Forces and the Laws of Motion Changes in Motion WHS Physics

Objectives  Students should be able to analyze situations in which a particle remains at rest, or moves with constant velocity, under the influence of several forces.  Students should understand the relation between the force that acts on an object and the resulting change in the object’s velocity  Students should understand how Newton’s Second Law,, applies to an object subject to forces such as gravity, the pull of strings, or contact forces  Students should be able to analyze situations in which an object moves with specified acceleration under the influence of one or more forces  Students should understand the significance of the coefficient of friction  Students should understand the effect of drag forces on the motion of an object  Students should be able to apply Newton’s Third Law in analyzing the force of contact between two objects that accelerate together

What we need to know…  How does force affect the motion of an object?  How do free-body diagrams help in analyzing forces and motion?  What are Newton’s Laws of Motion?  How can you determine forces required for equilibrium?  How is mass related to motion?  What are action-reaction pairs and why don’t they produce equilibrium?  What is the difference between mass and weight?  What is a normal force?  How can you determine the force due to friction?

What causes motion to change?  An object’s motion does not change without a cause  Something must force an object’s state of motion to change

A Little History…  Aristotle a famous Greek philosopher around 400 BC Natural Motion…  objects seek their natural resting places  Not caused by force Violent Motion…  Result of forces that push or pull an object  External cause… moving against nature  No force… No motion  Do you agree?

A Little History…  Galileo the foremost scientist of the late-Renaissance Asked how things moved rather than why Demolished the notion that force is necessary to keep an object moving Only when friction is present is force required to keep objects moving Without frictional force, an object’s motion will continue Open the door for Isaac Newton and his Laws of Motion

What do you think?  A ball is rolled across a counter top and rolls to a stop. How would you explain the motion of the ball? How would Aristotle explain the motion of the ball? How would Galileo explain it?

Force  Force causes changes in velocity Since acceleration is the rate of change in velocity… Force involves acceleration A force may cause a change in direction or a change in speed  The Meaning of Force A force is a push or pull upon an object Forces occur whenever there is an interaction between two objects When the interaction ceases, the two objects no longer experience a force. Forces only exist as a result of an interaction.

Force  All forces (interactions) between objects can be placed into two broad categories: Contact forces Field forces (or action-at-a- distance)

Contact Forces  Contact forces are types of forces in which the two interacting objects are physically in contact with each other

Field Forces  Field Forces (Action-at-a-distance) are types of forces in which the two interacting objects are not in physical contact with each other, but are able to exert a push or pull despite the physical separation.

Types of Forces Contact Forces Field Forces Frictional Force Gravitational Force Tensional Force Electrical Force Normal Force Magnetic Force Air Resistance Force Applied Force Spring Force

Units of Force  American Unit is the Pound, (lb)  SI Unit is the Newton, (N)  1N = lb

Force  Forces are Vectors Magnitude and direction Can be added as any vector Can be resolved into components  Force diagrams and Freebody diagrams help in analyzing forces Vertical Component Horizontal Component Force (F) 

Force and Free-body Diagrams Force Diagrams show all interacting objects and forces Free-body Diagrams isolate the object of interest and show only the forces acting on that object

Building a Free-body Diagram A car being towed by a tow truck…

Issac Newton  The English scientist Isaac Newton ( ) was born about the time of Galileo’s death.  By most accounts, he didn’t really display much genius until a plague in England closed the university he worked at for two years.  During those two years, Newton conducted the work that led to him publishing Principia, which is regarded by many as the greatest scientific work ever written.  In Principia, Newton showed that the motions of the sun, moon, stars, and any physical object were governed by the same physical principles: the Law of Gravity, and the three Laws of Motion.

Newton’s First Law The Law of Inertia  “An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force”.  What do think is meant by an unbalanced force?

Net Force  Net Force… The vector sum of all the external forces acting on an object Forces are vectors… If the sum of the force vectors is zero there is no net force… the forces are balanced… motion will not change If the sum of the force vectors is not zero there is a net force… the forces are unbalanced… motion will change

Net Force Balanced Forces… No Net Force Constant Velocity Unbalanced Forces… Net Force What happens? *Cart will Accelerate to the right

Equilibrium  Equilibrium… The state in which there is no change in an object’s motion The sum of all forces equals zero “the sum of” Balanced Forces… No Net Force Constant Velocity

Inertia  A Net Force causes an object’s state of motion to change… it causes acceleration.  Some objects are more resistant to change than others  Inertia… Is an object’s resistance to a change in motion it is the tendency of an object to maintain its state of motion  Mass is a measure of inertia More mass… more inertia Less mass… less inertia

Mass is not Weight…  Mass is a measure of an object’s inertia, or the amount of matter in an object It is the same everywhere… on earth, on the moon, or in space  Weight is the force on an object due to gravity It varies with location…on earth, on the moon, or in space Weight = mass x gravity or, W = mg

Calculating Net Force 1.An 3kg jacket is hung on a coat hanger. What is the force on the coat rack?

Calculating Net Force 1.An 3kg jacket is hung on a coat hanger. What is the force on the coat rack? 2.A 25kg child is on a swing. What is the tension in each chain holding up the seat?

Calculating Net Force 1.An 3kg jacket is hung on a coat hanger. What is the force on the coat rack? 2.A 25kg child is on a swing. What is the tension in each chain holding up the seat? 3.What force is required to keep the object below in equilibrium? 10N 38 o

Calculating Net Force 1.An 3kg jacket is hung on a coat hanger. What is the force on the coat rack? 2.A 25kg child is on a swing. What is the tension in each chain holding up the seat? 3.What force is required to the object below in equilibrium? 4.A sign is supported as shown. If the sign has a mass of 50 kg, what is the tension in each chain? Eat at Joe’s 30 o 10N 38 o

What we know…  How does force affect the motion of an object?  How do free-body diagrams help in analyzing forces and motion?  What are Newton’s Laws of Motion?  How can you determine forces required for equilibrium?  How is mass related to motion?  What are action-reaction pairs and why don’t they produce equilibrium?  What is the difference between mass and weight?  What is a normal force?  How can you determine the force due to friction?

Questions?