Home » Physics » Forces: Laws of Motion » Newton’s 2nd and 3rd laws of motion

# Newton’s 2nd and 3rd laws of motion

### Sir Isaac Newton # Mass Resists Acceleration

## Acceleration is inversely proportional to the mass

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The gravitational acceleration is constant

Twice the force, exerted on twice the inertia, produces the same acceleration, as half the force exerted on half of the inertia.

Both accelerate equally

Although the total masses in both cases is different, the total forces are also different. The “2”s cancel out, so the resulting acceleration is the same.

## http://slideplayer.com/slide/4555843/

# 6.3 Newton’s Second Law of Motion

## Force = mass x acceleration F = m•a F = ma # 6.5 Pressure is applying force over an area

## The honorary unit of pressure is the Pascal, named after the great French scientist Blaise Pascal (1600s) He was a mathematician, physicist, inventor, writer and Christian philosopher.

Common units of pressure.

If you divide a lot of pressure over a large amount of area, then the amount of pressure in any one area is small – hence the amazing bed of nails trick!

http://www.fas.harvard.edu/~scdiroff/lds/NewtonianMechanics/FakirPhysics/FakirPhysics.html

Here is an example of the power of air pressure – if a solid steel tank train car is sealed, and then evacuated, the air pressure inside is 0 Pascals. But the external air pressure is over 100,00 Pa, so this is the result! [……]

# 7.6 The Horse-Cart Problem

from https://www.lhup.edu/~dsimanek/physics/horsecart.htm

### A horse is harnessed to a cart. If the horse tries to pull the cart, then the horse must exert a force on the cart.

By Newton’s 3rd law, the cart exerts an equal & opposite force on the horse.

Newton’s 2nd law : acceleration is equal to the ∑ force divided by the mass.   a = F/m

Since the two forces are equal and opposite, they add to zero.

Thus the acceleration of the system must be zero.

If it doesn’t accelerate, and it started it rest, it must remain at rest.

Therefore no matter how hard the horse pulls, it can never move the cart?!  Nope….we’re obviously missing something here. What could it be? We need to think 🙂 Shh!  Quiet time – use your brain! List all the physical errors and mistakes in the above paragraph and explain why they are wrong. Show a free-body force diagram of the horse and cart, identifying all relevant forces, and then write a short paragraph describing this situation correctly.

### How can this sled move, if the forces are all in pairs?

Solution;

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• ### 100 kg is about a kN

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Lessons from The Physics Classroom:

1. Lesson 1 – Newton’s First Law of Motion
2. Lesson 2 – Force and Its Representation
3. Lesson 3 – Newton’s Second Law of Motion
4. Lesson 4 – Newton’s Third Law of Motion

## Misconceptions

Many students believe that an object cannot be moving unless there is a force on it, or that an object will naturally come to a stop, without any external force acting on it. Reality: when objects seem to come to a stop by themselves, there really are unseen friction forces acting to stop the object.

## Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a
mathematical model describing change in motion (the acceleration) of objects when
acted on by a net force.

HS-PS2-10(MA). Use free-body force diagrams, algebraic expressions, and Newton’s laws of motion to predict changes to velocity and acceleration for an object moving in one dimension in various situations

A FRAMEWORK FOR K-12 SCIENCE EDUCATION: Practices, Crosscutting Concepts, and Core Ideas
PS2.A: FORCES AND MOTION
How can one predict an object’s continued motion, changes in motion, or stability?

Interactions of an object with another object can be explained and predicted using the concept of forces, which can cause a change in motion of one or both of the interacting objects… At the macroscale, the motion of an object subject to forces is governed by Newton’s second law of motion… An understanding of the forces between objects is important for describing how their motions change, as well as for predicting stability or instability in systems at any scale.

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.

1.4 Interpret and apply Newton’s three laws of motion.
1.5 Use a free-body force diagram to show forces acting on a system consisting of a pair of
interacting objects. For a diagram with only co-linear forces, determine the net force acting on a system and between the objects.
1.6 Distinguish qualitatively between static and kinetic friction, and describe their effects on the motion of objects.
1.7 Describe Newton’s law of universal gravitation in terms of the attraction between two objects, their masses, and the distance between them.
1.8 Describe conceptually the forces involved in circular motion.