Mechanics

Unit 5: Motion and Forces

5.7 Newton’s Laws of Motion

...

Fundamentals of physics - Mechanics Newton’s First Law of Motion

----------------------------------------------------------------------------------------Every body continues in its state of rest, or a uniform speed in a straight line, unless acted on by a net force.

In plain English:An object at rest will stay at rest. A moving object will continue to move a straight line

Tablecloth trick

Sudden braking...

Fundamentals of physics - Mechanics Newton’s First Law of Motion

----------------------------------------------------------------------------------------Every body continues in its state of rest, or a uniform speed in a straight line, unless acted on by a net force.

Implications: We can say that there is a net force acting on an object if From rest it begins to move It is already moving it slows down or stop moving, speeds up or changes direction of motion From rest it begins to rotate It is already rotating it rotates slower or stop rotating, rotates faster or changes direction of rotation

In plain English:An object at rest will stay at rest. A moving object will continue to move a straight line

Fundamentals of physics - Mechanics Newton’s Second Law of Motion

----------------------------------------------------------------------------------------The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

In plain English:Greater force greater acceleration. Acceleration is in the direction of the force Greater mass, lesser acceleration...

Fundamentals of physics - Mechanics Newton’s Second Law of Motion

----------------------------------------------------------------------------------------

In plain English:Greater force greater acceleration. Acceleration is in the direction of the force Greater mass, lesser acceleration...

Fundamentals of physics - Mechanics Newton’s Second Law of Motion

----------------------------------------------------------------------------------------When you accelerate in the direction of your velocity, you speed up; when you accelerate against your velocity, you slow down; when you accelerate at an angle to your velocity, your direction changes..

Fundamentals of physics - Mechanics Newton’s Second Law of Motion

----------------------------------------------------------------------------------------Two ways of defining acceleration:Previously, we defined acceleration as the time rate of change of velocity a = [(change in v)/time] Now saying that acceleration it is the ratio of force to mass [a = F/ma)]

Question (1) A jumbo jet cruises at a constant velocity of 50 ms-1 when the thrusting force of its engines is a constant 100,000 N.

(a) What is the acceleration of the jet? (b) What is the force of air resistance on the jet?

Fundamentals of physics - Mechanics Newton’s Second Law of Motion

----------------------------------------------------------------------------------------Question (1) A jumbo jet cruises at a constant velocity of 50 ms-1 when the thrusting force of its engines is a constant 100,000 N. Answer

F = 0 (constant velocity – Newton’s First Law)No net force – Acceleration = 0

(F = 0 Far + Ft = 0

Taking the right to be positive Far + 100000N = 0NFar = -100000-ve sign indicates force is to the left

Fundamentals of physics - Mechanics Newton’s Second Law of Motion

----------------------------------------------------------------------------------------Doing the math.Newton’s Second Law of Motion can be written as F = maor a = F/m

Question (2) The engines of a jumbo jet at rest on the runway at produces a thrusting force of 100,000 N. If its mass is 10000kg, what is its acceleration when the brakes are released. Ignore any air resistance.

Answer: a = F/m a = 100000N / 10000kg a = 10ms-2

Fundamentals of physics - Mechanics Newton’s Third Law of Motion

----------------------------------------------------------------------------------------Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.We call one force the action force, and we can call the other the reaction force.The important thing is that they are coequal parts of a single interaction and that neither force exists without the other. Action and reaction forces are equal in strength and opposite in direction. They occur in pairs, and they make up a single interaction between two things

Fundamentals of physics - Mechanics Newton’s Third Law of Motion

----------------------------------------------------------------------------------------Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Fundamentals of physics - Mechanics Newton’s Third Law of Motion

----------------------------------------------------------------------------------------Action and Reaction on Different Masses.Quite interestingly, a falling object pulls upward on Earth with as much force as the Earth pulls downward on it. The resulting acceleration of the falling object is evident, while the upward acceleration of Earth is too small to detect.

Which falls toward the other, A or B? Do the accelerations of each relate to their relative masses?

Fundamentals of physics - Mechanics Newton’s Third Law of Motion

----------------------------------------------------------------------------------------When a cannon is fired, there is an interaction between the cannon and the cannonball. The sudden force that the cannon exerts on the cannonball is exactly equal and opposite to the force the cannonball exerts on the cannon.This is why the cannon recoils (kicks). But the effects of these equal forces are very different. This is because the forces act on different masses.The different accelerations are evident via Newton’s second law, a = F/mcannonball: A = F/MCannon a = F/mThus we see why the change in velocity of the cannonball isso large compared with the change in velocity of the cannon.

Fundamentals of physics - Mechanics Newton’s Third Law of Motion

----------------------------------------------------------------------------------------This is why the cannon recoils (kicks). But the effects of these equal forcesare very different. This is because the forces act on different masses. The different accelerations are evident via Newton’s second law,a = F/m

This is why the cannon recoils (kicks). But the effects of these equal forcesare very different. This is because the forces act on different masses. The different accelerations are evident via Newton’s second law,a = F/m