Forces & Motion

“Trust the Force Luke” Forces

Forces

The Cause of Forces A force is a push or pull, or an action that has the ability to change motion. Forces can increase or decrease the speed of a moving object. Forces can also change the direction in which an object is moving.

How do we label force??? F = ma Mass is measured in kg Acceleration is measured in m/sec 2 If we multiply these two units : 1 kg m/sec 2 = 1 newton (N)

Unit of Force Force is measured in newtons (N). 1 newton is the force needed to cause a 1 kg mass to accelerate at a rate of 1 m/s 2. 1 N = 1 kg m/s 2.

5.1 How are forces created? Forces are created in many ways. For example, your muscles create force when you swing a baseball bat.

Four Elemental Forces All forces in the universe come from only four basic forces. Strong nuclear force Electromagnetic force Weak force Gravity Electromagnetic forces are important to technology. Gravity is a universal force.

Newton’s Newton’s Law of Universal Gravitation Every object in the universe attracts every other object. Depends on two factors: Distance between the objects Mass of the objects

5.1 Units of force The pound is a unit of force commonly used in the United States. For smaller amounts, pounds are divided into ounces (oz.). There are 16 ounces in 1 pound.

5.1 Newtons Although we use pounds all the time in our everyday life, scientists prefer to measure forces in newtons. The newton (N) is a metric unit of force.

5.1 Unit conversions The newton (N) is a smaller unit of force than the pound (lb). If one pound of force equals newtons, then a 100 lb person weighs newtons.

5.1 Drawing a force vector The arrow points in the direction of the force.

Representing forces We often use arrows. Force is a vector. You can combine force vectors by adding them. Larger the arrow, the greater the force

Representing forces Combined balanced forces equal zero Normal force = the force acting perpendicular on an object in contact with another object.

5.1 The force vector The direction of a force makes a big difference in what the force does. That means force is a vector, like velocity or position. Arrows are often used to show the direction of forces in diagrams.

5.1 How forces act One way forces act is the result of direct contact. Matter directly touching other matter, such as wind acting to slow a parachute, transmits a contact force.

5.1 How forces act The force of gravity between Earth and Moon appears to be what people once called “action-at-a- distance”. Today, we know that the gravitational force is carried from the Earth to the Moon by a force field.

5.1 Contact forces from ropes and springs Ropes and springs are often used to make and apply forces. Ropes are used to transfer forces or change their direction. The pulling force carried by a rope is called tension. Tension always acts along the direction of the rope.

The winning person pushes harder against the ground than the losing person.

5.1 Spring forces The force created by a spring is proportional to the ratio of the extended or compressed length divided by the original (resting) length. If you stretch a spring twice as much, it makes a force that is twice as strong.

5.1 Gravity The force of gravity on an object is called weight. At Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass.

5.1 Weight vs. mass Weight and mass are not the same. Mass is a fundamental property of matter measured in kilograms (kg). Weight is a force measured in newtons (N). Weight depends on mass and gravity.

Weight depends on mass and gravity A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

5.1 Calculating weight

Solving Problems Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars. 1.Looking for: …weight of person in newtons on both planets Given: …mass = 60 kg; g = 3.7 N/kg on Mars; …implied g = 9.8 N/kg on Earth 2.Relationships: W = m x g 3.Solution: 60 kg x 9.8 N/kg = 588 N 60 kg x 3.7 N/kg = 222 N Sig. fig. = 600 N Sig. fig. = 200 N

Chapter 5.3 Learning Goals Determine the net force acting on an object. Define equilibrium. Draw free-body diagrams to represent all forces acting on a body.

5.3 Forces and Equilibrium The sum of all the forces on an object is called the net force. The word net means total but also means the direction of the forces has been taken into account. In what direction will this plane go?

5.3 Adding forces To figure out if or how an object will move, we look at ALL of the forces acting on it. Four forces act on a plane: 1.weight 2.drag (air friction) 3.the thrust of the engines, and 4.the lift force caused by the flow of air over the wings.

5.3 Equilibrium When several forces act on the same object: 1.The net force is zero, or 2.The net force is NOT zero. + +

5.3 Normal forces When the forces are balanced, the net force is zero. When the net force on an object is zero, we say the object is in equilibrium.

5.3 Equilibrium and normal forces A normal force is created whenever an object is in contact with a surface. The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight. The normal force is sometimes called the support force.

5.3 The free body diagram How do you keep track of many forces with different directions? Draw a free-body diagram that contains the objects, like a book on a table.

5.3 Solving equilibrium problems For an object to be in equilibrium, all the forces acting on the object must add up to zero. Is this object in equilibrium?

Solving Problems Two chains are used to support a small boat weighing 1,200 newtons. One chain has a tension of 600 newtons. What is the force exerted by the other chain?

Solving Problems 1.Looking for: …tension on chain 2 2.Given …weight boat = 1,200N; tension 1 = 600 N Implied: weight and tension are forces 3.Relationships: Net force on boat = zero

Solving Problems 4.Solution: Draw free body diagram  Upward force of chains = weight of boat  600 N + tension 2 = 1,200 N  tension 2 = 900 N

Balanced Forces Forces opposite in direction and equal in size. Net result: no change in motion.

Equilibrium Means "Zero Acceleration" Forces in balance: Equilibrium

No change in motion! Balanced forces will keep an object moving at a constant velocity Zero acceleration does not mean zero velocity ENGINE FRICTION

Unbalanced forces = Net force Unbalanced forces = acceleration

Assign Concepts 3-7, 11, Problems 1, 9, 10, 14, 16, 17, 18 Applying What you Know 2 and 6