Forces

Newton’s 1st Law Review Question Use Newton’s 1st Law of Motion to explain why the zebra’s stripes in the comic below ended up all on his head…

Inertia An object’s resistance to changing its velocity

Quick Recap Newton’s First Law of Motion: Inertia: Mass: An Object at rest stays at rest unless acted upon by an unbalanced force An Object in motion stays in motion at the same velocity unless acted upon my an unbalanced force Inertia: An Object’s resistance to changing it’s motion More Mass=More Inertia Less Mass=Less Inertia Mass: How much stuff makes up an object Book vs. Piece of Paper Say these aloud as a class. This is the exact phrasing you should use when I ask you to explain how Newton’s First Law applies. More mass=more inertia the more mass something has the more resistant it is to changing its motion Less mass=Less inertia  the less mass something has the more resistant it is to changing its motion

Force A force is a type of push or pull Force is measured in Newtons 1 Newton is the force is takes to accelerate a 1kg object 1m/s2 Force is a vector- They have a magnitude and direction

Free Body Diagrams These help us see all of the forces acting on an object Objects are represented by a box Forces are represented by arrows coming from (pulling) or going into (pushing) the box (they are vectors) The direction of the arrow is the direction of the force The length of the arrow represents how big the force is The diagram to the right would be an object with a force of 2 Newton’s upward and 4 Newton’s downward 2N 4N

Net Force Net force is the overall force acting on an object Right and up are positive, left and down are negative An object will accelerate in the same direction as the net force Balanced forces have a Net Force of 0 Newtons This will result in NO change in velocity Unbalanced forces will change an object’s velocity An unbalanced force is REQUIRED in order to change an object’s velocity (accelerate)

What is the net force? Example 1: Roy pushes a box to the left with a force of 30 N and Sally pushes the box to the left with a force of 50 N Step 1: write down givens (draw a FBD) ForceRoy= -30N ForceSally= -50N Step 2: Total Force=(ForceRoy)+(ForceSally) Step 3: Total Force= (-30)+(-50) Step 4: Total Force= -80N (OR 80N to the left) Sally (50N) Roy (30N) *Remember: forces to the LEFT or DOWN are negative

What is the net force? Example 2: Roy pulls a box to the left with a force of 30 N and Sally pulls the box to the left with a force of 50 N Step 1: write down givens (draw a FBD) ForceRoy= -30N ForceSally= -50N Step 2: Total Force=(ForceRoy)+(ForceSally) Step 3: Total Force= (-30)+(-50) Step 4: Total Force= -80N (OR 80N to the left) Sally (50N) Roy (30N) *The math works out the same! HOWEVER, note the arrows for the Free Body Diagram are pointing outward, not inward

Check YOUR Understanding: What is the net force? 1) Roy pulls a box to the right with a force of 30 N and Sally pushes the box to the left with a force of 50 N 2) Roy pulls a box to the left with a force of 30 N and Sally pushes the box to the right with a force of 50 N

Check YOUR Understanding What is the net force? Al pushes a box to the left with a force of 40 N, Bob pushes the box to the right with a force of 60 N and Carl pushes the box to the left with a force of 30 N

Types of Forces Compression Force (FCompression) a push force Think about a spring Applied Force (FApplied) a pushing force that you would put on an object to make it move Tension Force (FTension) a pull force Think about tug o’ war Gravitational Force (FGravity) pulling force that exists between any two objects that have mass

Frictional Force (FFriction) opposes the motion of an object that’s in contact with another object Kinetic Friction (FKineticFriction ) Friction felt by an object when it is moving Static Friction (FStaticFriction )  Friction felt by an object when it is not yet moving FFriction FTension

Normal Force (Fnormal) The force acting on an object, perpendicular to the surface where the object rests This is the force that keeps gravity from pulling you through your chair and the floor and the ground to the center of Earth! FNormal FWeight

If you aren’t moving are you exerting a force?

YES… standing on Earth anyway We are always exerting a force standing on Earth because we are always encountering another object (at the very least the ground) Gravity causes us to put a force on the ground or our chairs. This means we also have forces acting on us, the normal force.