Forces and Newton’s Laws

Physical ScienceUnit 8

Newton’s Laws• 1st Law: An object in rest will stay in rest and an

object in motion will stay in motion at a constant velocity unless acted on by an outside force.

• 2nd Law: Force equals mass times acceleration• 3rd Law: For every action there is an equal and

opposite reaction.• Law of Universal Gravitation: There is a

gravitational force between every two objects. They pull on each other equally.

Fist of all… What is a force?????

• A push or a pull that has the ability to change motion!

Newton’s 1st Law(Law of Inertia)

• An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced (outside) force.

Law of Inertia

• “An object will keep doing what it is doing.”

• An object resists changes in motion because of its inertia. We have to apply a force to change the motion.

Example: These pumpkins will not move unless they are acted on by an unbalanced (outside) force.

Law of Inertia

• ALL objects have inertia.

• Since all objects have inertia, do they all have the same tendency to resist change??– NO!– Inertia depends on the mass of the

object!!!!!!!!!!!!!!!!!!• More Mass = More Inertia• Example: It is easier to kick a soccer ball that has a mass of

2 kilograms than a bowling ball that has a mass of 12 kilograms.

Examples of Inertia…

• If we were to kick the bowling ball and the soccer ball which one would go farther?

The soccer ball!!!

Newton’s 2nd Law

• The force needed to move an object is equal to the object’s mass x its acceleration.

• Force = mass x acceleration (F = ma)

Measuring Force

• We measure force in Newtons (after Sir Issac Newton, founder of all of Newton’s Laws).

• This can be abbreviated to “N”.

• Therefore…– F = ma• N = (kg)(m/s2) • N = kg*m/s2

Practice…• If the mass of a helicopter is 4,500 kg. and the net force

on it is 18,000 N, what is the helicopter’s acceleration?

– F = ma• F = 18,000 N• m = 4,500 kg• a = ?

– 18,000 N = (4,500 kg)a

– a = 18,000 N / 4,500 kg

– a = 4 m/s2

Practice…

• What is the net force on a dragster with a mass of 900 kg if its acceleration is 32.0 m/s2?

• 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. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

Friction

• The force that opposes sliding when two objects are in contact.

Terminal Velocity• As an object falls through the air,

gravity accelerates that object

• As an object accelerates, the upward force of air resistance increases

• Terminal Velocity: highest falling speed an object will reach!

• F gravity = F air resistance

Gravity

• Attractive force between any two objects

• Depends on the masses of the objects and the distance between them

• Acceleration due to gravity on earth = 9.8 m/s2

Weight

• Gravity exerts a force on all objects

• Weight is the force of gravity on objects

• Weight = mass x gravity ( W = m x g)

• Weight will change if acceleration due to gravity changes!

Practice…

• On earth, what is the weight of a television having a mass of 75 kg?

• M = 75 kg g = 9.8 m/s2 W=?

• W = (75 kg) (9.8 m/s2)= 735 kg x m/s2

735 N

Practice…

• Find the acceleration due to gravity on Mars if a person with a mass of 60.0 kg weighs 22.2 N on Mars?

Review… What is the difference between weight and mass???

• Mass is the amount of matter in an object. This does not change in an object as it moves away from the earth.

• Weight is caused by gravity. Therefore, the greater the distance, the less the force of attraction. The less the distance the greater the force of attraction.

• Your weight is dependent upon the mass of the earth. If you were able to go to the planet, Jupiter, your weight would be 27 time greater, because the mass of Jupiter is 27 times greater.

What causes “weightlessness”?

• Even in the space shuttle, there is a force of attraction exerted by the earth on the shuttle and its contents.

• When the space shuttle temporarily “falls” toward the earth, the contents of the space shuttle appear to be weightless, but in fact they are falling with the space shuttle. This is called “freefall”.

What is centripetal force?

• “centripetal” means to move toward the center.

• Centripetal acceleration is the acceleration of an object toward the center of a curve or circular path.

• Centripetal force is a force toward the center of a circle which holds the moving object in its circular path.

Newton’s 3rd Law

• When an object exerts a force on second object, the second object exerts a force on the first that is equal in size but opposite in direction.

• A.K.A. action-reaction forces

• For every action there is an equal but opposite reaction.