Forces Chapter 12

Forces A force is a push or pull. This push or pull causes an object to change its velocity or acceleration.

Forces The combination of all the forces acting on an object is the net force. This combination will determine whether the velocity of the object will change. An object accelerates in the direction of the net force. An object won’t accelerate if the net force is zero.

Forces Balanced Forces are equal in strength and cancel each other out. Motion does not change. The net force is zero. Unbalanced Forces are not equal in strength and cause an object to change its velocity or acceleration. The net force is greater than zero.

Forces

Friction Friction is an unbalanced force that opposes motion. Friction resists the movement of one surface past another.

Friction There are three types of frictions: Sliding Friction: Occurs when a solid surface slides over another solid surface. Rolling Friction: Occurs when an object rolls across a solid surface. Static Friction: Occurs between two objects that touch but do not move.

Friction Air Resistance is another type of friction. It occurs between the surface of a moving object and the air molecules.

Newton’s 1st Law Newton’s First Law: an object at rest will remain at rest and an object in motion maintains its velocity unless it experiences a net force. Also known as inertia.

Newton’s 1st Law An object with a greater mass has a greater inertia; therefore, a larger force will be needed to change its motion.

Momentum Momentum is a quantity defined as the product of an object’s mass and its velocity. p = mv The more mass something has the more momentum it has. The more velocity something has the more momentum it has. If an object is not moving then its momentum is zero! momentum (kg*m/s) mass (kg) velocity (m/s)

Momentum The law of conservation of momentum states that the total amount of momentum in a system is conserved.

Momentum A pitcher in a professional baseball game throws a fastball, giving the baseball a momentum of 6 kg*m/s. Given that the baseball has a mass of 1 kg, what is its speed? p= m= v= 6 kg*m/s v = p / m p m * v v = 6 kg*m/s / 1 kg 1 kg v = 6 m/s ?

Momentum The lightest pilot-driven airplane ever built was the Baby Bird. Suppose the Baby Bird moves along the ground without a pilot at a speed of 25 m/s, Under these circumstances the momentum of the empty plane would be only 3000 kg*m/s. What is the mass of the plane? p= m= v= 3000 kg*m/s m = p / v p m * v m = 3000 kg*m/s / 25 m/s ? kg 25 m/s m = 120 kg

Momentum The largest animal ever to have lived on Earth is the blue whale. Consider a blue whale with a mass of 140,000 kg and a top swimming speed of 20 km/h. What is the momentum of this whale at this speed? p= m= v= ? kg*km/h p = mv p m * v p = 140000kg x 20km/h 140,000 kg p = 2800000 kg*km/h 20 km/h

Newton’s 2nd Law Newton’s Second Law says the unbalanced force acting on an object equals the object’s mass times its acceleration. F = m x a Force is measured in newtons (N). Mass is measured in kilograms (kg). Acceleration is measured in meters per second squared (m/s2). force mass acceleration

Newton’s 2nd Law Gravity is the universal force between any two objects. Every object in the universe exerts a gravitational force on all other objects.

Newton’s 2nd Law All matter is affected by gravity. The strength of the force of gravity is related to the masses of the objects and the distance between the objects. The more mass an object has, the greater the force of gravity it exerts. The greater the distance between two objects, the smaller the gravitational force is between them.

Newton’s 2nd Law The force on an object due to gravity is called weight. Weight and mass are different. Mass is simply how much matter your body contains, but weight is the amount of gravitational force exerted on you by Earth.

Newton’s 2nd Law The weight of an object on Earth is greater than the weight of the same object on the surface of the Moon, but the object’s mass stays the same.

free-fall acceleration Newton’s 2nd Law Weight is equal to mass times free-fall acceleration (the constant acceleration that all masses near the object experience). w = m x g One pound is equal to 4.448 N. Weight and mass are proportional but not equal. weight (N) mass (kg) free-fall acceleration (9.8 m/s2 on Earth)

Newton’s 2nd Law Free fall is an object falling under the influence of gravity. On Earth, g = 9.8 m/s2. In the absence of air resistance, all objects falling near Earth’s surface accelerate at the same rate regardless of their mass.

Newton’s 2nd Law Terminal velocity is the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite direction to the force of gravity. 320 km/h (200 mi/h) on Earth.

Newton’s 2nd Law Orbiting the Earth is a projectile motion. Projectile motion is a curved path that an object follows when thrown, launched or otherwise projected near the surface of the Earth.

Newton’s 2nd Law Two motions combine to make a curved path: Horizontal from object and vertical with freefall towards Earth. Cannon Projectile Motion Free Fall Acceleration 9.8 m/s2

Newton’s 2nd Law Astronauts in orbit are not weightless because they are too close to Earth. They are simply experiencing free fall.

Newton’s 2nd Law Assume that a catcher in a professional baseball game exerts a force of -50 N to stop the ball. If the baseball has a mass of 0.1 kg, what is its acceleration as it is being caught? F = m= a = -50 N a = F / m F m * a a = -50 N / 0.1 kg 0.1 kg a = -500 m/s2 ?

Newton’s 2nd Law F = m= a = ? F = m * a F m * a F = 1000 kg * 10 m/s2 In drag racing, acceleration is more important than speed, and therefore drag racers are designed to provide high accelerations. Suppose a drag racer has a mass of 1000 kg and accelerates at a constant rate of 10 m/s2. How large is the unbalanced force acting on the racer. F = m= a = ? F = m * a F m * a F = 1000 kg * 10 m/s2 1000 kg F = 10,000 N 10 m/s2

Newton’s 2nd Law F = m= a = 1 N m = F / a F m * a m = 1 N / 9.8 m/s2 ? The tallest man-made structure at present is the Warszawa Radio mast in Warsaw, Poland, This radio mast rises 646 m above the ground, nearly 200 m more than the Sears Tower in Chicago. Suppose a worker at the top of the Warszawa Radio mast accidentally knocks a tool off the tower. If the force acting on it is 1 N, and its acceleration is 9.8 m/s2, what is the tool’s mass? F = m= a = 1 N m = F / a F m * a m = 1 N / 9.8 m/s2 ? m = 0.1 kg 9.8 m/s2

Newton’s 2nd Law A textbook has a mass of about 2.25 kg on Earth. What is its weight? w = m= g = ? N w = m * g w m * g w = 2.25kg * 9.8 m/s2 2.25 kg w =22 N 9.8 m/s2

Newton’s 3rd Law Newton’s Third Law explains why forces act in pairs. For every action, there is an equal and opposite reaction. Forces always act in pairs.