Chapter 12 Forces and Motion

Chapter 12 Forces and Motion Section 12.1 Forces Section 12.2 Newton’s First and Second Laws of Motion Section 12.3 Newton’s Third Law of Motion and Momentum

Section 12.1 Forces • What is a Force? • Def.-a push or a pull that acts on an object • Key Concept: A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction • Ex. Figure 1 pg. 356; man walking in gusty wind- wind (force) can change his speed or direction

Section 12.1 Forces • Measuring Force • Most of the time force is easy to measure. • Figure 2; fruit acts as a force (weight) on the spring in a scale • Units of Force • Force is measured in Newtons (N) • 1 Newton-the force that causes a 1 kg mass to accelerate at a rate of 1 m/s2 or is in other words 1kg*m/s2 • Newton: after Sir Isaac Newton (scientist) who explained how force, mass, and acceleration are related

Section 12.1 Forces • Representing Force • Use an arrow to represent the direction and strength of a force. • Direction of arrow=direction of force; the length of arrow=the strength/magnitude of the force

Section 12.1 Forces • Combining Forces • You can combine force arrows to show the result of how forces combine. • Forces in the same direction add together; forces in opposite directions subtract from one another. • Net force-the overall force acting on an object after all the forces are combined

Section 12.1 Forces • Balanced Forces • Key Concept: When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion. • Ex. Tug of war and arm wrestling • An unlimited number of individual forces can act on an object and still produce a net force of zero.

Section 12.1 Forces • Unbalanced Forces • An unbalanced force is a force that results when the net force acting on an object is not equal to 0. • Key Concept: When an unbalanced force acts on an object, the object accelerates. • Forces acting in opposite directions can also combine to produce an unbalanced force (winners of tug of war). • The net force equals the size of the larger force minus the size of the smaller force.

Figure 4 Combining Forces Acting on an Object

Section 12.1 Forces • Friction • **All moving objects are subject to friction. • Def.-a force that opposes the motion of objects that touch as they move past each other • **Without friction, surfaces would be more slippery than ice. • Friction acts at the surface where objects are in contact (that includes solid objects that are directly touching each other and also objects moving through a liquid or a gas)

Section 12.1 Forces • Friction • Key Concept: There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction. • Static Friction • Def.-the friction that acts on objects that are moving • **It always acts in the direction opposite to that of the applied force (Taking steps, push off a step, between your shoes and the ground)

Section 12.1 Forces • Sliding Friction • Def.-A force that opposes the direction of motion of an object as it slides over a surface • **Sliding friction is less than static friction, so less force is needed to keep an object moving

Section 12.1 Forces • Rolling Friction • ** When a round object rolls across a flat floor, both the object and the floor are bent slightly out of shape. • Def.-the friction force that acts on rolling objects ( is about 100 to 1000 times less than the force of static or sliding friction) • Ex. Ball bearings (Figure 6 pg. 360)-used to reduce friction (rolling friction replaces sliding friction) inline skates, automobiles, bicycles, skateboards

Section 12.1 Forces • Fluid Friction • Water and a mixture of gases such as air are known as fluids. • Def.-the force that opposes the motion of an object through a fluid • Increases as the speed of an object moving through the fluid increases • Air resistance-fluid friction acting on an object moving through the air • At higher speeds, air resistance can be a significant force.

Section 12.1 Forces • Gravity • Def.-a force that acts between any two masses • Is an attractive force (pulls objects together) • **Gravity holds us to the ground.** • Does not require objects to be in contact for it to act on them • Key Concept: Earth’s gravity acts downward toward the center of Earth. • Upward force usually balances the downward force of gravity. (Book on desk; boulder on cliff)

Section 12.1 Forces • Falling Objects • Both gravity and air resistance affect the motion of a falling object. • Key Concept: Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration. **Fig. 8 flying squirrel • Large surface area maximizes the force of air resistance

Section 12.1 Forces • Falling Objects • Falling objects accelerate and gain speed=increase in air resistance • If an object falls for a long time, the upward force of air resistance becomes equal to the downward force of gravity. • Forces acting on the object are balanced, acceleration is 0, and the object continues to fall at a constant velocity. • Terminal velocity-the constant velocity of a falling object when the force of air resistance equals the force of gravity (2 objects w/ same mass fall at same rate)

Section 12.1 Forces • Projectile Motion • Ex. A ball that is thrown, follows a curved path • Def.-the motion of a falling object (projectile) after it is given an initial forward velocity • Air resistance and gravity are the only forces acting on a projectile. • Key Concept: The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

Section 12.2 Newton’s First and Second Laws of Motion • Aristotle, Galileo, and Newton • **It took about 2000 years to develop the understanding of the relationships between force and motion. • Aristotle • Incorrectly proposed that force is required to keep an object moving at constant speed • Error held back progress in the study of motion for almost 2000 years

Section 12.2 Newton’s First and Second Laws of Motion • Galileo • Studied how gravity produces constant acceleration • Concluded that moving objects not subjected to friction or any other force would continue to move indefinitely

Section 12.2 Newton’s First and Second Laws of Motion • Newton • Built on the work of other scientists (like Galileo) • Published his results many years later in a book entitled Principia (first had to define mass and force) • Then introduced his laws of motion

Section 12.2 Newton’s First and Second Laws of Motion • Newton’s First Law of Motion • **Newton summarized his study of force and motion in several laws of motion. • Key Concept: According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.

Section 12.2 Newton’s First and Second Laws of Motion • Newton’s First Law of Motion • Unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction. • Ex. Ball (at rest is kicked; slows down from friction between the ball and the ground) • First law aka the law of inertia. • Def.-the tendency of an object to resist a change in its motion

Section 12.2 Newton’s First and Second Laws of Motion • Newton’s Second Law of Motion • **How do unbalanced forces affect the motion of an object? • An unbalanced force causes an object’s velocity to change (the object accelerates). • The more force used, the more acceleration there is. • Newton: the acceleration of an object depends on its mass

Section 12.2 Newton’s First and Second Laws of Motion • Newton’s Second Law of Motion • Mass-a measure of the inertia of an object and depends on the amount of matter the object contains • Key Concept: According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object’s mass. • Ie. Doubling an object’s mass will cut its acceleration by half.

Section 12.2 Newton’s Second Law

Section 12.2 Newton’s First and Second Laws of Motion • Newton’s Second Law of Motion • The acceleration of an object is in the same direction as the net force. • Newton’s 2nd law also applies when a net force acts in the direction opposite to the object’s motion. (The force causes a deceleration that reduces the speed) • Ex. Seat belt, volleyball

Figure 13 Effects of a Force on Acceleration

Section 12.2 Newton’s First and Second Laws • Weight and Mass • **Mass and weight are not the same, but are related. • Weight-the force of gravity acting on an object • An object’s weight is the product of the objects mass and acceleration due to gravity acting on it. • Weight=Mass x Acceleration due to gravity, or W=mg; g=9.8 m/s2 • (F)orce or (W)eight is expressed in Newtons; Acceleration due to grativty (a or g) is expressed in m/s2 • Mass is expressed in kilograms.

Section 12.2 Newton’s First and Second Laws of Motion • Weight and Mass • Mass and weight are proportional; doubling mass, doubles the object’s weight • Key Concept: Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.

Section 12.3 Newton’s Third Law of Motion and Momentum • Newton’s Third Law • **A force can’t exist alone. Forces always exist in pairs. • Key Concept: According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. • The two forces are called: action and reaction.

Section 12.3 Newton’s Third Law of Motion and Momentum • Action and Reaction Forces • Action force-the force exerted by the first object • Reaction force- the force exerted by the second object • Both forces are equal in size and opposite in direction • Ex. Pushing on a wall

Section 12.3 Newton’s Third Law of Motion and Momentum • Action-Reaction Forces and Motion • Not all action and reaction forces produce motion (pushing a wall). • Action-Reaction Forces Do Not Cancel • **Net force is not zero with action reaction forces. • b/c action and reaction forces do not act on the same object (swimmer in water) • Only when equal and opposite forces act on the same object do they result in a net force of 0.

Section 12.3 Newton’s Third Law of Motion and Momentum • Momentum • Def.-the product of an objects mass and its velocity • **An object with more momentum is hard to stop. • Key Concept: An object has a large momentum if the product of its mass and velocity is large. • Momentum for any object at rest is 0. • Momentum= Mass x Velocity (kg * m/s)

Section 12.3 Newton’s Third Law of Motion and Momentum • Conservation of Momentum • What happens when objects collide? • Under certain conditions, collisions obey the law of conservation of momentum. • ***Conservation of momentum means that momentum does not increase or decrease. • If a system is closed the momentum is conserved • Closed system-other objects and forces cannot enter or leave a system

Section 12.3 Newton’s Third Law of Motion and Momentum • Conservation of Momentum • Objects within the system can exert forces on one another. • Law of conservation of momentum-law stating that the total momentum of a system does not change if no net force acts on the system • Key Concept: In a closed system, the loss of momentum of one object equals the gain in momentum of another object—momentum is conserved.

Figure 17A and 17B Conservation of Momentum

Figure 17C Conservation of Momentum

Chapter 12 Forces and Motion