Physics Review What Are Newtons Laws of Motion

Physics Review What Are Newton's Laws of Motion?

Forces h “Push or Pull” that acts between two bodies – – – – Tension Gravitational force Frictional force Air resistance Electrostatic force Strong nuclear force Weak nuclear force The SI unit force is the Newton (N). This unit is equivalent to Phabulous Physics: Newton’s Laws

Newton’s First Law h “Law of Inertia” h An object will continue in its state of motion unless compelled to change by a force impressed upon it. h What net force is required to maintain a 5000 kg object moving at a constant velocity of magnitude 7500 m/s? – Net force of 0 constant motion Phabulous Physics: Newton’s Laws

Newton’s Third Law h For every action, there is an equal, but opposite, reaction. Phabulous Physics: Newton’s Laws

Weight h The weight of an object is the gravitational force exerted on it by Earth (or whatever planetary mass the object is on). h What is the mass of an object that weighs 500 N? = Fw mg m= Fw 500 N = = 51 kg m g 9. 8 s 2 Phabulous Physics: Newton’s Laws

Other Examples h A book with a mass of 2 kg rests on a table. Find the magnitude of the force exerted by the table on the book. FN Fg = mg = FN ( Fg ) Fg = (2 kg) 9. 8 sm 2 = 20 N = FN Note: This force is called the Normal force because it acts perpendicular to the contact surface of the object. Phabulous Physics: Newton’s Laws

Other Examples h A can of paint with a mass of 6 kg hangs from a rope. If the can is to be pulled up to + a rooftop with a constant velocity of 1 m/s, direction what must the tension in the rope be? FT FT = Fg = mg ( ) F T = 6 kg 9. 8 sm 2 = 59 N Fg Phabulous Physics: Newton’s Laws

Friction h Friction is a contact force that is parallel to the contact surface and perpendicular to the normal force. h Static friction (Fs) occurs when a force tries unsuccessfully to set a body in motion. h Kinetic (sliding) friction (Fk) occurs when a force acts on a body in motion. h Generally Fs >Fk Phabulous Physics: Newton’s Laws

Friction Equations Fs (max) = m s FN Fk = m k FN h represents the coefficient of friction - a number that is related to the nature of the surfaces in contact with each other. h Fs has a range of values dependent on the magnitude of the horizontal force being applied. Phabulous Physics: Newton’s Laws

Friction Problems h A crate of mass 20 kg is sliding across a wooden floor. k between the crate and the floor is 0. 3. Determine the strength of the force acting on the crate. FN F k = m k FN = m k mg F k = (0. 3) (20 kg) ( 9. 8 Fk F m s 2 )= 59 N If the crate is being pulled by a force of 90 N (parallel to the floor), find the acceleration of the crate. Fg Phabulous Physics: Newton’s Laws

Friction Problems h A crate of mass 100 kg rests on the floor. s is 0. 4. If a force of 250 N (parallel to the floor) is applied to the crate, what is the magnitude of Fs on the crate? = m s FN = m s mg F s, max FN ( ) m ( )( = ) 100 kg 9. 8 = 390 N F s, max 0. 4 s 2 Fs Fg F NOTE: Remember that Fs represents a range of values. In this case, the applied force of 250 N is less than the maximum Fs so the actual magnitude of Fs is 250 N. Phabulous Physics: Newton’s Laws

Pulleys h Pulleys are devices that change the direction of the tension force in cords that slide over them. h In problems, we generally ignore the mass and friction associated with pulleys. Phabulous Physics: Newton’s Laws

Inclined Planes h When a mass sits on an inclined plane, its weight has two components: normal and parallel to the plane. m Weight mg Normal force Parallel force Phabulous Physics: Newton’s Laws

How do the laws of motion describe everyday events? Phabulous Physics: Newton’s Laws

Can the motion of any object be predicted? How? Phabulous Physics: Newton’s Laws