The Force Is with You

Vocabulary Motion - change in position Speed - rate of motion = distance / time Direction of Motion - where an object is going. Draw a straight line to represent where object was in the past and in the present.

More Vocabulary Velocity – the speed and direction of an object Acceleration – a change in either the speed or the direction of an object (speed up or SLOW DOWN) Work – force acting upon an object multiplied by the distance the object moves

Vocabulary Force – a push or pull on an object Equilibrium – a condition where all forces are balanced Friction- force when two surfaces touch. It is always in the opposite direction of motion (Newton’s 3 rd Law)

Misconceptions True or False? A force is necessary to keep object moving

Answer: False Newton’s First Law of Motion states: Objects in motion stay in motion in a straight line unless acted upon by an outside force. Outside forces such as friction, gravity, magnetic force, electrical, force, and air resistance slow/stop objects

True or False If an object is at rest, no forces are acting upon it

Answer: False There are many forces acting on it (gravity, friction, air pressure, etc) but they are all BALANCED

Newton’s 1 st Law An object in motion stays in motion in a straight line, unless acted upon by unbalanced force. A push or pull will cause object to speed up, slow down, or change direction.

Forces are Balanced Object at Rest V = zero m/s Objects in Motion V ≠ zero m/s Stay at Rest Stay in Motion (same speed and direction a = 0 m/s 2

Basically, objects just keep on doing whatever they are doing unless they are acted upon by an unbalanced force.

Common Examples Ketchup stays in the bottom (at rest) until you bang (outside force) on the end of the bottom. A headrest in a car prevents whiplash injuries during a rear-end collision ( your head goes forward and then jerks backward). Animation 1 – ladder truckAnimation 1 Animation 2 – no seatbeltAnimation 2

Inertia and Mass The natural tendency of any object is to resist changing state of motion. This is called inertia. For example, if an object is moving, it likes to keep on moving. If an object is stationary, it likes to remain stationary. It takes some measure of force to change this tendency. Are some objects capable of resisting change better than other objects?

YES, the more mass an object has, the greater its ability will be to resist change. For example, a big guy will be harder to push over than a small guy.

Check out my MASS! I have a LOT of INERTIA!

Question 1 Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that an astronaut in that place throws a rock. The rock will: a. gradually stop. b.continue in motion in the same direction at constant speed.

Answer b.continue in motion in the same direction at constant speed.

Question 2 An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

Answer Nothing, zero, nada…Zero Newtons. An object in motion will maintain its state of motion. The presence of an unbalanced force changes the velocity of the object.

Question 3 Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed, then it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why.

Answer Tosh is correct. Inertia is that quantity which depends solely upon mass. The more mass, the more inertia. Momentum is another quantity in Physics which depends on both mass AND speed. Momentum (p) is expressed as: mass X velocity

Question 4 Suppose you were in space in a weightless environment. Would it require a force to set an object in motion?

Answer Absolutely yes! Even in space, objects have mass; and if they have mass, they have inertia. That is, an object in space resists changes in its state of motion. A force must be applied to set a stationary object in motion. Newton's laws rule - everywhere!

Question 5 Mr. Wegley spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia? Explain.

Answer Mr. Wegley's inertia will increase! Mr. Wegley will increase his mass if he makes a habit of eating and lying on the sofa; and if his mass increases, then his inertia increases.

Question 6 Ben Tooclose is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. 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!

Balanced Forces

We call this NORMAL FORCE. The book is said to be at equilibrium since there are no unbalanced forces acting upon the book and it maintains its state of motion (it still sits on the table).

Lets say you are standing on the ground. Since the floor pushes upward on you and gravity is pulling down on you, you are at equilibrium.

Let’s say you give that book on the table a push… The book goes temporarily into motion, but friction from the table and gravity doesn’t allow it to go far.

ENGAGE Bill Ding Push Me, Pull Me –Flying Saucer story –Videos of cars jumping- name all forces acting upon car Penny Drop –Cart with Rocks

EXPLORE Dare You to Stop Me –How did you make one puck go faster? –How long does it take to travel 1m? 2m? 3m? Are these different rates?

EXPLAIN How do objects behave when there is no outside force? When the puck moves in a straight line, what forces are acting on it? Why doesn’t it move in a perfectly straight line?

Elaborate Knocked off Course Flag me Down

EVALUATION 1.How does an object behave when there are no forces acting upon it? 2.When the puck was moving in activity one what force(s) were acting upon it? 3.Why didn’t the puck move in a perfectly straight line? 4.Why did the puck not move each equal distance in exactly the same time? 5.Graph distance on y-axis, time on x-axis = speed 6.Graph speed on y-axis, time on x-axis = acceleration

Special thanks to the Physics Classroom Website which was used to prepare this lesson. S/Class/newtlaws/u2l2d.html