Forces and Newton’s Laws

Force

A Force is a push or a pull that one body exerts on another.

All Forces are measured in Newtons (N)

Balanced Forces

Force that are equal in opposite directions and act on an object to produce a net force of zero.

20N

20N

Net force = 0

Balanced Forces cause no change in motion

Unbalanced Forces

Forces acting on an object to produce a net force that is positive.

20N

15N

Net force = 5N

Unbalanced Forces cause a change in motion.

Box moves to the right

Some Types of ForcesFriction is the force between two objects that opposes the motions of either object.

Example: Rubbing your hands together.

Some Types of ForcesAir Resistance is the force that acts on objects as they move through air.

The amount of air resistance acting on an object depends on two factors:

1) Speed (Velocity). The faster an object moves, the more air resistance acts on it

More air resistance Less air resistance

2) Surface area. The larger the surface area of an object the more air resistance acts in it.

Crumpled paper Flat paper

Less air resistance More air resistance

Some Types of ForcesGravity is the force that holds everything in the universe together.

“Every object in the universe exerts the force of gravity on all other objects.”

1) Distance. The shorter the distance between two objects the greater the force of gravity between them

Less Gravity 2) Mass. The greater the mass of an object, the more gravity it exerts on other objects.

Gravity depends on two factors:

Less Gravity

More Gravity

More Gravity

Some Types of ForcesWeight is a force. It is the force that gravity exerts on an object due to the object’s mass.

“Like all forces weight is measured in Newtons.”

1) Mass. The more mass an object has, the more gravity acts on it and the more weight it has.

More Weight 5N

Weight depends on two factors: Weight = MassxGravity

10kg5kg

Less Weight 2.5 N

Some Types of Forces

weight = mass x acceleration of gravity

W = mg

W

m g

Some Types of Forces

2) The amount of Gravity on the planetary body the object is on.

More Weight 5N

10kg

Less Weight 2.5 N

Earth Jupiter

10kg

Sir Isaac Newton

Newton’s First Law

Newton’s First Law states, “An object at rest will remain at rest unless acted upon by an unbalanced force while an object in motion will remain in motion with constant speed anddirection unless acted upon by an unbalanced force”

It is often called the Law of Inertia.

What is inertia?An object’s tendency to resist a change in it’s motion.

• This means that an object:

• Will be hard to move if it is at rest

• Example: The plates on a set table.

• Or a tomato being shot.

Would the bowling ball andthe golf ball have the sameinertia?

NO…why?The more massive an object is, the greater itsinertia…

And the greater an objects inertia is, the greaterthe force required to put the object in motion

Examples of Inertia• Will be hard to slow an object down or speed it up if it is moving

• Example People involved in a car stopping suddenly fly forwards because they keep moving at the same speed as the car.

Only the seatbelt stops them.

Newton’s Second Law of MotionNewton’s Second Law states, “When a net force acts on an object the object will accelerate in the direction of the net force”.

The greater the mass (of the object being accelerated) the greater

the amount of force needed (to accelerate the object).

Everyone knows that heavier objects require more force to move the same distance as lighter objects

Newton’s Second Law of Motion

Newton’s Second Law gives us an exact relationship between

force, mass, and acceleration.

It Sates that the amount of force needed to move an object is equal to it’s mass multiplied by its acceleration

FORCE = MASS times ACCELERATION

Example of how Newton's Second Law works

Mike's car, which weighs 1,000 kg, is out of gas.

Mike is trying to push the car to a gas station,

and he makes the car go 0.05 m/s/s. How much force is Mike using?

F

AM

Answer = 50 newtons

Newton’s Third Law of MotionNewton’s Third Law states, “For every action there is an equal and opposite re-action”

What does this mean? This means that for every force there is a reaction force that is equal in

size, but opposite in direction.

That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard.

Newton’s Third Law of Motion

Explains how a rocket works

The rocket's action is to push down on the ground

with the force of its powerful engines, and the reaction

is that the ground pushes the rocket upwards with an equal force.

Quiz1) Who was the scientist who gave us the

Laws of Motion?

2 What is another name for the first law of motion?

3) Which law explains why we need to wear seatbelts?

4) Which law says that force is equal to mass times acceleration (F=MA)?