Introduction to Forces and Motion Chapter 1 Introduction to Forces and Motion

What is a force? Force – a push or pull exerted on an object in order to change the motion of an object Motion – the process of an object moving, changing place or changing position * Forces affect how objects move*

Lets focus on motion first.. What is a reference point? A place or object used for comparison to determine if an object is in motion It appears to stay in place

Measuring Motion Speed is one way to measure motion Speed is a measure of the distance traveled per unit of time. Speed = distance/time That means you can find the speed of any object that is in motion! Question * What are some possible units for measuring speed?

Example What is the speed of a cyclist that travels 30 kilometers in 2 hours? Calculation: Speed = distance / time Speed = 30 km / 2 hr Speed = 15 km/hr

Measuring Motion Velocity is another way to measure motion Simply put, velocity -
is the speed of an object with a direction included. Example : Runner's speed: 10 km/hr Runner's velocity: 10 km/hr to the East Question – What do you notice about the units for speed and velocity?

Why is velocity important? Have you ever been in an airplane? What would be the consequence of 
a pilot only knowing the speed of other nearby planes? Tornadoes travel at about 170 km/h. Question *
Why is knowing the velocity of a 
tornado important?

Measuring Motion Most objects do not travel at a constant speed. Acceleration is a way 
to measure changing motion. Do you walk at the exact same speed 
from class to class? What if you are late 
for class? Are you ever accelerating when you go to class? Acceleration occurs when there is a change in velocity. Remember, 
velocity is speed with direction. So acceleration occurs any time there is an increase in speed, a 
decrease in speed, or a change in direction.

Which car or cars (red, green, and/or blue) are undergoing an acceleration? Study each car individually in order to determine the answer.

Only the blue and green are gaining speed, the red moves at a constant rate and therefore isn’t accelerating

Force Forces have a size and direction ( size is the strength of the force ) Direction ( which way it is moving ) The unit used to express force is a newton (N) Usually, more than one force is acting on an object Net force – the combination of all the forces acting on an object Forces can push in same direction or in different directions

Balanced Force Balanced forces – forces in opposite directions and equal size , there is no change in motion , the net force = 0 N Example : 2 men pulling a rope with 100 N of force in opposite directions no movement takes place Unbalanced forces – forces produce a change in motion when the net force is not = 0 N , unbalanced forces are necessary to cause a nonmoving object to start moving Example : 2 dogs pulling a rope, 1 with 10N, 1 with 12N so the next force would = 2N to the right motion

Motion No Motion

When can we see unbalanced forces acting around us? Every time we stand When we lean on a wall When you hold a book

What are the unbalanced forces in this launch? Which is stronger? Look at the direction of motion

Forces acting on an object We can experience the force of friction and the force of gravity Friction - is a force that resists motion and we experience daily. The force of friction is caused by microscopic particles touching each other. These microscopic pieces on both surfaces cause friction Ex. Walking The force of friction is opposite to the motion stationary box force applied by 
person pushing box static friction 
force

Forces acting on an object Forces are present all around us and always act in pairs, so we usually 
experience more than one force. Gravity is a force that pulls objects towards each other. Question* How does gravity act on objects here on Earth?

Gravity Gravity causes all objects to accelerate toward earth at a rate of 9.8 m/s2 Law of universal gravitation- force of gravity acts between all objects in the universe gravity is always pushing objects down to the center of earth… that’s why you don’t float into space every time you jump! Gravity slows and stops motion (think about a falling baseball)

Newton and the Study of Gravity In 1665, British scientist Sir Isaac Newton made a discovery when, as legend has it, he saw an apple falling from a tree. Newton knew that unbalanced forces are needed to change the motion of objects. He concluded that an unbalanced force on the apple made the apple fall. He also reasoned that an unbalanced force on the moon kept the moon moving around the Earth. He proposed that these two forces are actually the same force––gravity.

What is the difference between weight and mass? Mass is the measure of the amount of matter in an object -(this never changes, no matter where the object is) Weight is a measure of the gravitational force on an object. Weight = mass and gravity

- Friction and Gravity

Lets talk about inertia What is inertia? -The tendency of an object to resist a change in motion. Example: the car stops, but your body moves forward The rollercoaster moves right, but your body moves left This will lead into the next part of our unit

Concept Check What did we learn so far?

18 Forces are all around us. Which of the following do you think are examples of forces? (choose all that apply) [This object is a pull 
tab] Answer A & B A Gravity B Friction C Muscles D Wind Answer – A & B

C Answer - C 19 What is the SI unit for force? A Pounds B Kilograms C [This object is a pull 
tab] Answer C A Pounds B Kilograms C Newtons Answer - C

A Answer – A 11 A dragster launches from rest to 50 mi/hr at the start of a race. How did the car accelerate? [This object is a pull 
tab] Answer A A increased speed B decreased speed C changed direction Answer – A

21 Friction acts more on objects in motion than it does on objects at rest. [This object is a pull 
tab] Answer False True False Answer - False