Forces Force- a push or pull Gravitational force- force between all objects. Any mass exerts a force upon any other mass Electromagnetic force- forces between charged particles. Strong Nuclear force- holds particles in nucleus of an atom together. (strongest) Weak force- a form of electromagnetic force from radioactive decay.
Newton’s 1st Law of Motion Otherwise known as Newton’s Law of Inertia States: an object with no force acting on it remains at rest or moves with constant velocity in a straight line. Inertia: the tendency of an object to resist a change in its state of motion. Inertia is dependent upon mass.
Newton’s 1st Law In other words, Newton’s 1st law: an object at rest will stay at rest and an object in motion will stay in motion at a constant velocity in a straight line unless acted upon by an outside force.
Newton’s 2nd Law of Motion Force = Mass x Acceleration Units: Newton’s Newton = kg m/s^2 Newton’s 2nd law is also known as the law of acceleration
Newton’s 2nd law Problem: What net force is required to accelerate a 1500 kg race car at 3 m/s2?
Newton’s 3rd Law of Motion When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude but opposite in direction. For every action there is an opposite and equal reaction. Action-Reaction
Mass and Weight Weight is the gravitational force exerted by the earth. Weight = Mass x Acceleration due to Gravity Ex. Find the weight of a 2.3 kg bag of sugar.
Friction The force that opposes motion between two surfaces that are in contact. It is usually parallel to the direction of motion. Static Friction – the force that opposes the start of motion. Ex. Pushing a box Sliding Friction – the force between surfaces in relative motion. Sliding friction is also called kinetic friction. Ex. Pushing a box
Net Forces The net force is the vector sum of all forces acting on a body. The acceleration of an object is always in the same direction as the net force.
Air resistance Air resistance to falling objects is called drag. Terminal velocity – when the drag force equals the force of gravity. Do you know the terminal velocity for the average person?
Questions A skydiver in the spread eagle position opens the parachute. Is the diver accelerated? Which direction? A 3 kg object is pulled to the right with a force of 30 N and to the left with a force of 40 N. Find the net force acting on the object and the object’s acceleration.
Friction Continued Normal Force Applied Force Friction Force Coefficient of Friction – a constant that depends upon the 2 surfaces in contact. The higher the coefficient of friction, the greater the frictional force will be between the two surfaces Equation: Ff = µ Fn Force of Friction = Coefficient of Friction x Normal force (perpendicular force) Normal Force Friction Force Applied Force Force of Weight
Force applied: what you are pushing or pulling with Force Friction: What is opposing you Force gravity: weight of the box (means w=mg) Force normal: Normal force is the same as Force gravity
Problem Solving Sketch the drawing Draw the arrows representing the forces Label each arrow with the force it is representing. Be Specific!
Example Problems Ex. Elevator Fnet = Fapplied + Wt. Net force is equal to the sum of the applied force + the weight. Example: A spring scale hangs from the ceiling of an elevator. It supports a package that weighs 25 N. A) what is the upward force the scale exerts when the elevator is not moving? B) what force must the scale exert to accelerate the package 1.5 m/s2?