Newton’s Laws of Motion

Newton’s Laws of Motion • Newton’s Laws • Forces • Mass and Weight Serway and Jewett 5.1 to 5.6 Physics 1D03 - Lecture 6

Newton’s First Law (Law of Inertia) An isolated object, free from external forces, will continue moving at constant velocity, or remain at rest. (This law was actually due to Galileo.) Earlier, Aristotle said objects were “naturally” at rest, and needed a continuing push to keep moving. Galileo realised that motion at constant velocity is “natural”, and only changes in velocity require external causes. Objects in equilibrium (no net external force) also move at constant velocity. Physics 1D03 - Lecture 6

Forces • A force is a push or pull that tends to cause motion (more exactly, changes in motion) • From the Second Law, force should have units of • Force is a vector. • In Newton’s dynamics, all influences on a particle from its surroundings are expressed as forces exerted on that particle. Physics 1D03 - Lecture 6

Newton’s Second Law Fnet (or Ftotal) is the vector sum of all forces acting on the particle of mass m: The acceleration a is parallel to the total force, and proportional to it. The proportionality constant is the particle’s mass. Newton defines mass as a measure of an object’s inertia. Physics 1D03 - Lecture 6

Contact Forces : direct contact is required examples - normal forces, friction, air resistance, buoyancy, ... Non-Contact Forces : gravity, electromagnetic, weak and strong forces The gravitational force is also called weight and is measured in newtons. Weight is proportional to mass : Fw = mg, where g is the gravitational field (and is also the acceleration of an object in free fall). Physics 1D03 - Lecture 6

Weight and Mass Weight is a force; it can be measured using a spring scale On the moon, a baseball weighs 0.40 N On Earth, a baseball weighs 2.40 N Physics 1D03 - Lecture 6

Mass is a measure of inertia : on the earth or on the moon, a 24.5 N force applied to the baseball will give it an acceleration of 100 m/s2 (its mass is m = F/a = 0.245 kg) • We can compare masses with a balance, because of the remarkable property : Weights are equal when masses are equal Physics 1D03 - Lecture 6

r m Newton’s Law of Gravitation Gravity is a universal attractive force between masses. The force is proportional to each mass, and inversely proportional to the the square of the distance between them (from center of mass): M Units of G ? What is g ??? Physics 1D03 - Lecture 6

Quiz Astronauts in orbit are seen floating motionless inside the space shuttle. They can do this because: • They have no mass in space. • There is no gravity in space. • The shuttle is not an inertial reference frame. Physics 1D03 - Lecture 6

Newton’s Third Law (action and reaction) If object A exerts a force on object B, object B exerts an equal, opposite force back on A. Physics 1D03 - Lecture 6

1.) Gravity (of block) pulls earth up Gravity pulls block down 2.) Block pushes down on table (contact) Table pushes up on block Newton’s Third Law : examples What is the “reaction” to the following forces? Physics 1D03 - Lecture 6

Pressure forces on balloon 3.) Balloon pushes air backwards Physics 1D03 - Lecture 6

Quiz A 140-kg wrestler and a 90-kg wrestler try to push each other backwards out of the ring. At first they are motionless as they push; then the large wrestler moves the other one backwards. Compare the forces they exert on each other. Which statement is correct? • The forces are always equal. • The larger wrestler always exerts a larger force. • When they are motionless, the forces are equal; they start to move when the large wrestler exerts a larger force on his opponent than his opponent exerts back on him. Physics 1D03 - Lecture 6

Contact Forces Forces on Block Examples : A heavy block on a table • The table must push up on the block to prevent it from falling • The type of contact force is called a normal force if it is perpendicular (normal) to the surfaces in contact. • The normal force will be as large as necessary to hold the block (until the table breaks) Physics 1D03 - Lecture 6

If we look closely, the normal force arises from the table being bent : as the table tries to straighten, it pushes back. This is really an elastic force; the table behaves like a spring. At the atomic level, all contact forces are due to electromagnetic forces. Physics 1D03 - Lecture 6

normal force : is perpendicular to the surfaces in contact • friction : is parallel to the surface • We divide the contactforce into two components” friction has a more complex behaviour than the normal force (next lecture) Physics 1D03 - Lecture 6

Summary • Newton’s Three Laws • Objects naturally move at constant velocity in a straight line • Accelerations are caused by unbalanced forces • Forces come in action-reaction pairs, with the ‘action’ and ‘reaction’ always on different objects. • Gravity : • Weight is a force, mass is a measure of inertia. • All familiar forces require direct contact, except for gravity and electromagnetism (and nuclear forces). Physics 1D03 - Lecture 6

Practice: Chapter 5 Questions 5, 7, 10 Problems 1, 3, 11(5th ed) Questions 1, 9, 18 Problems 1, 3, 15 Physics 1D03 - Lecture 6

Newton’s Laws of Motion