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##### Chapter 5 Forces

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**Forces**• Kinematics: Describes how an object moves • Dynamics: Describes why an object moves • Newton’s three laws of motion, describe the “dynamics” of why objects move**Forces and Interaction**• Force – a “push or pull” • Contact Force – you physically push on a wall • Long-range Force – like magnets or gravity • Force – a vector quantity. Has what two parts. • SI Unit – the Newton abbreviated as “N” • Superposition of Forces : forces combine according to vector addition**4 Types Forces**• Gravitational forces • The Earth pulls and holds the moon in orbit • The moon pulls and causes tide changes • Electromagnetic forces • due to electric charges, both static and moving. • Strong Nuclear Forces • holds particles in the nucleus together.(Strongest of the 4) • Weak Nuclear Forces • Radioactive decay**Physics**= Phun Newton’s Three Laws of Physics Why did the apple accelerate? The apple was at rest, what started it in motion?**Newton’s 1st Law**• An object at rest will stay at rest unless a force acts on it. • Here, the girl is at rest until acted on by the force imposed by the cannonball.**Newton’s 1st Law**• An object at rest will stay at rest unless a force acts on it. • Here, the skateboard/log combination is at rest until acted on by the force imposed by the rocket.**Newton’s 1st Law**• An object in motion will continue in a straight line until acted on by some “outside” force. • Consider a baseball that is pitched to a batter. What force causes the ball to change direction? Hit**Newton’s 1st Law**• An object in motion will continue in a straight line until acted on by some “outside” force. • Consider the space shuttle turning while in space flight. What produces the forces?**Newton’s 1st law**• An object in motion will continue in a straight line until acted on by some “outside” force. • Consider the space shuttle when landing. What produces the forces?**Multimedia**• The car and the wall • The motorcyclist • The truck and the ladder**Weight and Mass**Mass: A measure of the amount of matter in an object has. Symbol (m) Unit (kg) Weight: The gravitational force exerted on a body Symbol (W) Unit (N Newton)**Weight**A Motor cycle weights 2450N. What is its mass?**Homework**• WS 5a 1-10 • WS 5a (b) 1-11**Newton’s 2nd Law**• Acceleration is directly proportional to the magnitude of the net force. • Acceleration is inversely proportional to the mass of the object.**Newton’s 2nd Law**• Acceleration depends on both mass and the net force**Acceleration depends on net force**• A force of 10N accelerates the box**Acceleration depends on net force**• A force of 20N accelerates the box twice as fast**Acceleration depends on mass**• A force of 10N accelerates the box**Acceleration depends on mass**• A force of 10N accelerates the smaller box faster**Free-body diagrams WS 5b #1**Draw the free-body diagram for a book is at rest on a table top.**Free-body diagrams WS 5b #2**A girl is suspended motionless from a bar which hangs from the ceiling by two ropes. A free-body diagram for this situation looks like this:**Free-body diagrams WS 5b #3**An egg is free-falling from a nest in a tree. Neglect air resistance. A free-body diagram for this situation looks like this:**A rightward force is applied to a book in order to move it**across a desk at constant velocity. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:**Net Force**• If there is no movement • The net force is zero • If there is no acceleration • The net force is zero • If there is acceleration • The net force is not zero.**Homework**• WS 5b (FBD) • 3-10**Newton’s 1st Law Equilibrium Situation**• An object at rest will stay at rest unless a force acts on it. • An object in motion will continue in a straight line until acted on by some “outside” force.**Newton’s Second Law – Non-Equilibrium Situation**• The result of a Non-Equilibrium situation is that the body, will begin to move.**Newton’s Second Law – Non-Equilibrium Situation**• Two 50g masses are placed 40cm on either side of a fulcrum. What is the Net force? • If left mass slides 20cm right, what happens? • If the left mass slides 20cm right and grows to 100g, what type of situation is this? Equilibrium Non-Equilibrium**Newton’s 2nd law e.g.**• What net force is required to accelerate a 1500kg race car at 3.0m/s2?**4 x 4**Newton’s 2nd law e.g. A truck with a mass of 710kg starts from rest and travels 40m in 3.0s What Fnet acts on the car?**Newton’s 2nd law e.g.**A 873kg dragster, starting from rest, accelerates to a speed of 26.3m/s in .59s a) Find the acceleration b) Find the average force on the dragster**Newton’s 2nd law WS5d #1**• An artillery shell has a mass of 55kg. The shell leaves the 1.5m long barrel at a velocity of 770m/s. • What is the force the gun applies to the shell? • Step 1 Find the acceleration • Step 2 Find the Net Force**Newton’s 2rd Law WS5d #1**• What is the force the gun applies to the shell?**Newton’s 2rd Law WS 5c #5**• A car, mass 1225kg traveling at 105km/hr slows to a stop in 53m. • What is the size and direction of the force that acted on the car? • Step 1 Unit conversion km/hr to m/s • Step 2 Find the acceleration • Step3 Find the Force**Newton’s 2nd Law WS 5d #5**Given: • mass 1225kg • velocity 105km/hr • distance 53m.**Homework**• WS 5c • 1-5 • WS 5d • 1,2**Newton’s 3rd Law**• 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 equal and opposite re-action. • Action – Reaction force pair**Newton’s 3rd LawExample**• A book rests on a table • The force from the weight of the book pushes down. • The table provides a supportive force up. • Normal force (N) is perpendicular to the surface**Friction**• Friction is a force like any other force • Friction acts on materials that are in contact with each other • Friction slows down motion • Forces due to friction are always in the opposite direction of the motion.**Friction**• There are two types of friction: • Kinetic (Sliding) friction: The force that opposes the motion of a moving object • Static friction: The force that oppose the start of motion.**Friction**• The force of friction depends on • The force pushing down (weight) • The surface materials in contact with each other**Friction Example**• A horizontal force of 30N pushes a 12kg crate across a floor at a constant velocity. Find the coefficient of sliding friction.