ed2.5 newtons laws of motion

Dr Pusey

Learning Outcomes

Recall Newtons Three Laws of Motion

Define inertia and the physical quantities that determine the inertia of an object

Use Newton’s 2nd Law of Motion (F = ma) to solve problems

Analyse real world situations in terms of Newton’s second law of motion (e.g. heavier car with same engine will accelerate slower)

Identify action-reaction force pairs in different situations

Analyse real world situations in terms of Newton’s third law of motion (e.g. person in a lift feeling heavier)

An object at rest remains at rest, or if moving keeps moving in a straight line if no external

forces act upon it

If an external force is applied, the object's motion will change either magnitude or direction, and

the rate of change of the motion (its acceleration) when multiplied by the object's

mass is equal to the applied force (F=ma)

For every action there is an equal and opposite reaction

State Newton’s first law of motion

Define:

inertia

equilibrium

Identify that inertia depends upon the mass of an object

Analyse real world situations in terms of Newton’s first law of motion (e.g. braking in car, body pushes against seatbelt)

Newton’s 1st Law of motion An object at rest stays at rest and an object in motion

stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Sometimes referred to as the Law of Inertia

www.physicsclassroom.com

Newton’s 1st Law of motionExample 1

When travelling in a car that suddenly stops what happens to your body with and without a seatbelt?

Newton’s 1st Law of motionExample 1

When travelling in a car that suddenly stops what happens to your body with and without a seatbelt?

Without Car stops, body continues travelling forward

With Seatbelt provides an external force which stops body from continuing forwards

Inertia Objects have a natural tendency to resist changes in

their state of motion.

Inertia: the resistance an object has to a change in its state of motion

This includes resistance to a change in velocity and a change in acceleration

The inertia of an object varies with mass

E.g. The more mass an object has the more inertia/resistance to change it has

Newton’s 2nd Law The second law states that the acceleration of an

object is dependent upon two variables - the net force acting upon the object and the mass of the object.

The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object.

Newton’s 2nd Law According to Newton, an object will only accelerate

if there is a net or unbalanced force acting upon it. The presence of an unbalanced force will accelerate an object - changing its speed, its direction, or both its speed and direction

www.physicsclassroom.com

Newton’s 2nd LawHave a go:

In a physics lab, Kate and Rob use a hanging mass and pulley system to exert a 2.45 N rightward force on a 0.5 kg cart to accelerate it across a low-friction track. If the total resistance force to the motion of the cart is 0.72 N, then what is the cart's acceleration?

Hint: Draw a free body diagram

Newton’s 2nd Law

Newton’s 2nd LawExample 2

Explain (using Newton’s 2nd law) why heavier vehicles, such as 4WD’s and trucks, have more powerful engines.

Newton’s 2nd LawExample 2

Explain (using Newton’s 2nd law) why heavier vehicles, such as 4WD’s and trucks, have more powerful engines.

Greater mass, so need greater net force (from engine) in order to accelerate same amount as a smaller car.

Newton’s 3rd Law For every force applied to an object there is an equal

and opposite force exerted back by the object

According to Newton, whenever objects A and B interact with each other, they exert forces upon each other.

E.g. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

Newton’s 3rd Law Forces always come in pairs - equal and opposite

action-reaction force pairs.

Draw the other half of the action-reaction pairs below:

Newton’s 3rd Law Newton’s 3rd law seems to contradict the idea of Newton’s

2nd law and the idea of a net/unbalanced force

Newton’s 3rd law is a different way of thinking about problems

The 3rd law focuses on two objects interacting

Equilibrium

(balanced/unbalanced forces)

Newton’s 1st Law

Newton’s 2nd Law

Action-reaction

(force pairs)Newton’s 3rd Law

How did you go?

Recall Newtons Three Laws of Motion

Define inertia and the physical quantities that determine the inertia of an object

Use Newton’s 2nd Law of Motion (F = ma) to solve problems

Analyse real world situations in terms of Newton’s second law of motion (e.g. heavier car with same engine will accelerate slower)

Identify action-reaction force pairs in different situations

Analyse real world situations in terms of Newton’s third law of motion (e.g. person in a lift feeling heavier)