An Introduction to Forces Dorsey DE Physics

A Definition A force is a push or a pull. get a student to demonstrate being forceful

A Definition A force is a push or a pull. It is a vector quantity and is symbolized by: Before going to the next slide ask the students if they remember the equation for newton’s force. Write it up on the board, ask them what the units for each are. Then what the units are.

A Definition A force is a push or a pull. It is a vector quantity and is symbolized by: = m * a In the SI system, force is measured in Newtons (N). 1 N = 1 kg m/s2 Remind them F=ma and that is where this unit comes from

https://www.youtube.com/watch?v=8yis7GzlXNM&list=RD8yis7GzlXNM

Applied Force Applied Force is a general term for any contact force, e.g.

Applied Force Applied Force is a general term for any contact force, e.g. Tension

Applied Force Applied Force is a general term for any contact force, e.g. Tension Friction

Applied Force Applied Force is a general term for any contact force, e.g. Tension Friction Normal Force

Tension Force exerted by strings, ropes, cables, etc. attached to an object. Tension force is generally constant. It can only pull

Friction Friction acts to oppose any motion. And attempted motion. Ask the student to think about different types of friction that they know about and then cold call students for responses. Friction to stop things from starting to move Friction to slow things down that are moving (rolling, kinetic, fluid) Air friction

Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move

Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion

Friction Friction acts to oppose any (attempted) motion. Static friction: the force that prevents a stationary object from starting to move Kinetic friction: the force that acts against an object’s motion Air resistance (drag): friction on an object moving through air Many physics problems will neglect air resistance. What would happen differently with motion if we did put it in the calculations ?

Normal Force The normal force acts to keep objects apart. Think back to the force of the book acting on the table. Normal force comes from the atoms keeping things from moving through them. What happens if the force gets too large

Normal Force The normal force acts to keep objects apart. Fn = mg*cos(angle) The angle is from horizontal on a ramp. Think back to the force of the book acting on the table. Normal force comes from the atoms keeping things from moving through them. What happens if the force gets too large

Normal Force i.e. push on a wall, the wall will exert a normal force on your hand that prevents your hand from moving through the wall

Action-at-a-Distance Forces What does this saying make you think cold call.

Action-at-a-Distance Forces Contact between objects is not necessary. Get students to come up with some on their own. Give them less than a min to think of some in their groups. Ask some groups what they got. Talk about how this is the coolest force because

Action-at-a-Distance Forces Contact between objects is not necessary. These are action-at-a-distance forces

Action-at-a-Distance Forces Contact between objects is not necessary. These are action-at-a-distance forces. One example is gravitational force, the force of attraction between all objects with mass.

Action-at-a-Distance Forces Contact between objects is not necessary. These are action-at-a-distance forces. One example is gravitational force, the force of attraction between all objects with mass. (The gravitational force the Earth exerts on an object is called its weight.)

Free-Body Diagrams Usually an object will have more than one force acting upon it.

Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object

Free-Body Diagrams Usually an object will have more than one force acting upon it. A free-body diagram (FBD) shows all the forces acting on an object – and only the forces acting on the object. The object is drawn in the centre of the diagram

Free-Body Diagrams The forces acting on it are drawn as arrows pointing outwards.

Free-Body Diagrams The forces acting on it are drawn as arrows pointing outwards. The arrows must be labelled!

FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball. Get students to state the forces acting on the ball in real life

FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball. Fg

FBD: Example 1 A ball is falling downward through the air. Draw a FBD for the ball. Fair Fg

FBD: Example 1 What would the net force be??? Fair Fg

FBD: Example 1 What would the net force be??? + = Fair Fg

FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book.

FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FA

FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. Ff FA

FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. Ff FA Fg

FBD: Example 2 A book is being pushed rightward across a table. Draw a FBD for the book. FN Ff FA Fg

Net Force FBDs are drawn to help determine the net force (the sum of all forces) acting on an object. Resolve all vertical and horizontal vectors and determine the resultant.

FBD: Example 2 with Numbers A book is being pushed across a table, The applied force is 5 N [right]. The force of friction is 2 N [left], The gravitational force is 10 N [down], The normal force is 10 N [up]. Find the net force on the book. Split everything into x and y components then sum the x and y Draw a triange with the x tot and y tot Solve the hyp and angle of the triangle. Ask kids to solve it. Then put the method on the board

FBD: Example 2 with Numbers The applied force is 5 N [right]. The force of friction is 2 N [left], The gravitational force is 10 N [down], The normal force is 10 N [up]. FA = 5 N Fg = 10 N Ff = 2 N FN = 10 N Answer 3 N right

Mr. Dorsey In the Door Try and draw out the force diagram you think are involved with Mr. Teacher in a door Ignore friction.

Greg In the Door Applied Force

Inv 7.3 Forces in Two Dimensions Investigation Key Question: How do forces balance in two dimensions? 46

7.3 Forces in Two Dimensions Force is also represented by x-y components.

7.3 Forces in Two Dimensions What are the components of this force?” What is the x acceleration if m= 10. kg?

7.3 Forces in Two Dimensions What are the components of this force?” F=ma so A=F/m = 30/10=3 m/s^2

7.3 Force Vectors Equilibrium, is when all forces are balanced and the net force is zero. In two dimensions, then all of the forces in the x-direction and y-direction balance separately.

7.3 Equilibrium and Forces It is much more difficult for hold his arms out at a 45° Because each arm must still support 350 newtons vertically to balance the force of gravity.

7.3 Forces in Two Dimensions Use the y-component to find the total force in the gymnast’s left arm.

7.3 Forces in Two Dimensions The force on both arms must be 495 newtons because it also has a vertical component of 350 N.

7.3 Forces in Two Dimensions When the gymnast’s arms are at an angle, only part of the force from each arm is vertical. The total force must be larger because the vertical component of force in each arm must still equal half his weight.

Practice

Steps for Complex Problems Draw all forces on FBD Split every force into x and y components then sum the x and y Draw a triangle with the x tot and y tot Solve the hyp to find net force magnitude and angle of the triangle. Easy Example Angle Example