Newton’s Laws of Motion

Newton’s First Law of Motion:

An object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.

Part I: Objects at Rest-Objects at rest mean they are not moving

Ex: chair on the floor, a plane on a runway-Objects will not start moving until a push or a pull is

exerted on them.Ex: You tend to fall backward when your school bus starts to move.

Part II: Objects in Motion

-An object moving at a certain velocity will continue to move forever at the same speed and in the same direction unless some unbalanced force acts on it.

Ex: Riding in a bumper car…..car stops, but you continue to move forward until your seat belt stops you.

www.physicsclassroom.com/mmedia/newtlaws/il.html

Friction and Newton’s First Law

Friction often prevents us from observing Newton’s First Law on everyday objects. (meaning: we don’t see things moving forever)

Ex: You want to give your desk a push so you can see it move forever across the floor. But when you do, it quickly comes to a stop.

WHY?Because an unbalanced force acted on the desk to stop its motion. That unbalanced force is FRICTION!!!

Inertia is Related to Mass

Newton’s first law is sometimes called the law of inertia

Inertia: the tendency of all objects to resist any change in motion

Inertia is the reason objects at rest remain at rest and objects in motion remain in motion.

Inertia is the reason you slide toward the side of a car when the driver makes a sharp turn.

Mass Is a Measure of Inertia

An object with small mass has less inertia

An object with large mass has greater inertia

This is why it is easier to start and to change the motion of an object with a small mass.

Which of these two cars would be easier to push and stop once it was moving?

Matchbox car Elvis’s pink cadillac

Newton’s Second Law of Motion

The acceleration of an object depends on the mass of the object and the amount of force applied.

Part I: Acceleration Depends on Mass

Acceleration decreases as its mass increases

Acceleration increases as mass decreases

Example: You are pushing a shopping cart at the grocery store. At the beginning of your shopping trip, you exert a small force on the cart to accelerate it. (smaller mass = greater acceleration)

Exert the same amount of force when the cart is full and the cart will not accelerate as much. (greater mass = smaller acceleration)

http://www2.hawaii.edu/~kobatake/secondlaw4.html

Part II: Acceleration Depends on Force

AccelerationAcceleration increases as the force on it increases

AccelerationAcceleration decreases as the force on it decreases

Example: When pushing the full shopping cart, if you push harder (greater force), the cart will move faster.

If you push the full shopping cart with less force, the cart will move slower.

**The acceleration is always in the same direction as the force applied. The shopping cart moved forward because the push was in a forward direction

Expressing Newton’s Second Law Mathematically

Newton’s second law shows the relationship between:

1. Acceleration = (a)2. Mass = (m)

3. Force = (F)

Formula to find acceleration: (Force divided by mass)

a = F /m

Formula to find Force: (mass times acceleration)

F = m * a

Formula to find mass: (Force divided by acceleration)

m = F/a

http://www2.hawaii.edu/~kobatake/chpt3review.html

Newton’s second law explains why objects fall to Earth with the same acceleration (9.8 m/s)

Less mass

Less Gravitational force

Less inertia = easier to move

More mass

More Gravitational force

More inertia = harder to move

Second-Law Problems

a=F/m F=m*a m=F/a

1. What is the acceleration of a 7kg mass if a force of 68.6 N is used to move it toward Earth?

A=F/m

A= 68.6 / 7kg

A=9.8 m/s/s

a=F/m F=m*a m=F/a

2. What force is necessary to accelerate a 1,250 kg car at a rate of 40 m/s/s?

F = m*a

F = 1,250 kg * 40 m/s/s

F = 50,000 N

a=F/m F=m*a m=F/a

3. What is the mass of an object if a force of 34 N produces an acceleration of 4 m/s/s?

m = F/a

m = 34 N / 4

m = 8.5 kg

Newton’s Third 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.

*All forces act in pairs (action and reaction)

*If a force is exerted, another force occurs (equal in size and opposite in direction).

For every action, there is an equal and opposite reactionSimply stated:

Action and reaction force pairs occur even if there is no motion.

Your weight pushing down on the chair is the action force

The force exerted by the chair that pushed up on your body and is equal to your weight is the reaction force.

Force Pairs Do Not Act on the Same ObjectRemember:

A force is always exerted by one object on another object.

If action and reaction forces acted on the same object, the net force would always be zero and nothing would ever move.

Action force was exerted on the water by the swimmer’s hands and feet

Reaction force was exerted on the swimmer’s hands and feet by the water

Example:

More Examples of Action and Reaction Force Pairs

Momentum Is a Property of Moving Objects

Momentum: is a property of a moving object that depends on the object’s mass and velocity.

The more momentum an object has, the harder it is to stop the object or change its direction.

If an object has more mass, it has more momentum.

If an object has a greater velocity, it has more momentum.

http://www2.hawaii.edu/~kobatake/thirdlaw2.html

http://www.physicsclassroom.com/class/newtlaws/U2L4b.html

Momentum is Conserved

When a moving object hits another object, some or all of the momentum is transferred to the other object.

Example: You are playing pool. You hit the cue ball. It travels across the pool table and hits another ball. That ball starts moving, but the cue ball stops.

All of the cue ball’s momentum was transferred to the other pool ball. That ball now moved away with the same amount of momentum the cue ball had.

Law of conservation of momentum: any time two or more objects interact, they may exchange momentum, but the total amount of momentum stays the same.

Explain the conservation of momentum in this photo

The bowling ball rolls down the lane with a certain amount of momentum

When the ball hits the pins, some of the ball’s momentum is transferred to the pins and the pins move off in different directions.

Some of those pins that were hit with the ball will go on to hit other pins, transferring the momentum again.

A Review of Newton’s Laws of Motion

Newton’s First Law:

An object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.

Newton’s Second Law:

The acceleration of an object depends on the mass of the object and the amount of force applied.

Newton’s Third Law:

For every action, there is an equal and opposite reaction

A Check-Up on Newton’s Laws

1. How does Newton’s second law explain why it is easier to push a bicycle than to push a car with the same acceleration?

The bicycle has a smaller mass, so a smaller force is required to give it the same acceleration as the car.

2. What are two ways that you can increase the acceleration of a loaded grocery cart?

You can increase the force applied to the cart, or you can decrease the mass of the cart by removing some of the objects from it.

3. How does Newton’s third law explain how a rocket takes off?The hot gases expelled from the back of the rocket produce a reaction force on the rocket that lifts and accelerates the rocket.