# Newton’s 2nd Law of Motion

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Newton’s 2nd Law of Motion

Flashback: Newton’s Laws of Motion
Sir Isaac Newton was a central figure in the Scientific Revolution during the 17th century. His ideas went against the more accepted ideas of motion. The popular idea before Newton was that a moving object, if left on its own, would eventually come to rest, and a force was necessary to keep the object moving. We now know this to be false, thanks to Isaac Newton.

Flashback: Newton’s Laws of Motion
Today, Newton’s three laws of motion are considered to be the backbone of physics. These ideas were major breakthroughs that set history on a new course. Because of these ideas we now have safety restraints (seat belts) in multiple types of travel.

Flashback: Newton’s 1st Law of Motion
An object at rest remains at rest, and an object in motion remains in motion at a constant speed and in a straight line unless acted on by an unbalanced force. Newton’s 1st law is sometimes called the law of inertia. Inertia is the tendency of all objects to resist any change in motion.

Tennis ball wants to keep sitting still
Law of Intertia Tennis ball wants to keep sitting still Baseball wants to keep moving!

Law of Intertia, continued
Mass is a measure of inertia Small mass = less inertia Large mass = more inertia

Don’t let this be you. Wear seat belts!!
Law of Intertia Don’t let this be you. Wear seat belts!! Because of inertia, objects (including you) resist changes in their motion. When the car going km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.

Law of Intertia

Flashback: Newton’s First Law Balanced Forces = No Change in Velocity
A stationary object will never move without unbalanced forces acting on it Balanced forces = no change in speed A moving object will never stop moving without unbalanced forces acting on it 0N

Combining forces to form NET FORCE
If the forces are in the same direction, then the net force is the sum between the two forces. 12 N 8 N NET FORCE = 12N + 8N = 20N, left

Combining forces to form NET FORCE
If two forces are in opposite directions, then the net force is the difference between the two forces, and it is in the direction of the larger force. 12 N 8 N NET FORCE = 12N – 8N = 4N, right

Flashback: Types of Friction – Static Friction
Static friction is the type of friction that prevents an object from moving when a force is applied. For example, when you push or pull on the refrigerator and at first it doesn’t want to move but then it suddenly starts to move.

Flashback: Types of Friction – Static Friction
Generally, the rougher the surface or the heavier the object, the more static friction.

Flashback: Types of Friction - Sliding Friction
Sliding friction slows down an object that slides. Sliding friction is due to microscopic roughness of two surfaces. Brakes on a car are an example of sliding friction.

Flashback: Types of Friction - Rolling Friction
Rolling friction is needed to provide grip to make a wheel or a tire turn. Rolling friction occurs between the ground and the part of the tire touching the ground.

Flashback: Types of Friction - Fluid Friction
Fluid Friction occurs between an object and air or water. For example, fluid friction occurs when skydivers jump out of a plane, or when boats travel through the water

Flashback: Mass vs. Weight
Mass is the amount of matter in an object and DOES NOT change with gravity. Mass is measured in Kilograms (Kg). Weight is a force measured by the pull of the Earth’s gravity. Weight is measure in Newtons (N).

Flashback: Mass vs. Weight
Your weight would change if you were standing on a planet other than Earth, as shown. If your mass was kg, what would your weight in Newton’s be on Earth? On Jupiter?

Flashback: Force and Acceleration
When the motion of an object changes, the object is accelerating. Acceleration occurs any time an object speeds up, slows down, or changes its direction of motion.

Newton’s 2nd Law of Motion
Newton’s 2nd law of motion connects force, acceleration, and mass. In the presence of a NET FORCE, an object experiences acceleration in the direction of the force. The acceleration of an object depends on the mass of the object and the amount of force applied (f=ma)

Newton’s 2nd Law of Motion
With any given force: an object with less mass accelerates quicker (i.e. volleyball) An object with more mass accelerates slower (i.e. bowling ball)

Question 1 Your _______ on Earth is the gravitational force between you and Earth. A. acceleration B. inertia C. mass D. weight

Question 1 Answer The answer is D, “weight”. Since weight is dependant on gravity, your weight would be different if you were standing on a planet other than Earth.

Question 2 Explain Newton’s second law of motion.

Question 2 Explain Newton’s second law of motion. ANSWER
Newton’s Second Law states that an object acted upon by a net force will accelerate in the direction of the force. And, that the net force is equal to the acceleration times the mass of the object (f=ma).

Newton’s 2nd Law Formulas
f = ma m = f / a a = f / m ALL THESE ARE DIFFERENT WAYS TO EXPRESS THE SAME EQUATION!! In these equations, a is the acceleration (m/s2), m is the mass (kg), and f is the net force (N). Force is measured in Newtons, abbreviated N.

f = ma Calculations, Force 8 N
Newton’s second law of motion can be used to calculate NET FORCE. A baseball has a mass of 0.2 kg. If its acceleration is 40 m/s2, what is the net force on the baseball? f = ma f = ? m = 0.2kg a = 40m/s2 f = 0.2kg * 40m/s2 f = 8 N

Calculations, Force A 6 kg cat accelerates at 1.5 m/s2. Find the net force. f = ma f = ? m = 6kg a = 1.5m/s2 f = 6kg * 1.5m/s f = 9 N

f = ma A 2 kg book is pushed along a table with a thrusting force of 4 N. Find the frictional force on the book if the book’s acceleration is 1.5 m/s2 f = ma f = ? m = 2kg a = 1.5m/s2 f = 2kg * 1.5m/s f = 3 N (net force) 4 N (thrusting force) – 3 N (net force) = 1 N (frictional force)

Calculations, Acceleration
f = ma Newton’s second law of motion can be used to calculate ACCELERATION. Suppose you pull a 10-kg sled so that the net force on the sled is 5 N. What is the acceleration? f = ma f = 5N m = 10kg a = ? 5N = 10kg * a a = 5N/10kg = 0.5 m/s2

Calculations, Acceleration
f = ma A net force of 4,500 N acts on a car with a mass of 1,500 kg. What is the acceleration of the car? f = ma f = 4,500N m = 1,500kg a = ? 4500N = 1500kg * a a = 4500N/1500kg = 3.0 m/s2

Calculations, Acceleration
f = ma A book with a mass of 2.0 kg is pushed along a table. If the net force on the book is 1.0 N, what is the acceleration of the book? f = ma f = 1.0N m = 2.0kg a = ? 1.0N = 2.0kg * a 0.5 m/s2 a = 1.0N/2.0kg =

f = ma Calculations, Mass 6 kg
Newton’s second law of motion can also be used to calculate MASS. A basketball is thrown with a net force of 9 N and accelerates at 1.5 m/s2, what is the mass of the basketball? f = ma f = 9N m = ? a = 1.5 m/s2 9N = m * 1.5m/s2 6 kg m = 1.5m/s2 / 9N =

f = ma Calculations, Mass 3 kg
An object acted on by a force of 3.6 N has an acceleration of 1.2 m/s2. What is the mass of the object? f = ma f = 3.6N m = ? a = 1.2 m/s2 3.6N = m * 1.2m/s2 m = 3.6N / 1.2m/s2 = 3 kg