Forces Chapter 3

What is a Force? A force is a push or a pull Any action that causes a change in motion

Balanced Forces Force does not always change velocity. When two or more forces act on an object at the same time, the forces combine to form the net force. Choose one direction to be positive and one to be negative

Balanced Forces The net force on the box is zero because the two forces cancel each other. Forces on an object that are equal in size and opposite in direction are called balanced forces. -NO CHANGE IN MOTION OCCURS What about unequal forces?

Unbalanced Forces They are considered to be unbalanced forces. CAUSES A CHANGE IN MOTION The net force that moves the box when unequal forces are applied will be the difference between the two forces because they are in opposite directions.

Unbalanced Forces The students are pushing on the box in the same direction. These forces are combined, or added together, because they are exerted on the box in the same direction. The net force that acts on this box is found by adding the two forces together.

Inertia and Mass Inertia is the tendency of an object to resist any change in its motion. If an object is moving, it will have uniform motion. It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it. If an object is at rest, it tends to remain at rest. Its velocity is zero unless a force makes it move. The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.

Force Mass and Acceleration Sir Issac Newton ( ) Physicist and mathematician He formulated the basic laws of mechanics, law of universal gravitation and invented calculus Newtons discoveries helped to answer many questions such as: what causes tides, how do the planets move and why do objects of different masses fall at the same rate? He was able to state rules that describe the effects of forces on the motion of objects. These rules are known as Newton’s Law of Motion (3 total)

Newton's First Law of Motion Newton's first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it. If an object is at rest, it stays at rest unless an unbalanced net force acts on it. This law is sometimes called the law of inertia.

What happens in a crash? When a driver slams on the brakes any passenger not wearing a safety belt continues to move forward at the same speed the car was traveling. The demonstrates the Law of Inertia

Safety Belts The force needed to slow a person from 50 km/h to zero in 0.1 s is equal to 14 times the force that gravity exerts on the person. The belt loosens a little as it restrains the person, increasing the time it takes to slow the person down. This reduces the force exerted on the person. The safety belt also prevents the person from being thrown out of the car. Air bags also reduce injuries in car crashes by providing a cushion that reduces the force on the car's occupants.

REVIEW: Inertia Inertia is related to the mass of an object. The greater the mass, the more inertia an object will have. Mass is measured in kilograms 1 kilogram = 2.2 pounds

What’s different about throwing a ball horizontally as hard as you can and tossing it gently? When you throw hard, you exert a much greater force on the ball. Newton’s 2 nd Law How are Force and Acceleration related?

If you throw a softball and a baseball as hard as you can, why don’t they have the same speed? The difference is due to their masses. Mass and Acceleration

If it takes the same amount of time to throw both balls, the softball would have less acceleration. Mass and Acceleration The acceleration of an object depends on its mass as well as the force exerted on it. Force, mass, and acceleration are related. If mass is kept constant: Acceleration and Force are directly related A F If Force is kept constant: Acceleration and mass are indirectly related. A M

Newton’s Second Law –The acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass. This law describes how Force, Mass and Acceleration are related. the acceleration of an object is in the same direction as the net force on the object

Using Newton’s 2 nd law F=ma a=F/m (acceleration = force / mass) F = ma (Force = mass x acceleration) M = F/a F M A Units Force = Newtons Acceleration: meters/sec2 Mass = kilograms

Newtons Second Law says: Force causes acceleration Mass resists acceleration How is it used? Used for cars – a car engine produces force and you can calculate the acceleration when you know this force, so if you want to increase the acceleration you must decrease the mass or increase the force of the engine

Weight vs Force Pounds and Newtons are units of force (different from mass: grams and kilograms) 4.48 Newtons per pound So a person weighing 100 pounds (force) would have a force of 448 newtons. A Newton is force and motion – a force of 1 Newton causes 1 Kilogram to accelerate at 1 meter per second squared

What is a Newton? 1 N = 1kgm/s 2

What is gravity? Gravity is an attractive force between any two objects that depends on the masses of the objects and the distance between them. This force increases as the mass of either object increases or as the objects move closer

4 basic forces Gravity Electromagnetic force – reason behind electricity and magnetism, chemical interactions Strong nuclear force – act on particles in the nuclei of atoms Weak Nuclear Force – act on particles in the nuclei of atoms

The Law of Universal Gravitation Law that is written as an equation that enables the force of gravity to be calculated The gravitational force between two masses decreases rapidly as the distance between the masses increases EX: if the distance between two objects doubles, the gravitational force between them becomes one fourth as large. The gravitational force between objects never completely disappears. Because of this Gravitational force is sometimes called a long-range force.

Earth’s Gravitational Acceleration All objects near Earth’s surface would fall with the same acceleration The acceleration of a falling object in free fall is about 9.8m/s 2 Acceleration is given the symbol g and is sometimes called acceleration due to gravity

Newton’s 2 nd Law and Gravity Newtons 2 nd law of motion – the force of Earth’s gravity on a falling object is the objects mass times the acceleration of gravity Force of gravity (N) = mass (kg) x acceleration of gravity (m/s2) So: F = ma is also F= mg

Weight The gravitational force exerted on an object is called the objects weight Weight and mass Weight and mass are not the same but they are related Weight = force Mass = amount of matter an object contains Weight increases as mass increases Mass is consistent through the universe, weight is not.

Weightlessness and Free Fall in a Space Shuttle As you move away from Earth’s surface, the force of gravity is not as strong To measure your weight you stand on a scale, you push on the scale and the scale pushes back. If you were in an elevator and were moving down, your weight would be zero. The scale would not push back on you because you were in free fall. A space shuttle in orbit is in free fall, but it is falling around Earth, rather than straight down. Objects in the shuttle seem to be floating because they are all falling with the same acceleration

So what does it mean to say that something is weightless in orbit? Floating in Space When you stand on a scale you are at rest and the net force on you is zero. The scale supports you and balances your weight by exerting an upward force.

The dial on the scale shows the upward force exerted by the scale, which is your weight. Floating in Space Now suppose you stand on the scale in an elevator that is falling. If you and the scale were in free fall, then you no longer would push down on the scale at all. You would have zero weight.

Projectile Motion Anything that’s thrown or shot through the air is called a projectile. Earth’s gravity causes projectiles to follow a curved path. Horizontal and Vertical Motion: How the curved path is created Example: Throwing a ball Horizontal motion – hand pushes ball forward giving ball horizontal motion. After you let go of the ball, no force accelerates it forward and velocity is constant ignoring air resistance Vertical Motion – gravity pulls it downward giving it vertical motion Both a thrown ball and a dropped ball will hit the ground at the same time if dropped from the same distance from Earth

Projectile Motion

RECALL: HOW DO WE KNOW THAT ACCELERATION OCCURS? Acceleration occurs when there has been a 1. Change of direction 2. object speeds up 3. object slows down

When a ball enters a curve, even if its speed does not change, it is accelerating because its direction is changing. Centripetal Force When a ball goes around a curve, the change in the direction of the velocity is toward the center of the curve.

Acceleration toward the center of a curved or circular path is called centripetal acceleration. Centripetal Force

According to the second law of motion, when a ball has centripetal acceleration, the direction of the net force on the ball also must be toward the center of the curved path (recall unbalanced forces). Centripetal Force The net force exerted toward the center of a curved path is called a centripetal force.

Anything that moves in a circle is doing so because a centripetal force is accelerating it toward the center. Centripetal Force and Traction EX: 1. Earth and Moon 2. Car around Curve

Suppose you give a skateboard a push with your hand. Friction According to Newton’s first law of motion, if the net force acting on a moving object is zero, it will continue to move in a straight line with constant speed. Does the skateboard keep moving with constant speed after it leaves your hand? No it slows because there must be a net force acting on it.

Friction Friction is the force that opposes the sliding motion of two surfaces that are touching each other. The amount of friction depends on two factors – the kind of surface and the force pressing the surface together. Force that works against the motion that creates them, caused by two surfaces working against one another Force of friction is always opposite the motion F in Newtons Law also includes friction so it must be included in the net force Smooth objects are not “smooth”. Microwelds exist as a bump on the surface that comes in contact with other objects. The larger the force pushing the two surfaces together is, the stronger these microwelds will be.

Microwelds

Types of Friction Static Friction – frictional force that prevents two surfaces from sliding past each other. Sliding Friction – force that opposes the motion of two surfaces sliding past each other. Caused by the microwelds constantly breaking and then forming again as an object slides along the floor Rolling Friction – frictional force between a rolling object and the surface it rolls on Air Friction – frictional force caused by air moving around an object (air resistance) Viscous Friction – frictional force caused by objects moving in water or fluid Friction is always opposing force, force will cause an object to stop eventually unless force is continually supplied ALL machines experience Friction

Static Friction/Sliding Friction

When an object falls toward Earth, it is pulled downward by the force of gravity. Air Resistance However, a friction-like force called air resistance opposes the motion of objects that move through the air. Air resistance causes objects to fall with different accelerations and different speeds. The amount of air resistance on an object depends on the speed, size, and shape of the object.

Air resistance acts in the opposite direction to the motion of an object through air. Air Resistance If the object is falling downward, air resistance acts upward on the object.

As an object falls, the downward force of gravity causes the object to accelerate. Air resistance will increase. Terminal Velocity – greatest falling speed This causes the net force on a sky diver to decrease as the sky diver falls. Finally, the upward air resistance force becomes large enough to balance the downward force of gravity. This means the net force on the object is zero. Then the acceleration of the object is also zero, and the object falls with a constant speed called the terminal velocity.

Newton’s Third Law of Motion – describes action- reaction pairs: When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first object. Another way of stating it: to every action force there is an equal and opposite reaction force Action and Reaction forces don’t cancel each other out. Action and reaction forces act on different objects. Even though the forces are equal they are not balanced because they act on different objects. Ex: a swimmer in water. The swimmer push on the water and the water pushes back on the swimmer

For example, a swimmer “acts” on the water, the “reaction” of the water pushes the swimmer forward. Thus, a net force, or unbalanced force, acts on the swimmer so a change in his or her motion occurs. Action and Reaction Forces Don’t Cancel

Action Reaction Pairs

Rocket Propulsion In a rocket engine, burning fuel produces hot gases. The rocket engine exerts a force on these gases and causes them to escape out the back of the rocket. By Newton’s third law, the gases exert a force on the rocket and push it forward.

Momentum Momentum is the product of its mass and velocity Momentum (kgm/s) = mass (kg) x velocity (m/s) P = mv Law of conservation of momentum – as long as objects are not influenced by outside forces then the total amount of momentum cannot change. Momentum can be transferred from one object to another. EX: Momentum is transferred in collisions: pool cue ball When talking about momentum you need to use positive and negative to tell the direction of motion.

When Objects Collide If the pucks are speeding toward each other with the same speed, the total momentum is zero. WHY?