Forces. Examples –A force causing an object to start moving –A force causing an object to stop moving –A force causing an object to change direction.

ForcesForces

ExamplesExamples– A force causing an object to A force causing an object to

start movingstart moving– A force causing an object to A force causing an object to

stop movingstop moving– A force causing an object to A force causing an object to

change directionchange direction

Force:Force: The cause of an acceleration, or The cause of an acceleration, or the change in an object’s velocity.the change in an object’s velocity.

Newton (N):Newton (N): The amount of force that, The amount of force that, when acting on a 1kg mass, produces an when acting on a 1kg mass, produces an acceleration of 1m/sacceleration of 1m/s22

1 N = 1kg x 1m/s1 N = 1kg x 1m/s22

Two Types of ForcesTwo Types of ForcesContact Forces:Contact Forces: Physical contact between Physical contact between two objects, results in the change in velocity two objects, results in the change in velocity (that is – acceleration)(that is – acceleration)– Example: pull on a spring, the string will moveExample: pull on a spring, the string will move– Pull on a wagon, the wagon will movePull on a wagon, the wagon will move– Catching a football, the ball will stopCatching a football, the ball will stop– Kick a soccer ball, the ball will moveKick a soccer ball, the ball will move– Your turn – let’s come up with 2 more examples.Your turn – let’s come up with 2 more examples.

How do you know that these are forces?How do you know that these are forces?

Field Forces: Field Forces: Force that can exist between Force that can exist between objects, even in the absence of physical contact objects, even in the absence of physical contact between the objects.between the objects.– Example: gravityExample: gravity

magnetsmagnets static electricitystatic electricity

Force DiagramsForce DiagramsThe effect of a force depends on its The effect of a force depends on its magnitudemagnitude and and direction (sound familiar?)direction (sound familiar?)

Forces are vector quantities Forces are vector quantities

Force DiagramsForce Diagrams: : a diagram of the objecta diagram of the objectss involved in a situation and the forces exerted involved in a situation and the forces exerted on the objecton the objectss. . – Arrows represent forcesArrows represent forces– Attached to the object on which the force is actingAttached to the object on which the force is acting

Force diagram vs. Free body Force diagram vs. Free body diagramdiagram

(b) Free Body Diagram: only shows the car and the forces acting on it

Force diagram shows the forces exerted by the car on other objects

Free Body DiagramsFree Body Diagrams

Free Body Diagram (FBD):Free Body Diagram (FBD): shows forces shows forces affecting the motion of a single object. affecting the motion of a single object. – A free body diagram of the car will show all the forces A free body diagram of the car will show all the forces

acting on the car as if the forces are acting on the acting on the car as if the forces are acting on the center of the car.center of the car.

-- To draw the free-body -- To draw the free-body diagram, you must first diagram, you must first isolate and identify all the isolate and identify all the forces acting on the car.forces acting on the car.

How to draw a free body diagram.How to draw a free body diagram.

Force exerted on the car by the tow truck

5800 N

14,700 N

Road exerts upward force on the car

The interaction between the road and the car’s tires, the road also exerts a backward force of friction on the car

775 N

Give it a try!Give it a try!

Draw a free-body Draw a free-body diagram (FBD) of diagram (FBD) of a football being a football being kicked. Assume kicked. Assume only forces acting only forces acting on it are gravity on it are gravity and the force of and the force of the kicker.the kicker.

Draw a diagram of a Draw a diagram of a crash-test dummy in a crash-test dummy in a car at the moment of car at the moment of collision. Assume that collision. Assume that the forces acting on the the forces acting on the car are 19600 N car are 19600 N downward, 17800 N downward, 17800 N forward, 2500 N forward, 2500 N backward. The forces backward. The forces on the dummy are 585 on the dummy are 585 N downward, 175 N N downward, 175 N backward, and 585 N backward, and 585 N upward.upward.

Now make 2 FBD – one for car, one for dummy

ResolvingResolving

Derek leaves his Derek leaves his physics book on top physics book on top of a drafting table that of a drafting table that is inclined at a 35 is inclined at a 35 degree angle. The degree angle. The free body diagram to free body diagram to the right shows the the right shows the forces acting on the forces acting on the book. Find the net book. Find the net external force acting external force acting on the book, and on the book, and determine whether determine whether the book will remain the book will remain at rest in this position. at rest in this position. F gravity on book = 22 N

F table on book = 18 N

F friction = 11 N

Select a coordinate Select a coordinate plane to work with.plane to work with.

Try to get the most Try to get the most forces in the x & y forces in the x & y plane, then resolve plane, then resolve the ones left on an the ones left on an angle.angle.

+ & - direction is + & - direction is importantimportant

yx

11 N

18 N

22 N

Find the x and y components of all vectorsFind the x and y components of all vectors

yx

Θ =35o

θ

F table on book (normal):

X = 0 N

Y = 18 N

F friction:

X = -11 N

Y = 0 N

F gravity on book :

X = ?

Y = ?

= ?θ

yx

13 N

18 N

11 N18 N

22 N

XX YY

ΣΣFFxx = = FFg,xg,x + F + Ffrictionfriction ΣΣFFyy = = FFg,yg,y + F + Fnormalnormal

ΣΣFFxx = = 13N+ -11N13N+ -11N ΣΣFFyy = - = -18N + 18N18N + 18N

ΣΣFFxx = = 2 N2 N ΣΣFFyy = = 0 N0 N

The net external force is 2 NThe net external force is 2 N

Find the net external force in both the x & y directions

Evaluate your answerEvaluate your answer

The book experiences an acceleration in The book experiences an acceleration in the downhill direction and it will slide off the downhill direction and it will slide off the table the table

Give it a try - Give it a try p. 133 #1-4Give it a try - Give it a try p. 133 #1-4