The Laws of Motion

The Laws of Motion Chapter 2

2.1: Gravity and Friction • A force is a push or pull that causes a resting object to move, or it can accelerate a moving object by changing its speed or direction. • Unit = Newton (N) • 1 kg m/s2 • Combination of all forces acting on an object is called the net force.

2.1: Gravity and Friction • Contact forces • A contact force is a push or pull on one object by another object that is touching it • However, a contact force does not guarantee movement!

2.1: Gravity and Friction • Noncontact forces • Noncontact forces are forces that one object can apply to another withouttouching it • Sometimes we cannot see the forcesthat are in place • Gravity • Magnetism • Static electricity

2.1: Gravity and Friction • Balanced Force • When the forces on an object are balanced, the net force on an object is zero and there is no change in the object’s motion. • Unbalanced force • When the forces on an object are unbalanced, there is a net force and the object accelerates.

2.1: Gravity and Friction • Affect of force on moving object • Change in speed or direction • Ex. When soccer ball is passed to another player and is kicked • Affect on force on stationary object • Cause a nonmoving object to start moving. • Ex. Stationary soccer ball kicked

2.1: Gravity and Friction • Gravity is a force of attraction that acts between objects with mass. • Objects close to Earth accelerate at 9.8m/s2. • All objects fall at the same rate.

2.1: Gravity and Friction • If gravity affects all objects with mass – why aren’t all objects stuck together? • Masses of most objects are too small, you can’t detect this force • Earth has a huge mass, so the gravitational force of Earth is large. It pulls everything toward the center of the Earth. – Dropped objects fall to the floor.

2.1: Gravity and Friction • Law of Universal Gravitation • All objects in the universe attract each other through gravitational force. • The size of the force depends on the masses of the objects and the distance between the objects. • Greater the mass the greater the force. • Smaller the distance, the greater the force.

2.1: Gravity and Friction Mass vs. Weight • Mass is the amount of matter in an object. • Unit = kilogram • Measuring device is a balance. • Weight is a measure of gravitational force on an object. • Unit – Newton • Measuring device is a spring scale. • Weight = mass x gravity N = (kg) x (m/s2)

2.1: Gravity and Friction • Kyle’s mass is 40.0kg. Gravity on earth is 9.8m/s2. What is kyle’s weight on earth? • What is Bob’s weight on the moon is 320N. If gravity is 5.0 m/s2, what is his mass?

2.1: Gravity and Friction • Friction - A force that opposes motion between two surfaces that are in contact. • Friction causes moving objects to slow down and eventually stop. • What causes friction? • When the hills and valleys of one surface come in contact with the hills and valleys of the other surface

2.1: Gravity and Friction • Rough surfaces have more hills and valleys than smooth surfaces do. • The rougher the surface, the greater the friction. • Ex. A soccer ball rolling on grass, verses a hockey puck on ice. • The greater the weight of the object, the greater the friction will be

2.1: Gravity and Friction Types of Friction • Static Friction is the friction force that acts on a stationary object. It opposes the applied force. • Kinetic Friction is the force that opposes the direction of motion of an object as it slides over a surface. • Solid over solid

2.1: Gravity and Friction • Rolling Friction is the force of friction felt on rolling objects • Example: wheels and balls. • Fluid friction opposes the motion of a moving object in a fluid (gas or liquid) • Example: Air resistance

2.1: Gravity and Friction Is friction helpful or harmful? • Helpful • Movement of tires • Walking • Eraser • Harmful • Machine wear • Erosion • Burns/blisters

2.1: Gravity and Friction • Decreasing Friction • Lubricants- Substances that are applied to surfaces to reduce the friction between the surfaces. • Ex: motor oil, wax, grease • Lubricants are usually liquids, but can be solids or gases. (Air in air-hockey) https://www.youtube.com/watch?v=eg9foFo6vHk

2.1: Gravity and Friction • Switching from sliding friction to rolling friction. • EX. Ball bearings are placed between the wheels and axles of skates • Make surfaces that rub against each other smoother.

2.2: Newton’s First Law • Calculating New Force • Forces in the same direction • Add forces to determine net force

2.1: Newton’s First Law • Calculating Net Force • Forces in opposite directions • Subtract forces to determine net force

2.2: Newton’s First Law • Newton’s First Law of Motion • An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force. • What does this mean? • This means that there is a natural tendency of objects to keep on doing what they're doing. All objects resist changes in their state of motion. In the absence of an unbalanced force, an object in motion will maintain this state of motion.

2.2: Newton’s First Law • What is the motion in this picture? Forward motion – rolling • What is the unbalanced force in this picture? Rock • What happened to the skater in this picture? Fall off the skateboard • What other force do you know that can stop an object from moving? Friction, gravity

2.2: Newton’s First Law • Newton’s 1st Law is also the reason you wear a seatbelt! • Newton’s 1st Law is also known as inertia • Inertia is the tendency of an object to _resist_a _change_ in its _motion_. • Mass is a measure of inertia. • An object that has a small mass has less inertia than an object that has a large mass

2.2: Newton’s First Law • Think about it………….. • If Wile E. Coyote and a boulder fall off a cliff at the same time, which do you think will hit the ground first? Would it matter if the cliff were very high or particularly low? How could Mr. Coyote slow down his fall?

2.2: Newton’s First Law • Acceleration due to Gravity • Due to gravity, all objects accelerate toward earth at a rate of 9.8 m/s/s, or 9.8 m/s2 • Every second, velocity increases by 9.8 m/s

2.2: Newton’s First Law • Equation: • v = g x t • EX 1: A penny at rest is dropped from the top of a tall stairwell. What is the penny’s velocity after it has fallen for 2 s? • EX 2: The same penny hits the ground in 4.5s. What is the penny’s velocity as it hits the ground?

2.2: Newton’s First Law • EX 3: A marble at rest is dropped from a tall building. The marble hits the ground with a velocity of 98 m/s. How long was the marble in the air? • EX 4: A rock falls off of a cliff on Mercury. The rock takes 5 seconds to fall and has a velocity of 18.5 m/s when it lands. What is the gravity on Mercury?

2.2: Newton’s First Law • Air Resistance and Falling Objects • Demo: Which falls faster? • Crumpled paper or flat sheet of paper? • Why did this happen?

2.2: Newton’s First Law • Air Resistance - force that opposes the motion of objects through air

2.2: Newton’s First Law • Acceleration Stops at Terminal Velocity • As the speed of a falling object increases, air resistance increases. The upward force of air resistance continues to increase until it is equalto the downward force of gravity. The object then falls at a constant velocity called the terminal velocity. • Is terminal velocity a good thing or bad thing?

2.2: Newton’s First Law • Free Fall Occurs When There Is No Air Resistance • An object is in free fall only if gravity is pulling it down and no other forces are acting on it. • What are the 2 places that have no air and free is possible? Vacuum and space

2.2: Newton’s First Law • Orbiting Objects are in Free Fall • Two Motions Combine to Cause Orbiting • A space shuttle moves forward at a constant speed and it is in constant free fall towards the earth causing orbiting • The unbalanced force that causes objects to move in a circular path is centripetal force (ex. Moon orbiting the earth, earth around sun.)

2.2: Newton’s First Law Projectile Motion and Gravity Projectile Motion- the curved path that an object follows when thrown, launched, or otherwise projected near the surface of earth. Horizontal Motion - When you throw a ball, the forward force your hand exerts on the ball Vertical Motion- Pulled down towards the earth by gravity

2.3 Newton’s Second Law of Motion • Acceleration is produced when a force acts on a mass. The greater the mass of the object being accelerated the greater the amount of force needed to accelerate the object. • What does this mean? The heavier objects require more force to move the same distance as lighter objects

2.3: Newton’s Second Law • Unbalanced Forces and Velocity • Only unbalanced forces change an object’s velocity • For objects at rest • When unbalanced forces act on an object at rest, the object begins to move in the direction of the net force.

2.3: Newton’s Second Law • Unbalanced Forces and Velocity • Only unbalanced forces change an object’s velocity • For objects in motion • If net force is in the same direction, object will speed up • If net force is in the opposite direction, object will slow down • If net force is in a different direction, object will change direction • ALL EXAMPLES OF ACCELERATION

33: Newton’s Second Law

2.3 Newton’s Second Law of Motion • The 2nd Law gives us an exact relationship between force, mass, and acceleration. It can be expressed as a mathematical equation: • or FORCE = MASS times ACCELERATION • SI Unit for force = Newtons (N)

2.3Newton’s 2nd law of motion • Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s2. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

2.3Newton’s 2nd law of motion • What is the acceleration of a 3 Kg mass if a force of 14.4 N is used to move the mass? • 50,000N of force was applied to a car for it to move at a rate of 40m/s2. What was the mass of the car?

2.4 Newton’s third law • For every action there is an equal and opposite re-action. • What does this mean? This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard

2.4 Newton’s third law • Let's study how a rocket works to understand Newton's 3rd Law. • The rocket's actionis to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force. • Newton’s Cradle

2.4 Newton’s third law • Momentum - Depends on the object’s mass and velocity. • The more momentum an object has, the harder it is to stop the object or change directions. • What would have more momentum? • A bowling ball or a basketball rolled at the same velocity? • Bowling ball b/c it has more mass • A tractor trailer or a Honda civic at the same velocity? • Tractor trailer

2.4 Newton’s third law • Equation: Momentum = mass x velocity p = m x v (Kg m/s) (Kg) (m/s) • Example: • A 100 Kg car falls off a cliff from rest and hits the ground with a velocity of 35 m/s. What is the car’s momentum when it hits the ground? • A 35 Kg bowling ball was thrown at Tommy and had a momentum of 525 kg m/s. What was the bowling ball’s velocity

2.4 Newton’s third law What is the mass of a cat that is moving at 10 m/s down the alley toward the mouse with a momentum of 60 kg m/s? • An 85 Kg man is jogging with a velocity of 2.6 m/s to the north. Nearby, a 65 Kg person is skateboarding and is traveling with a velocity of 3 m/s north. Which person has a greater momentum?

2.4 Newton’s third law • Law of conservation of momentum • Any time objects collide, the total amount of momentum stays the same. This is true for all collisions if no other forces act on the colliding objects • Conservation of Momentum can be explained by Newton’s third law of motion. Action Force Reaction Force 5

The Laws of Motion