Net Force Problems Find the net force for the following problems: = ???

Net Force Problems Find the net force for the following problems: 5N Left - 1N Right = ??? 10NLeft - 5N Right = ??? 34N Right - 20N Left = ??? I 40N Left - 20 N Right I = ??? I 100N Left - 200N Right I = ???

Newton’s First Law of Motion

First we need to define the word FORCE: The cause of motion (what causes objects to move) Two types of forces Pushes Pulls Slide from www.science-class.net

Forces may be balanced or unbalanced Balanced forces – all forces acting on an object are equal There is NO MOTION Unbalanced forces – one or more forces acting on an object are stronger than others There is MOTION A NET FORCE

Objects at Rest Objects at rest tend to stay at rest unless acted upon by a force. [push or pull] Newton described this tendency as inertia. Inertia can be described as the tendency of an object to keep doing whatever’s it’s doing.

Difference between Weight and Mass??? Difference Between Mass and Weight??? Mass is a measure of how much matter an object has. Weight is a measure of how strongly gravity pulls on matter. The MORE MASS an object has, the MORE INERTIA the object has.

Which vehicle has more inertia? Mass & Inertia Mass is the amount of matter in an object. The more MASS an object has, the more INERTIA the object has. Bigger objects are harder to start & stop Which vehicle has more inertia? Slide from www.science-class.net

What about objects that are already in motion? Newton stated that objects in motion tend to stay in motion until acted upon by a force (or hits it.)

Newton’s 1st Law (also known as the law of inertia) A moving object moves in a straight line with constant speed unless a force acts on it. The tendency of an object at rest to remain at rest and an object in motion to remain in motion unless acted upon by an unbalanced force. Objects do not change their motion unless a force acts on them

The truck is in motion. What is the force that causes it to stop? The push of the stopped car. The car is at rest. What is the force that causes it to move? The push of the truck. Slide from www.science-class.net

Newton’s Second Law of Motion How fast does it go?

Acceleration An unbalanced force causes something to accelerate.

Acceleration Acceleration is directly related to the size of the force and the direction of the force. It accelerates in the direction you push or pull it.

In other words…. Large Force = Large Acceleration F a

So….if you push twice as hard, it accelerates twice as much. In other words…. Small Force = Small Acceleration F a So….if you push twice as hard, it accelerates twice as much.

But there is a twist…. Acceleration is INVERSELY related to the mass of the object.

In other words…..using the same amount of force…. Small acceleration a Large Mass Large acceleration F a Small Mass

Newton’s Second Law Newton, that brilliant genius, observed those “rules” of acceleration and came up with his second law of motion. It is both a formula & a law.

Newton’s Second Law The acceleration of an object is directly proportional to the net force & inversely proportional to it’s mass. Force = Mass x Acceleration F = ma

Okay then… First, you need to know the units of Force, Mass & Acceleration. The units used for force are Newtons (N) The units used for mass are kilograms (kg) The acceleration units are meters per second squared (m/sec2).

Next, what is speed?

Average Speed Comparison of time and distance Distance traveled per unit time

Next, what is acceleration?

Acceleration A change in velocity Speeding up Slowing down Positive acceleration Slowing down Negative acceleration Deceleration Changing direction

Calculating Speed Given Distance & Time Speed = Distance ÷ Time

Let’s Practice Riley chunks a water balloon at a guy. Running for his life, he travels 100m in 9.83s. What was his average speed? S= Distance/time S= d/t S= 100m/9.83s S=10.17 mps (meters per second)

Calculating Distance Given Speed & Time Multiply Speed and Time Distance = Speed X Time Speed X Time = Distance

Let’s Practice If you ran 15 km/h for 20 h, how much distance would you cover? Distance = Speed x time D=ST D=15 km/h x 20 h D = 300 km

Calculating Time Given Distance and Speed Divide Distance by Time Distance ÷ Speed = Time Time = Distance ÷ Speed

Let’s Practice Marcy doesn't want to be late for class so she rushes to McDonald’s at lunch. How much time would it take Marcy to walk 2 km to McDonalds for a Big Mac if she walked at a rate of 4.5 km/h? Time = distance/speed T=DS T = 2km / 4.5 km/h T= .44 hours or 26.4 minutes

Calculating Acceleration Acceleration Equation Acceleration = (final speed (in m/s) – initial speed (in m/s)) Time (in seconds) A= (sf-si) t

Let’s practice… Rob is really bored one Saturday night and goes outside to study the nocturnal habits of mice in the hayfield. He sees a mouse sniffing along at 0.1 m/s. but it hears and starts to scurry for safety. In just 3.7 s it accelerates to 0.9 m/s. Find its acceleration.

Let’s Practice A= (sf-si) t Acceleration = (final speed (in m/s) – initial speed (in m/s)) Time (in seconds) A= (sf-si) t A= 0.9m/s – 0.1 m/s 3.7s A= 0.8m/s 3.7s A= 0.22 m/s2

Let’s Practice A roach moves down the hall at 1.2 m/s. When he sees the janitor coming down the hall, he begins to run. After 3.2 s, he is moving at 3.6 m/s. What is his acceleration?

Let’s Practice While waiting for his Mom to come out of the hairdresser's, Sean accidentally puts the car in gear and it begins to roll forward. How far would the vehicle travel if it moved at 34 m/s for 2.5 s?

Let’s Practice While showing off for some girls at the skate park, Josh D crashes.. After the crash he tumbled 30 m in 4.2 s, what was his speed in m/s

Let’s Practice Colin skateboards down the sidewalk in front of the school, traveling at 24 km/h. How much time would it take him to travel 6.0 km?

Newton’s 3rd Law For every action…..

Newton’s Third Law 3 Action and Reaction Newton’s third law describes something else that happens when one object exerts a force on another object. According to Newton’s third law of motion, forces always act in equal but opposite pairs.

Newton’s Third Law 3 Action and Reaction Another way of saying this is for every action, there is an equal but opposite reaction. This means that when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted.

Action and Reaction Forces Don’t Cancel Newton’s Third Law 3 Action and Reaction Forces Don’t Cancel The forces exerted by two objects on each other are often called and action-reaction force pair. Either force can be considered the action force or the reaction force. Action and reaction force pairs don’t cancel because they act on different objects.

Action and Reaction Forces Don’t Cancel Newton’s Third Law 3 Action and Reaction Forces Don’t Cancel You constantly use action- reaction force pairs as you move about. When you jump, you push down on the ground. The ground then pushes up on you. It is this upward force that pushes you into the air.

Action and Reaction Forces Don’t Cancel Newton’s Third Law 3 Action and Reaction Forces Don’t Cancel When a bird flies, its wings push in a downward and a backward direction. This pushes air downward and backward. By Newton’s third law, the air pushes back on the bird in the opposite directions— upward and forward. This force keeps a bird in the air and propels it forward.

Large and Small Objects Newton’s Third Law 3 Large and Small Objects When you walk forward, you push backward on the ground. Your shoe pushes Earth backward, and Earth pushes your shoe forward.

Large and Small Objects Newton’s Third Law 3 Large and Small Objects Earth has so much mass compared to you that it does not move noticeably when you push it. If you step on something that has less mass than you do, like a skateboard, you can see it being pushed back.

Newton’s Third Law 3 A Rocket Launch When the rocket fuel is ignited, a hot gas is produced. As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.

A Rocket Launch 3 This downward push is the action force. Newton’s Third Law 3 A Rocket Launch This downward push is the action force. The reaction force is the upward push on the rocket engine by the gas molecules. This is the thrust that propels the rocket upward.

Newton’s Third Law 3 Weightlessness You might have seen pictures of astronauts floating inside a space shuttle as it orbits Earth. The astronauts are said to be weightless. Yet the force of gravity on the shuttle is almost 90 percent as large as at Earth’s surface. Newton’s laws of motion can explain why the astronauts float as if there were no forces acting on them.

Newton’s Third Law 3 Measuring Weight When you stand on a scale, your weight pushes down on the scale. This causes the scale pointer to point to your weight.

Newton’s Third Law 3 Measuring Weight At the same time, by Newton’s third law the scale pushes up on you with a force equal to your weight. This fore balances the downward pull of gravity on you.

Free Fall and Weightlessness Newton’s Third Law 3 Free Fall and Weightlessness Now suppose you were standing on a scale in an elevator that is falling. A falling object is in free fall when the only force acting on the force is gravity. You and the scale are both in free fall.

Free Fall and Weightlessness Newton’s Third Law 3 Free Fall and Weightlessness Because the only force acting on you is gravity, the scale no longer is pushing up on you. According to Newton’s third law, you no longer push down on the scale.

Free Fall and Weightlessness Newton’s Third Law 3 Free Fall and Weightlessness So the scale pointer stays at zero and you seem to be weightless. Weightlessness is the condition that occurs in free fall when the weight of an object seems to be zero.