Newton’s Laws of Motion PPMF102 – Lecture 1 Newton’s Laws of Motion

Newton

Newton’s First Law of Motion Every object continues in its state of rest or of uniform speed in a straight line unless acted on by a nonzero force The tendency of a body to maintain its state of rest or of uniform motion in a straight line is called inertia. Newton’s first law is also called the law of inertia.

Applications of Newton's first law of motion “Wait until the bus stopped” “Fasten your seat belt” in cars, aeroplanes, or roller coasters. Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

Car seat belt Seatbelt for roller coaster

Applications of Newton's first law of motion 4. The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.

Applications of Newton's first law of motion 5. To dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted.

Applications of Newton's first law of motion 6. Blood rushes from your head to your feet when riding on a descending elevator which suddenly stops.

Applications of Newton’s 1st law 7. A tablecloth is whipped from beneath dishes placed on a table top.

Applications of Newton’s 1st law 8. Flip a coin in high- speed car, bus, or plane and catch the vertically moving coin as if the vehicle were at rest.

Applications of Newton’s 1st law

Conceptual Question: Newton’s first law. A school bus comes to a sudden stop, and all of the backpacks on the floor start to slide forward. What force causes them to do that?

Force A force is a push or pull. An object at rest needs a force to get it moving. A moving object needs a force to change its velocity.

Force Force is a vector, having both magnitude and direction. The magnitude of a force can be measured using a spring scale.

Various names of force Force Symbol Unit Weight W N Friction f Normal force Tension in a string T These are all forces with the same unit Newton (N).

Force causes velocity of object to change It doesn’t take a force to keep an object moving in a straight line—it takes a force to change its motion or velocity. It takes a force to change either the direction or the speed of an object.

Newton’s Second Law of Motion The net force acting on an object equals the mass times the acceleration of the object. F = ma

Newton’s Second Law of Motion F = ma F  net force acting on an object [F  net force  summation of all forces  resultant force  total force] m  mass of the object a  acceleration of the object

Newton’s Third Law Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first To every action there is an equal and opposite reaction  “action” force and “reaction” force are acting on different objects

Newton’s Third Law

Newton’s Third Law

Newton’s Third Law

Newton’s Third Law of Motion Conceptual question: What exerts the force to move a car? Response: A common answer is that the engine makes the car move forward. But it is not so simple. The engine makes the wheels go around. But if the tires are on slick ice or deep mud, they just spin. Friction is needed. On firm ground, the tires push backward against the ground because of friction. By Newton’s third law, the ground pushes on the tires in the opposite direction, accelerating the car forward.

Force = mass x acceleration F = ma Unit: Newton (N) 1 N = 1 kgm/s2

Mass A property of a body Quantity of matter A measure of the inertia of a body SI unit is kilogram (kg) Does not depend on location

Weight—the Force of Gravity; and the Normal Force Weight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight of an object of mass m is: where

Weight—the Force of Gravity; and the Normal Force An object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there? The force exerted perpendicular to a surface is called the normal force. It is exactly as large as needed to balance the force from the object. N W Figure 4-14. Caption: (a) The net force on an object at rest is zero according to Newton’s second law. Therefore the downward force of gravity (FG) on an object at rest must be balanced by an upward force (the normal force FN) exerted by the table in this case. (b) F’N is the force exerted on the table by the statue and is the reaction force to FN by Newton’s third law. (F’N is shown in a different color to remind us it acts on a different object.) The reaction force to FG is not shown.

Weight—the Force of Gravity; and the Normal Force Example 1: Weight, normal force, and a box. A box of mass 10.0 kg is resting on a smooth (frictionless) horizontal surface of a table. Determine the weight of the box and the normal force exerted on it by the table. Figure 4-15. Caption: Example 4–6. (a) A 10-kg gift box is at rest on a table. (b) A person pushes down on the box with a force of 40.0 N. (c) A person pulls upward on the box with a force of 40.0 N. The forces are all assumed to act along a line; they are shown slightly displaced in order to be distinguishable. Only forces acting on the box are shown. Answer: The box is always at rest, so the forces must always add to zero. In (a), the normal force equals the weight; in (b), the normal force equals the weight plus the 40.0 N downward force; in (c), the normal force equals the weight minus the 40.0 N upward force.

(b) Now the box is pushed down on with a force of 40. 0 N (b) Now the box is pushed down on with a force of 40.0 N. Again determine the normal force exerted on the box by the table. (c) If the box is pulled upward with a force of 40.0 N, what is the normal force exerted on the box by the table?

Weight—the Force of Gravity; and the Normal Force Example 2: Accelerating the box. What happens when a person pulls upward on the box in the previous example with a force greater than the box’s weight, say 100.0 N? Figure 4-16. Caption: Example 4–7. The box accelerates upward because FP > mg. Answer: Now there is a net upward force on the box of 2.0 N, and the box accelerates upward at a rate of 0.20 m/s2.

Weight A force A force of gravity acting on a body Weight = mass x gravity SI unit is Newton (N) Depends on location i.e. strength of gravity

Friction – standard model Force of resistance to the relative motion of two solid objects when the two surfaces are in contact. Friction is directly proportional to normal force. Static friction – friction without motion. fs = s N s  coefficient of static friction

Kinetic friction – friction when there is relative motion between surfaces. fk = k N k  coefficient of kinetic friction Friction is in fact a very complex phenomenon which cannot be represented by a simple model.