Forces and Newton’s Laws

Physics

The Concepts of Force and Mass

• Contact forces- push or pull that arises from physical contact between 2 objects– Example: basketball player pushes basketball

• Noncontact forces (action-at-a-distance forces)- 2 objects exert a force on each other even though they are not touching– Example: force of gravity

Types of ForcesContact Forces Action-at-a-Distance

Forces

*Frictional Force *Gravitational Force

*Tensional Force Electrical Force

*Normal Force Magnetic Force

Air Resistance Force

Applied Force

*Spring Force

Newton’s First Law

• An object at rest remains at rest, an object in motion remains in that same motion, unless acted upon by an outside force.

• Often called Law of Inertia

• Inertia is the tendency of an object to resist changes in its velocity.

• Net force- vector sum of all forces acting on an object

Galileo’s Experiments17th Century

Galileo’s Conclusion

Balanced or Unbalanced Forces

•Balanced Forces: forces acting on an object that combine to produce a net force equal to zero –forces cancel each other out = no movement or constant v

Newton’s 1st LawDemonstration of Inertia(Don’t try this at home!)

Balanced or Unbalanced Forces

• Unbalanced Forces: forces acting on an object that combine to produce a net nonzero force–When one force is stronger than the other, causing movement in the direction of the stronger force.

             Provide explanations for each application of Newton’s 1st Law:

•blood rushes from your head to your feet when riding on a descending elevator which suddenly stops. •the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. •a brick is painlessly broken over the hand of a physics teacher by slamming the brick with a hammer. (CAUTION: Do not attempt this at home!) •to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted. •headrests are placed in cars to prevent whiplash injuries during rear-end collisions. •while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard.

Newton’s 2nd Law

maF

F: The net external force

m: mass of the object

a: acceleration of the object

* The direction of the acceleration is the same as the direction of the net force

Good Luck!

• 6 Question Quiz

Quiz

• 1. State Newton’s 1st Law.

• 2. Give an example of the 1st Law.

• 3. How much force is needed to keep a 3 kg object at a constant v?

• 4. Name the 2 main categories of force.

• 5. What is the definition of a force?

• 6. Write the 2nd Law equation

• Forces are vector quantities: magnitude and direction

• Mass- the amount of matter in an object

• SI unit for mass is kg

• Mass is a scalar quantity

Derived Unit of force

F= ma

= (kg) (m/s2)

Unit of force is the newton (N)

1N = 1 kg*m/s2

Frictional Force

• Only exists if an object moves or attempts to move along a surface

2 Types:1. Static frictional force- The force it takes to

apply to an object at rest in order to make it move.

2. Kinetic frictional force- Friction that opposes motion of an object in motion

Normal Force

• A force in which a surface exerts on an object perpendicular to the surface.

Tensional Force• Force that is applied by means of cables or ropes that

are used to pull an object. Spring Force• The spring force is the force exerted by a compressed

or stretched spring.Gravitational ForceForce of attraction exerted between any 2 objects,

depending on the mass and distance between them

Free Body Diagrams

• Diagram that represents the objects and the forces that act on it.

The size of the arrow in a free-body diagram is reflective of the magnitude of the force.

The direction of the arrow reveals the direction in which the force acts.

Weight (4.7)

• The weight of an object on the earth is the gravitational force that the earth exerts on the object.

• Weight acts downward, toward the center of the earth.

• SI unit: NewtonW=mg(Just like F=ma)

4.5 Newton’s 3rd Law of Motion

• Whenever one body exerts a force on a second body, the second body exerts an equal and oppositely directed force on the first body.

• “Action-Reaction Pairs”

Newton’s Law of Universal Gravitation

• F=G m1m2

r2

m1= mass of 1 particle

m2= mass of another particler= distance between the particlesG= 6.67259 x 10-11 N*m2/kg2

• Every particle in the universe exerts an attractive force on every other particle. A particle is a piece of matter, small enough in size to be regarded as a mathematical point.

G: Universal Gravitational Constant

• G was first measured in an experiment by English scientist Henry Cavendish (1731-1810), more than a century after Newton proposed his law of universal gravitation.

• P. 94

Universal Gravitation

• What is the magnitude of the gravitational force that acts on each particle when the mass of one particle is 12 kg, the mass of the second particle is 25 kg, and the distance between them is 1.2m?

• Put your answer in scientific notation.

Answer

• 1.4 x 10-8 N

If it’s not a particle…?

• This law can be applied to objects that are spherical, but the distance is between the center of each of the spheres. If they are not uniform spheres, than it is a good approximation.

Problem

• Determine the force of gravitational attraction between the earth (m = 5.98 x 1024 kg) and a 70-kg physics student if the student is standing at sea level, a distance of 6.37 x 106 m from earth's center.

Problem

• Determine the force of gravitational attraction between the earth (m = 5.98 x 1024 kg) and a 70-kg physics student if the student is in an airplane at 40000 m above earth's surface. This would place the student a distance of 6.38 x 106 m from earth's center