Motion and Forces

Sir Isaac Newton (1643 – 1727) One of the world’s greatest scientists Developed the 3 Laws of Motion Inertia F = m x a Every action has an equal but opposite reaction

What is a Force? A force is a push or pull When one object pushes or pulls another object, the first object is exerting a force on the second object Forces are described by strength and direction

Forces are measured in Newtons SI unit of force Symbol: N Measured by using a spring scale

There are two types of forces Balanced Forces (No change in motion) No Acceleration! Unbalanced Forces (Change in Motion) Acceleration!

Net Force We calculate forces using net force. The overall force on an object after all the forces acting on it are added is called the net force. Net Force = 3N

Net Force If you earn $2000/mo, but taxes, ins, retirement and other reductions total $350/mo then your: Gross Pay = $2000/mo - $350 Net Pay = $1650/mo $350 Net Force= $1650 $2000 12/16/04

Balanced Forces and Motion Are equal forces acting on one object in opposite directions. Equal forces acting on an object will not change the object’s motion. The net force is 0. Net force = 0 (push) Net force = 0 (pull)

Balanced Forces All forces acting on an object are equal There is no change in motion

Balanced Forces 2 or more forces acting on an object Equal in size Opposite in direction No change in motion occurs

Unbalanced Forces and Motion Can cause an object to: start moving stop moving change direction or speed. When two forces act in the same direction, they add together. When forces act in opposite directions they also add together. If one force is greater than the other force, the overall force is in the direction of the greater force.

Unbalanced Forces All forces acting on an object are not equal When a net force greater than 0 acts on an object the forces are UNBALANCED. One or more force is stronger than others Change in Motion occurs

Unbalanced Forces 2 or more forces acting on an object Unequal in size Change in motion occurs

Net Force

Let’s Review

Balanced Forces Forces cancel each other out – No change in motion! We know this because the arrows are the same size.

Unbalanced Forces Forces are not balanced – so there is a change motion! We know this because the arrows are different sizes. Upward Force is bigger so – weights move up Downward Force is bigger so – weights move down

Calculate = 13 N Unbalanced 225 N Unbalanced 0 N Balanced Forces and Net Force Balanced =

Review – Question 1 What are the differences in how balanced and unbalanced forces affect motion? Balanced forces cancel out and do not change the motion of the object. Unbalanced forces cause an object to start moving, stop moving, or change speed or direction.

Question 2 Two children are pulling on opposite ends of a rope. The result is a standoff. Explain this in terms of the net force. Because the rope does not move, each child must be pulling with the same amount of force. Because the forces are balanced, the net force equals 0.

Newton’s Laws of Motion 1st Law – Law of Inertia 2nd Law – Force = Mass x Acceleration 3rd Law – Action-Reaction Everything to do with motion, forces, gravity, speed, or acceleration is governed by these laws.

Newton’s First Law of Motion “Law of Inertia” An object will stay at rest unless acted on by an unbalanced force An object in motion will stay in motion unless acted on by an unbalanced force

Inertia Bike Race Crash

Inertia Inertia – the tendency of an object to resist change in motion (whether at rest or moving) Table Cloth Inertia Egg Drop Inertia

What does this mean? An object will “keep doing what it was doing” unless acted on by an unbalanced force If the object was sitting still, it will remain stationary If it was moving at a constant velocity, it will keep moving It takes force to change the motion of an object

Mass and Inertia The amount of inertia an object has depends on its mass The greater the mass the greater the inertia Mass is measured in grams or kilograms.

The soccer ball has a little mass and therefore a little inertia When kicked it is easy to put it into motion because it has low inertia

The brick wall has a large mass and therefore a lot of inertia When kicked it is difficult to put it into motion because it has high inertia

Objects at rest remain at rest unless acted on by a net force. Since the train is so heavy, it is difficult to change its speed. Since the baby carriage is so small, it is very easy to change its speed.

Which was easier to put into motion? Flick a pencil Flick a binder Which was easier to put into motion?

Objects in motion remain in motion in a straight line (unless acted upon by an outside force). Since the train is so heavy, it is difficult to stop it once it is moving. Since the soccer ball is so small, it is very easy to stop it once it is moving.

Which was easier to stop? Stop a pencil Stop a binder Which was easier to stop?

Two crates, one heavy and one light, are at rest on a waxed floor Two crates, one heavy and one light, are at rest on a waxed floor. Which crate will need a greater force to provide the same acceleration? Use the concept of inertia to explain your answer. The heavy crate has more inertia, so it requires more force to make it accelerate.

Section Review What is inertia? How is it related to Newton’s first law of motion? Inertia is the tendency of an object to resist any change in its motion. Newton’s first law is called the law of inertia. The law states that an object in rest stays in rest and an object in motion stays in motion unless acted on by an outside force.

Friction and Gravity Newton’s First Law of Motion states that an object in motion will stay in motion unless acted on by an outside force. Two forces that act on objects are friction and gravity.

Friction Friction acts in a direction opposite to the object’s direction of motion Without friction, the object would continue to move at a constant speed forever The strength of the force of friction depends on two factors: the types of surfaces how hard the surfaces push together.

Types of Friction 1. Sliding Friction – solid surfaces slide over each other. 2. Rolling Friction – when an object rolls over a surface. Force of friction is less in rolling friction than with sliding friction. 3. Fluid Friction – when an object moves through a fluid. Force of friction is less in fluid friction than with sliding friction.

Explain the role of friction in walking on a sidewalk versus walking on a sidewalk covered with ice. Friction between your shoe and the sidewalk allows you to push against the sidewalk. Ice decreases this friction.

Gravity Gravity – is the force that pulls objects toward each other. Force and Gravity Gravity – is the force that pulls objects toward each other.

Air Resistance Objects falling through air experience a type of friction called air resistance As surface area increases, air resistance increases. Eventually, air resistance equals gravity The greatest velocity an object reaches is called terminal velocity In a vacuum, all objects fall at the same rate Resultant Force Podcast

Weight vs. Mass Weight – a measure of the force of gravity on an object. (weight is a force) (Weight on the moon is 1/6 of your weight on Earth) Mass – a measure of the amount of matter in that object. Weight = Mass x Acceleration due to gravity Units of Measure: Weight – measured in Newtons Mass – measured in Kg Acceleration – measured in m/s2 (50 Kg person weighs 50kg x 9.8m/s2 = 490 Newtons) Your Weight on Other Worlds Newton’s 1st Law of Motion

Some Examples from Real Life A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion. Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

Check Your Understanding Ben Tooclose is being chased through the woods by a moose. If Ben takes a zigzag pattern through the woods, he will be able to use the large mass of the moose to avoid being caught. Explain this in terms of inertia and Newton's first law of motion.

Answer The large mass of the bull moose means that the bull moose has a large inertia. Thus, Ben can more easily change his own state of motion (make quick changes in direction) while the moose has extreme difficulty changing its state of motion. Physics for better living!

Force, Mass, and Acceleration

Newton’s Second Law of Motion The acceleration of an object is inversely proportional to the mass of the object and directly proportional to the force Units of measure: Acceleration = m/s2 Mass = Kg Newton = Kg x m/s2 Newton - is the force needed to accelerate one kg of mass at 1 meter/sec 2 (1 N = 1kg x 1m/s2) Force = mass x acceleration F = ma Newton’s 2nd Law

Inversely Proportional As the mass increases, the acceleration decreases They both do opposite things

Directly Proportional As the force increases, the acceleration increases They both do the same thing Push the shopping cart

Shopping Cart Example

Graphing Newton’s 2nd Law For the same force: more mass causes less acceleration less mass causes more acceleration

Graphing Newton’s 2nd Law For the same mass: more force causes more acceleration less force causes less acceleration

Graphing Newton’s 2nd Law For the same acceleration: more mass needs more force less mass needs less force

What does F = ma say? F = ma basically means that the force of an object comes from its mass and its acceleration. Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force. Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.

A Little Practice How much force will you need to accelerate a 200kg wagon at 5m/s2? 1000 N How much force will you need to accelerate a 10,000kg train at 5m/s2 50,000 N Newton’s Second Law Summary

Question 1 When the net force on an object increases, how does the object’s acceleration change? The acceleration increases.

Question 2 Suppose you know the acceleration of a shopping car as it rolls down a supermarket aisle. You want to find the net force with which it was pushed. What other information do you need in order to find the force? You need to know the mass of the shopping cart.

Question 3 Suppose you doubled the force acting on an object. In what way could you change its mass to keep its acceleration? If you double the net force acting on an object, you have to double the mass of the object to keep the acceleration unchanged. Newton’s Second Law

Action and Reaction

Newton’ s 3rd Law of Motion Reaction All forces occur in pairs, and these two forces are equal in strength and opposite in direction. For every action there is an equal and opposite reaction Action

What does this mean? Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

Equal but Opposite Ice Skaters (push off of each other with equal force in opposite directions): The speed at which the two skaters move depend on their masses. (the greater the mass the slower the speed)

Think about it What happens if you are standing on a skateboard and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

Check Your Understanding While driving down the road, an unfortunate bug strikes the windshield of a bus. The bug hits the windshield and the windshield hits the bug. Which of the two forces is greater: the force on the bug or the force on the bus?

Answer Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the bug splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.

Check Your Understanding A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...

Answer The force on the gun equals the force on the bullet. (Newton’s 3rd Law) Yet, acceleration depends on both force and mass. (Newton’s 2nd Law) The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional. (Newton’s 2nd Law)

Question 3 Suppose you and a friend, who has exactly twice your mass, are on skates. You push away from your friend. How does the force with which you push your friend compare to the force with which your friend pushes you? How do your accelerations compare? The force with which you push your friend is equal and opposite to the force with which your friend pushes you. You accelerate faster than your friend and roll back further because your mass is less. Newton’s 3rd Law Lego - Newton’s Laws of Motion Laws of Motion Rap

Review (F = ma). Newton’s First Law (Inertia): Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law (F=ma): Force equals mass times acceleration (F = ma). Newton’s Third Law: For every action there is an equal and opposite reaction.