Chapter 3 Forces
What is force? • A force is a push or pull • Sometimes it is obvious that a force has been applied; But other forces aren’t as noticeable • A force can cause the motion of an object to change. • Force does not always change velocity • Remember that velocity is speed and direction
Balanced Forces • When two or more forces act on an object at the same time, the forces combine to form the net force. • Forces on an object that are equal in size and opposite in direction are called balanced forces. The students are pushing on the box with equal force but in opposite directions. The forces are balanced. Net force = zero and the box does not move.
Unbalanced Forces • The net force that moves the box will be the difference between the two forces because they are in opposite directions • They are considered to be unbalanced forces • P. 73 Table 1 Rule 2 These students are pushing on the box with unequal forces in opposite directions. The forces are unbalanced. The net force equals the difference of the two forces and the box will move in the direction of the larger force. In this case, the box will move to the right.
Unbalanced Forces cont… • The net force that acts on this box is found by adding the two forces together. • These forces are combined, or added together, because they are exerted on the box in the same direction. • P. 73 Tabel 1 Rule 1 The students are pushing on the box in the same direction. The forces are unbalanced. The net force equals the sum of the two forces. The box will move in the direction that the students push (to the right).
Review • A force is a ___________. • A force is a push or pull. • Forces are always noticeable. • False. Forces are not always noticeable. • When are forces on an object balanced? • When forces are equal in size and opposite in direction and the net force is zero.
The 3 previous examples were noticeable forces. We saw people using force to move a box. • BUT! Sometimes, there are forces that we cannot see or are unnoticeable. • I need 2 volunteers! Each of you needs your textbook
Why did the textbook stop moving? What is this unseen, unbalanced force that acts on an object in motion? • There are four main types of friction: • Sliding friction: ice skating • Rolling friction: bowling • Fluid friction (air or liquid): air or water resistance • Static friction: initial friction when moving an object Friction!
The book comes to a rest because of the presence of a force - that force being the force of friction - which brings the book to a rest position.
In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least until their was another force that stopped it).
IP • Section 1 reinforcement • On the back of your worksheet, answer question 7 on page 79 of your text.
Newton’s Laws of Motion I. Law of Inertia II. F=ma III. Action-Reaction
While most people know what Newton's laws say, many people do not know what they mean (or simply do not believe what they mean).
Newton’s Laws of Motion • 1st Law– An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. • 2nd Law – Force equals mass times acceleration. • 3rd Law – For every action there is an equal and opposite reaction.
1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.
1st Law • Inertia is the tendency of an object to resist changes in its velocity • Basically, objects tend to keep doing what they are already doing, unless a force interrupts it These pumpkins will not move unless acted on by an unbalanced force.
Knowledge Check • Scenario: • You are riding a bus to school. You get on the bus and before you can find your seat, the bus driver presses the gas pedal. • What will happen to you? • Why?
1st Law • Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!
1st Law • Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.
Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction.
Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.
Mass and Inertia The smaller the mass, the less the inertia Small/light objects are easier to move, or stop from moving. Large/heavy objects are harder to move, or stop from moving.
Demonstration • #1 I need: • 1 chair • 2 student volunteers • #2 I need: • 2 textbooks
The Science of Football • https://www.youtube.com/watch?v=08BFCZJDn9w
Newton’s 2ndLaw Force and Mass determine Acceleration
2nd Law F = m x a
2nd Law The net force of an object is equal to the product of its mass and acceleration, or F=ma.
2nd Law • When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N). • One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.
The key points of Newton’s second law are that the acceleration of an object is: - Directly proportional to the force acting on the object - Inversely proportional to the mass of the object - In the same direction as the net force acting on the object Newton’s second law is summed up by the equation Force = mass times acceleration or F=ma. Write the following formulas on your “cheat sheet”: To calculate Force: F=ma To calculate Acceleration: a=F/m To calculate Mass: m=F/a
F=ma If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration. Example 1 Mass = 10 kg, Acceleration = 5 m/s/s, Force = 5 N Mass= 10 kg, Acceleration = 10m/s/s, Force = 10 N Example 2 Mass = 5kg, Acceleration = 10m/s/s, Force = 20 N Mass = 10kg, Acceleration = 5 m/s/s, Force = 20 N
Newton’s 2nd Lawproves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s
Let’s Watch! • https://www.youtube.com/watch?v=q1n-XgNKY2I
2nd Law (F = m x a) • How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? • 1. Write the formula • F = m x a • 2. Fill in given numbers and units • F = 1400 kg x 2 meters per second/second • 3. Solve for the unknown • 2800 kg-meters/second/second or2800 N
Sample Problem - Calculating Force Mass: m=F/a Force : F=ma Acceleration: a=F/m Two sumo wrestlers face off. The wrestler on the left has a mass of 130 kg and accelerates at a rate of 1 m/s2. The wrestler on the right has a mass of 30 kg and accelerates at a rate of 32 m/s2. Who will generate more force and push his opponent outside the circle?
Sample Problem #2 - Calculating Force F=ma F=ma F = 130 X 1 F = 30 X 32 F = 130 N F = 960 N 130 N 960 N m= 130 kg a= 1 m/s2 m= 30 kg a= 32 m/s2 Confused? Watch this! https://www.youtube.com/watch?v=Wk0Z3V4VQPk
acceleration = 2 m/s2 Force = 1 N Sample Problem - Calculating Mass Force : F=ma Mass: m=F/a Acceleration: a=F/m A rocket man is accelerating at 2 m/s2. The force on it is 1 N (Newton). What is the mass of the rocket man? Mass: m=F/a F m = a 1 N m = 2 m/s2 m = .5 kg
Check Your Understanding • 1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object? • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. • 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? • 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
Check Your Understanding • 1. What acceleration will result when a 12 N net force applied to a 3 kg object? 12 N = 3 kg x 4 m/s/s • 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. 16 N = 3.2 kg x 5 m/s/s • 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? 66 kg-m/sec/sec or 66 N • 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? • 9800 kg-m/sec/sec or 9800 N
3rd Law • For every action, there is an equal and opposite reaction.
Newton’s third law relates action and reaction forces. The key points to Newton’s third law are that when objects A and B interact: - The forces of A on B equals the force of B on A - The forces are opposite in direction The two forces occur simultaneously (at the same time).
3rd Law When you sit in your chair (object A), your body (object B) exerts a downward force on the chair and the chair exerts an upward force on your body.
3rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.
Example: When you push down on a table, the force from the table’s resistance increases to match force. A = B Action/reaction force pairs occur when any two objects interact, not just through contact forces. Force A: Action Force B: Reaction
Remember our balloon racing demonstration? Not only does it apply to newton’s second law, but it also applies to newton’s third law!
Newton’s 3rd Law in Nature • Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. • The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).
3rd Law Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.
Other examples of Newton’s Third Law • The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).
3rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.