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:Force: any any influence that influence that tends to tends to accelerate an accelerate an object.object.Forces produce accelerations

Ex: a push or a pullUnit: Newtons (N)

NEWTON’S FIRST LAWNewton’s 1Newton’s 1stst Law of Law of Motion:Motion: An object in motion will An object in motion will continue in motion at the same speed continue in motion at the same speed and in the same direction or an object and in the same direction or an object at rest will remain at rest until at rest will remain at rest until another force changes its motionanother 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

INERTIAInertia:Inertia: the property of a body to the property of a body to resist change in motion.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 AND INERTIAMass:Mass: the amount of matter the amount of matter present in an objectpresent in an objectThe 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 areUnits: kilograms (kg)Ex: your mass is 10 kg on Earth and on the moon

CHECK YOUR UNDERSTANDINGCHECK 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).

WEIGHTWeight:Weight: the force of gravity on an the force of gravity on an object’s massobject’s massEx: your weight on Earth is 100 N, but on the moon it is 16 N

Unit: Newtons (N)Equation: W = mgW = mg• W = weight (N)• m = mass (kg)• g = acceleration due to gravity (10m/s2)

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 UNDERSTANDINGCHECK YOUR UNDERSTANDINGA ball has a mass of 10 kg on A ball has a mass of 10 kg on Earth. Will its mass be more or Earth. Will its mass be more or less on the moon?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?What about the weight?The ball will weigh more on the Earth than the moon because there is more gravity on Earth.

CHECK YOUR UNDERSTANDINGCHECK YOUR UNDERSTANDINGA girl has a mass of 25 kg. What is her A girl has a mass of 25 kg. What is her

weight on Earth?weight on Earth?

W = ?m = 25 kg

g = 10 m/s2

W = mgW = (25)(10)W = 250 N

VIDEOS

• The Mass Vs Weight Song • https://www.youtube.com/watch?v=1whMAIGNq7E

• The Difference Between Mass and Weight • https://www.youtube.com/watch?v=_Z0X0yE8Ioc

WARM UP – REMEMBER TO WRITE THE QUESTION BEFORE YOU ANSWER

• 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•A force is a vector

It takes into account direction

Net force:Net force: the combination of all the combination of all forces acting on an object.forces acting on an object. It is the net force that changes an object’s state of motion.

SI 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.

• 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:Equilibrium: when the net force is when the net force is equal to zero.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 UNDERSTANDINGCHECK YOUR UNDERSTANDINGA girl pushes a cart with a force of 10 N, A girl pushes a cart with a force of 10 N,

however her mischievous friend pushes the however her mischievous friend pushes the same cart in the opposite direction with 5 N same cart in the opposite direction with 5 N of force. What is the net force?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 UNDERSTANDINGCHECK 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.

WARM UP 10/171. What is the formula for finding the

weight of an object?2. How much does an object weigh if it has

a mass of 12 g?3. How will the mass of that object be

different if it were on the moon?4. How would that object’s weight be

different?5. 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 amount2F = 2a3F = 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 2Newton’s 2ndnd Law Law:: The acceleration on an The acceleration on an object is directly object is directly proportional to the proportional to the magnitude of the net force, magnitude of the net force, and is inversely proportional and is inversely proportional to the mass of the object.to the mass of the object.

Forces produce accelerationsEx: The harder you push something, the faster it goes

Equation: a = f/ma = f/m•F = force (N)•m = mass (kg)•a = acceleration (m/s2)

CHECK YOUR UNDERSTANDINGCHECK YOUR UNDERSTANDINGA boy pushes on a 10 kg cart with a A boy pushes on a 10 kg cart with a force of 50 N. What is the force of 50 N. What is the acceleration of the cart?acceleration of the cart?

a = ?F = 50 N

m = 10 kgF = ma

50 = (10)aa = 50/10a = 5 m/s2

CHECK YOUR UNDERSTANDINGCHECK YOUR UNDERSTANDINGHow much force is needed to accelerate a bike 10 m/s/s if the bike has a mass of 50 kg?

WHITE BOARD PRACTICE

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

WHITE BOARD PRACTICE 2

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

WHITE BOARD 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?

WARM UP 10/21 – COMPLETE IN YOUR NOTES

•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?

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 3Newton’s 3rdrd Law of Law of Motion: Motion: Whenever one Whenever one object exerts a force on a second object exerts a force on a second object, the second object exerts object, the second object exerts an an equal and opposite force equal and opposite force on on the first object.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.

CHECK YOUR UNDERSTANDINGCHECK YOUR UNDERSTANDINGEarth and the moon are “connected” to each Earth and the moon are “connected” to each

other by a gravitational force. Is Earth other by a gravitational force. Is Earth pulling on the moon, or is the moon pulling pulling on the moon, or is the moon pulling on Earth?on Earth?

Both! They are part of the same interaction. They pull on each other in an action-reaction pair.

CHECK YOUR UNDERSTANDINGCHECK YOUR UNDERSTANDINGIdentify the action and reaction pair of forces Identify the action and reaction pair of forces

for the case of a bat interacting with a ball.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 Action/reaction of a ball falling towards the EarthEarth

Question: when a ball falls towards the Earth Question: when a ball falls towards the Earth is it pulling up on the Earth?is it pulling up on the Earth?

Answer: Yes! The action force is gravity Answer: Yes! The action force is gravity pulling down on the ball, the reaction is the pulling down on the ball, the reaction is the ball pulling up on the Earthball pulling up on the Earth

Question: Why can’t we see the Earth Question: Why can’t we see the Earth accelerating (moving) up to meet the ball accelerating (moving) up to meet the ball then?then?

Answer: Think about Newton’s second law: Answer: Think about Newton’s second law: acceleration is inversely related to mass – acceleration is inversely related to mass – the Earth is so massive that its acceleration the Earth is so massive that its acceleration towards the ball is tiny and we can’t see ittowards the ball is tiny and we can’t see it