Newton’s 1st and 2nd laws of Motion

4 Forces of the Universe Nuclear : Strong forces holding the nucleus of an atom together (gluon) Weak : Forces as a result of some atoms that decay the nucleus ( radioactive ) Electromagnetic : a force based on electricity, magnetism and light properties Gravitational : The weakest force caused by the 2 factors, MASS and DISTANCE

Mechanical Forces Inertia: Newton’s First Law, net or unbalanced force is needed to change the state of motion Friction: the contact force that acts to oppose sliding motion between surfaces Normal : the contact force exerted perpendicular to the surface contact Tension : the pull exerted by a cable when attached to a body Compression : the push exerted to the object Weight : force of attraction due to distance from the center of the earth and mass of the object Units are Measured in NEWTONS ( N )

Newton's First Law of Motion ( inertia ) An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Stationary and Moving objects

Balanced and Unbalanced Forces The book is said to be at equilibrium. There is no unbalanced force acting upon the book and thus the book maintains its state of motion

Balanced and Unbalanced Forces The book is not at equilibrium and subsequently accelerates

Newton’s Second law of motion The acceleration of a body is directly proportional to the net force on it and inversely proportional to the mass. ( F = ma ) Unit of Force = kg-m or N (Newton) s2 Calculation: mass of an apple is 100g or .1 kg F = ma = (.1kg) (10 m/s2) = 1 kg-m/s2 = 1 N

Sample Force Problem #1 Using Newton’s Second law of Motion ( the acceleration of a body is directly proportional to the net force on it and inversely proportional to the mass, F = ma). What net force is required to accelerate a 1500 kg car at 3.00 m/s2?

Solution Problem #1 F = m a F = 1500 kg x 3.00 m/s2 F = 4500 kg-m/s2 or F = 4500 N Given: m = 1500 Kg a = 3.00 m/s2 Find: F = ?

Static Balance (free body) Problem #2 Forces of 10 N and 15N are arranged according to diagram. What is the net force on this object. What is the rate of acceleration of the 12 Kg object? 10 15 12 Kg

Solution #2 Given: Force 10 N and 15 N in opposite directions Mass = 12 Kg Find : Net Force Acceleration Net Force = F1 - F2 = 15N – 10N = 5 N F = ma 5N = 12 Kg (a) 5N = a 12 Kg 0.417 m/s2 = a

Sample Question #3 The figure shows a block that is being pulled along the floor. According to the figure, what is the acceleration of the block?

Solution Problem #3 F = m a 40N = 20 kg (a) 2 m/s2 = a Find: a = ? Given: m = 20 Kg Ffriction= 10 N Fpull = 50 N Find: a = ?

Sample problem #4 A 50-kg child on a skateboard experiences a 75-N force as shown. What is the expected acceleration of the child?

Solution Problem #4 F = m a 75 N = 50 kg (a) 1.5 m/s2 = a Find: a = ? Given: m = 50 Kg Fpush= 75 N Find: a = ?

Uniform Circular Motion Circular motion at a constant speed HAS an ACCELERATION! Keeping the magnitude of the velocity constant, but changing the direction will create a change in acceleration Using vector addition, a centripetal = v2 r

Sample Problem #3 During a hammer throw, an athlete rotates a 7.00 kg hammer tied to the end of a 1.3 m chain. The hammer moves at the rate of 3 m/s in a circle. What is the centripetal acceleration of the hammer What is the tension in the chain? If the chain breaks at the point shown which direction will the hammer fly?

Solution Problem #3 m = 7.00 kg v c = 3.0 m/s r = 1.3 m a c = v2 / r a c = (30 m/s)2 / 1.3 m a c = 900 m2/s2 / 1.3 m a c = 692.3 m/s2 F c = m a c F c = 1.3 kg ( 692.3 m/s2) F c = 900 N @ the tangent (direction 2)

Summary Forces are invisible measurements that explain the states of motion Newton derived laws of motion that explained the way things move. Forces can be balanced (equilibrium) or unbalanced (acceleration). Uniform circular motion is accelerating because of the constant change in direction