Forces and Newton’s Laws

Forces Forces are push or pull acting on an object so vector! Contact forces Noncontact

Free body diagrams We draw free body diagrams to show forces acting on an object- magnitude and direction Here are forces acting on a block being pulled across the table- can you ID them?

5 common forces Weight Normal force Tension Friction Applied forces

Weight and Normal Weight starts at center of mass and points to center of earth Normal starts at point of contact with surface and points perpendicular to surface, passing through object’s center of mass

Tension and Friction Tension forces act along string, wire, rope- begin at point of contact and point in direction of pulling Friction forces start at same place as normal force but act parallel to contact surface and always against the motion of object

Applied Forces Applied forces is a generic catch-all for any other forces

Forces at an angle When forces act at an angle we break them up into their x and y components to work with them

Equilibrium Static equilibrium- object at rest Dynamic equilibrium- object at constant velocity

Equilibrium and Free Body Diagrams Object is in static or dynamic equilibrium if forces acting to the right= forces acting to the left and forces acting up= forces acting down If not, object will accelerate in direction of unbalanced force

Unbalanced Forces A net unbalanced force leads to acceleration in that direction a ~ F/m So a is directly proportional to F and indirectly proportional to m Easier to accelerate a penny than an elephant!

Law of Inertia Objects at rest will stay at rest and objects in motion will stay in motion unless compelled to change that state by a net force Or- object will maintain constant v unless acted upon by outside net force What are some forces that act to slow/stop objects? How do objects overcome these forces and continue to move at constant v?

Law of Inertia Remember- v is speed and direction so any change in v comes from unbalanced forces A net force acts to change velocity, not maintain it

Inertia and Mass Inertia is the tendency of an object to remain at rest or in motion at constant v (speed and direction) Mass= measure of inertia SI unit = kg

Inertial Frame of Reference In order for Newton’s Laws to appear valid, frame of reference must not be accelerating!

Law of Acceleration ∑F=ma Net force=mass*accel Units of force=newtons N=kg*m/s2 Direction of F= direction of a

Vector Nature of F and a Force and acceleration are both vectors so can be broken into their x and y components to solve ∑Fx =max ∑Fy =may

Action-Reaction, Newton’s 3rd If object A exerts a force on B, then B exerts an equal magnitude force on A in the opposite direction For every action, there is an = and opposite reaction When you push on balloon, it pushes back

Action-Reaction and mass If female skater were to push off from her partner, he would exert an equal and opposite force on her Would their accelerations be equal and opposite as well? No- he has greater inertia (mass) so he would not accelerate as much as she would.

Problem Solving using Newton’s 3rd Law Remember = and opposite forces does not mean = and opposite acceleration If you know the force acting on one object, the force acting on the other is just the opposite sign/direction