Forces and Newton’s Laws of Motion

Forces In order to make an object at rest move, you need to apply a push or a pull, otherwise known as a force. A force can make an object: Speed up Slow down Change direction …AKA change velocity or accelerate

Force: any influence that tends to accelerate an object. Forces produce accelerations Ex: a push or a pull Unit: Newtons (N)

Newton’s First Law Newton’s 1st Law of Motion: An object in motion will continue in motion at the same speed and in the same direction or an object at rest will remain at rest until another force changes its motion aka the Law of Inertia An object at rest stays at rest; an object in motion stays in motion Ex: An object moving in space will keep moving at a constant speed

Inertia: the property of a body to resist change in motion. Things tend to keep doing what they’re already doing. Unit: kilograms (kg) Ex: you keep moving when a car stops b/c you have inertia (you were moving with the car, but the brakes didn’t affect you)

Objects in a state of rest, remain at rest. Ex: pulling the tablecloth out from under a table full of plates and cups Table cloth trick video Objects in motion, stay in motion. Only if moving at a constant velocity in a straight line. Ex: A car you are sitting in stops, but you keep moving forward (this is why we wear seatbelts)

Mass: the amount of matter present in an object Mass and Inertia Mass: the amount of matter present in an object The more mass, the greater the inertia…and the greater the force it takes to change the state of motion. Mass remains the same wherever you are Units: kilograms (kg) Ex: your mass is 10 kg on Earth and on the moon

Check Your Understanding Why is it more difficult to stop a rolling car than a rolling toy car? The car has more mass and therefore more inertia. The inertia an object has, the more is needed to change its state of motion (liking making something stop).

Weight: the force of gravity on an object’s mass Ex: your weight on Earth is 100 N, but on the moon it is 16 N Unit: Newtons (N) Equation: W = mg W = weight (N) m = mass (kg) g = acceleration due to gravity (10m/s2)

Weight varies with location, based on gravity. Mass and Weight Mass and weight are used interchangeably in everyday language, but they are NOT the same thing in physics! Weight varies with location, based on gravity. Mass is the same everywhere; weight is not.

Check Your Understanding A ball has a mass of 10 kg on Earth. Will its mass be more or less on the moon? Neither, the mass will be the same in both locations because the mass of an object does not change. What about the weight? The ball will weigh more on the Earth than the moon because there is more gravity on Earth.

Check Your Understanding A girl has a mass of 25 kg. What is her weight on Earth? W = ? m = 25 kg g = 10 m/s2 W = mg W = (25)(10) W = 250 N

Videos The Mass Vs Weight Song https://www.youtube.com/watch?v=1whMAIGNq7E Force Sports Video http://science360.gov/obj/video/70fadaa8-c3d4-4132-ba1f-c98be5caeb14/science-nfl-football-newtons-first-law-motion The Difference Between Mass and Weight https://www.youtube.com/watch?v=_Z0X0yE8Ioc

Quick Check 1.  A space shuttle with a mass of 250 kg on Earth lands on the moon, what is it's mass on the moon? 2.  What is the same shuttle's weight on Earth?  What is it's weight on the moon (the moon's gravity is 1.6 m/s/s)?

Net force: the combination of all forces acting on an object. A force is a vector It takes into account direction Net force: the combination of all forces acting on an object. It is the net force that changes an object’s state of motion. Unit: Newtons (N) Ex: If you push on a cart with 10 N of force and someone else pushes in the opposite direction with 4 N, the net force is 6 N.

Equilibrium: when the net force is equal to zero. If an object is resting on the table, the table is pushing on it with the same force that the book is pushing on the table, the object is in equilibrium. Equilibrium: when the net force is equal to zero. An object in equilibrium has only inertia, no force acting on it and is moving at a constant velocity or not moving at all Ex: You push on the wall with 10 N of force, the wall pushes on you with 10 N of force to equal 0 N.

Check Your Understanding A girl pushes a cart with a force of 10 N, however her mischievous friend pushes the same cart in the opposite direction with 5 N of force. What is the net force? Since they are going in opposite directions, you subtract the forces to find the net force. 10 N – 5 N = 5N

Check Your Understanding A hockey player hits a hockey puck across the ice. 10 seconds after he hits it, it is still moving down the ice. Is the puck in equilibrium? Yes! Even though it is still moving, there is no net force being exerted on it, so it is moving at a constant velocity and only inertia is allowing it to keep moving.

Quick Check What is the formula for finding the weight of an object? How much does an object weigh if it has a mass of 12 g? How will the mass of that object be different if it were on the moon? How would that object’s weight be different? What would be the object’s weight on the moon?

Net force is directly proportional to acceleration This means that as one increases, the other increases by the same amount 2F = 2a 3F = 3a

Mass and Acceleration Mass and acceleration are inversely proportional As one increases, the other decreases by the same amount 2m, ½a ½m, 2a 10m, 1/10a 1/10m, 10a

Newton’s Second Law of Motion Newton’s 2nd Law: The acceleration on an object is directly proportional to the magnitude of the net force, and is inversely proportional to the mass of the object.

Forces produce accelerations Ex: The harder you push something, the faster it goes Equation: f = ma F = force (N) m = mass (kg) a = acceleration (m/s2)

Sports video http://science360.gov/obj/video/03b99e87-3efc-4e80-b53f-c66b8a0fd239/science-winter-olympics-blade-runners

Check Your Understanding A boy pushes on a 10 kg cart with a force of 50 N. What is the acceleration of the cart? a = ? F = 50 N m = 10 kg f = ma 50 =10(a) 50 = a 10 a= 5 m/s2

Check Your Understanding How much force is needed to accelerate a bike 10 m/s/s if the bike has a mass of 50 kg?

practice What is the acceleration of a .3 kg ball that is hit with a force of 25 N?

practice 2 What is the force on a 1000 kg elevator that is in free fall because the cable holding it up has broken?

practice 3 What is the mass of an object that needs a force of 4500 N to accelerate it at a rate of 5 m/s/s?

How much force is needed to make a toy car increase its speed from 2 m/s to 4 m/s in 4 seconds if the car has a mass of 5 kg?

Everything in nature comes in pairs. Balance in Nature Everything in nature comes in pairs. Forces also always come in pairs. A force is not just a push or pull, but a part of an interaction between one thing and another. One object is acting on the other object

Newton’s Third Law Newton’s 3rd Law of Motion: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. aka The Law of Action and Reaction Ex: You hit the wall, but the wall is hitting you back (hence why it hurts)

One force is the action force and the other force is the reaction force. Neither coexists without the other. You can’t touch something without being touched.

Force Video https://www.youtube.com/watch?v=GmlMV7bA0TM

Check Your Understanding Earth and the moon are “connected” to each other by a gravitational force. Is Earth pulling on the moon, or is the moon pulling on Earth? Both! They are part of the same interaction. They pull on each other in an action-reaction pair.

Check Your Understanding Identify the action and reaction pair of forces for the case of a bat interacting with a ball. The bat pushes on the ball and the ball pushes on the bat.

Action/reaction of a ball falling towards the Earth Question: when a ball falls towards the Earth is it pulling up on the Earth? Answer: Yes! The action force is gravity pulling down on the ball, the reaction is the ball pulling up on the Earth Question: Why can’t we see the Earth accelerating (moving) up to meet the ball then? Answer: Think about Newton’s second law: acceleration is inversely related to mass – the Earth is so massive that its acceleration towards the ball is tiny and we can’t see it