down an object.
Superman pushed the train, the train slows down.

A force can Change Direction for an object.
The person pulls the steering wheel, the car turns – changes direction.

There are 2 ways to apply a force to an object.
Contact Force Needs physical contact. Field Force Does not need physical contact.

Contact Force Forces between people and objects.
Forces between air molecules and objects. Wind Air pressure

Field Force Gravity: Magnetism (Magnetic)
An attraction between two masses. Magnetism (Magnetic) An attraction or repulsion between two poles of a magnet (north and south pole) Electrostatics (Static Electricity) An attraction or repulsion between two charges (positive charge and negative charge)

Concept Check What is a force?
A push or a pull. A force can change the motion of an object. What are the 3 ways a net force can affect the motion of an object? Speed up. Slow down. Change direction.

What are the two ways a force can be applied?
Contact Force Physical contact between objects (they touch!) Field force No physical contact between objects (they don’t touch!)

General Forces Information
Force is a Vector Vector: Amount and direction Example: Force of 50 N East Force: Push or Pull Unit: Newton (N) Remember, acceleration “a” causes an object to speed up, slow down or change direction. This means that a net force causes an object to accelerate. A Net Force means the forces are “unbalanced”.

Balanced Unbalanced

Force Diagram or Free-Body Diagram
A picture of the forces on an object. Box represents object. Arrows represent forces. A cow has two forces on it… 34N East, 50N West F= 50N F= 34N Force Diagram

Circle which box above has “unbalanced” forces on it?
When the forces on an object are “unbalanced” the object will accelerate.  Circle which box above has “unbalanced” forces on it?

Net Force Fnet= F1 + F2 + F3 + …
To determine if the forces on an object are “unbalanced” or “balanced” find the “Net Force”. Net Force: The sum of all the forces on an object. Fnet= F1 + F2 + F3 + …

Fnet = F1 + F2 Fnet = 450 N + (-200 N) Fnet = +250 N
Example 1: What is the net force on a box that is pulled west with a force of 200N and pulled to the east with a force of 450N. What is the net force on the box? Which way does the box move? Fnet = F F2 Fnet = N + (-200 N) Fnet = N The box accelerates to the right.

Fnet = F1 + F2 Equilibrium The forces on an object add to zero.
Unbalanced forces make an object accelerate. What happened when the forces are “balanced”? F = 68 N F = 68 N Equilibrium The forces on an object add to zero. Fnet = 0 The forces on the object are “balanced” Fnet = F1 + F2

Equilibrium Conditions for Equilibrium
The sum of the forces on an object add to zero. The Net Force is zero An object in equilibrium can: Not move (motionless) Move with a constant velocity Equilibrium The net force on an object adds to zero. Fnet = 0 The forces on the object are “balanced” Motionless Constant Velocity An object in equilibrium… Does not Speed Up Does not Slow Down Does not Change Direction

Forces Net Force and Equilibrium Review
Define Force:   A Push or a Pull. Can change the motion of an object. Define Net Force: The sum of all the forces on an object. An object with a Net Force on it will… Speed up, slow down, change direction. Define Equilibrium: The sum of all the forces is zero. Forces are balanced. Fnet = 0 An object in Equilibrium can be… Motionless (not moving), moving with at constant velocity

Define Force and Net Force. List 3 ways a Net Force can change motion.
Define Equilibrium. What are the 2 types of motion an object in equilibrium can have? Which of the following boxes are in “equilibrium”. How do you know the box is in equilibrium? What is the net force on the box shown? (Show your work) What is the net force on a box moving with a constant velocity of 60m/s? A cup sits motionless on a table, what is the net force on the cup? If an object moves with a constant velocity what is the net force on the object? Draw a force diagram for two objects being affected by air resistance and gravity… A man falling and accelerating downwards. A man with a parachute falling and moving down at a constant velocity. F2 = 40N F1 = 70N

Push or a Pull The sum of all the forces on an object. Speed Up
Define Force: Define Net Force: List 3 ways a Net Force can change motion: Push or a Pull The sum of all the forces on an object. Speed Up Slow Down Change Direction

The forces on the object add to zero. FNet=0
Define Equilibrium: An object in Equilibrium can have 2 types of motion: Which of the following boxes are in “equilibrium”. How do you know the box is in equilibrium? The forces on the object add to zero. FNet=0 The forces on the object are balanced. Motionless (not moving) Moving with a Constant Velocity (not speeding up, not slowing down, not changing direction) Because the forces on the box are equal and opposite. The forces on the box are balanced. The forces cancel out. The Net Force would be zero (Fnet = 0 ).

FNet = 0 FNet = 0 FNet = 0 What is the net force on the box below?
What is the net force on a box moving with a constant velocity of 60m/s? A cup sits motionless on a table, what is the net force on the cup? If an object moves with a constant velocity what is the net force on the object? F1 = 70N F2 = 40N Fnet = F1 + F2 Fnet = -70N + 40N Fnet = -30N FNet = 0 constant velocity FNet = 0 FNet = 0 Remember If an object is motionless or moving at a constant velocity, it is in Equilibrium. The Net Force on the object is zero.

Draw a force diagram for two objects being affected by air resistance and gravity…
A man fall and accelerating downwards. A man with a parachute falling and moving down at a constant velocity. Fair resistance b) a) Fair resistance Constant velocity Same sized arrows balanced forces Equilibrium Accelerating Different sized arrows unbalanced forces (Net Force) Fgravity Fgravity

Solve the problem, show a force diagram and all work.
What is the net force on an object if it is pulled west with a force of 500N and toward the east with a force of 300N. Which way does the object accelerate? A cup sits motionless on a table, what is the net force on the cup? Ms Sully pulls the last box of paper with a force of 70 N left and Ms Bucci helps her, pulling to the left with a force of 60 N If Dr. Castro pulls to the right with a force of 100N, what is the net force on the box of paper? Which teachers get the paper? What is the net force on a boat that is pushed to the right with a force of 3000N by the water current and pushed to the left by the wind with a force of 3000N? If an object moves with a constant velocity what is the net force on the object? FNet = -200N, accelerates West FNet = 0 FNet = -30N Bucci + Sully FNet = 0 FNet = 0

Unit 01 “Forces and the Laws of Motion” Introduction to Forces Stations Lab
Purpose: To introduce and begin to identify movement caused by different types of forces through hands-on activities. Directions: Move from station to station throughout the class period. At each station, follow the directions on the card at the table and make observations. Record them on your lab sheet.

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