NEWTON'S FIRST LAW OF MOTION

ARISTOTLE’S (384-322 B.C.) IMPETUS THEORY OF MOTION Objects fall toward the Earth because it wants to return to its “natural place”. Heavy objects fall faster than lighter ones. Objects fall faster in air than in water due to the resisting force of the medium. Objects move away from their natural force . This “violent motion” is caused by an outside force.

Trajectories of Projectiles According to Aristotle’s Impetus Theory

REASONS FOR THE PERSISTENCE OF ARISTOTLE’S NOTION FOR FORCE The theory was consistent with common sense and observation. No other theories about motion were known during the time. The quantitative method of observation to prove the validity of the theory was not yet developed

GALILEO’S EXPERIMENTAL FINDINGS Both heavy and light objects fell and struck the Earth at the same time. When a ball was rolled down an incline at a fixed angle θ, the ratio of the distance covered to the square of the corresponding time is always the same. When the angle of inclination was changed, the constant also changed. The constant, d/t2, is also the constant for a falling object.

GALILEO’S FINDINGS d = k t2 HEAVY light

Figure 4-3 Galileo’s experiment A ball rolls farther along the upward incline as the angle of incline is decreased. On a smooth, horizontal surface, the ball rolls a greater distance before coming to rest. How far would the ball travel on an ideal, perfectly smooth surface? (The ball would slide in this case because of the absence of friction.) A ball rolls farther along the upward incline as the angle of incline is decreased. On a smooth, horizontal surface, the ball rolls a greater distance before coming to rest. How far would the ball travel on an ideal, perfectly smooth surface? (The ball would slide in this case because of the absence of friction.)

NEWTON’S LAWS OF MOTION - Established the relationship between motion and force

FIRST LAW – LAW OF INERTIA Every object tends to remain at rest or move with a constant velocity unless acted upon by a net force. NET FORCE – summation of all forces acting on a given body INERTIA – measure of the mass of an object Examples: a. Passenger tends to move forward when a car suddenly stops. b. A moving car in uniform motion slows down when the brake is applied.

Figure 4-4 A difference in inertia The larger punching bag has more mass and hence more inertia, or resistance to a change in motion. The larger punching bag has more mass and hence more inertia, or resistance to a change in motion.

FIRST LAW – LAW OF INERTIA

Figure 4-5 Newton’s second law The relationships among force, acceleration, and mass shown here are expressed by Newton’s second law of motion (assuming no friction). The relationships among force, acceleration, and mass shown here are expressed by Newton’s second law of motion (assuming no friction).

More Mass = More Inertia

Who has the higher inertia?

Figure 4-6 The newton (N) A net force of 1.0 N acting on a mass of 1.0 kg produces an acceleration of 1.0 m/s2 (on a frictionless surface). A net force of 1.0 N acting on a mass of 1.0 kg produces an acceleration of 1.0 m/s2 (on a frictionless surface).

Figure 4-7 Newton’s second law and free fall In free fall, all objects fall with the same constant acceleration g. An object with twice the mass of another has twice as much gravitational force acting on it. But with twice the mass, the object also has twice as much inertia, so twice as much force is needed to give it the same acceleration. In free fall, all objects fall with the same constant acceleration g. An object with twice the mass of another has twice as much gravitational force acting on it. But with twice the mass, the object also has twice as much inertia, so twice as much force is needed to give it the same acceleration.

Figure 4-8 Force and acceleration See Example 4.1.

The net force acting on an object is the vector sum of all the forces acting on it. Examples: 8 N 6 N 9 N 8 N 3 N 4 N 8 N ? 7 N 7 N 5 N 4 N 4 N 12 N If an object remains at rest, it is incorrect to assume that there are no forces acting on the object. We can only conclude that the net force on the object is zero.

MOTION OF AN OBJECT BASED ON DIFFERENT PERSPECTIVES INSIDE OUTSIDE

Force is either a push or pull

Application of the Law of Inertia

Determine the resultant, or net force, exerted on the stationary elephant by the two clowns in the figure below. What is the tension in the rope attached to the elephant?

This is a 3-4-5 triangle. Therefore, the resultant is 500 N This is a 3-4-5 triangle. Therefore, the resultant is 500 N. Now we have to calculate the angle. Tangent will help us calculate that angle. Now the final answer is The tension in the rope is 500 N, because the net force exerted by the clowns is transmitted to the rope.

CALVIN AND HOBBS’ LOOPHOLES IN NEWTON’S FIRST LAW OF MOTION