Force, Friction & Gravity Notes

Transcription

1 Force, Friction & Gravity Notes

2 Key Terms to Know Speed: The distance traveled by an object within a certain amount of time. Speed = distance/time Velocity: Speed in a given direction Acceleration: The rate at which velocity changes In other words, acceleration refers to increasing speed, decreasing speed, or changing direction

3 What is a force? A force is a push or a pull, which causes the energy of motion. When an object is at rest, its potential energy is at its greatest. When an object is in motion, potential energy is changing into kinetic energy. Energy changes from potential to kinetic when a force is applied. Force is described by its strength and by the direction in which it acts. Force strength is measured in newtons. Direction & strength of force is represented by an arrow, which points in the direction of the force. The length of the arrow tells you the strength of the force the longer the arrow the greater the force.

4 Combining Forces Often more than a single force acts on an object at one time. The combination of all forces acting on an object is called the net force. The net force determines whether an object moves and in which direction it moves.

5 Unbalanced Forces Unbalanced forces acting on an object result in a net force and cause a change in the object s motion. In other words an unbalanced force has a net force greater than 0. Unbalanced forces can occur in the same direction or opposite directions.

6 Balanced Forces Equal forces acting on one object in opposite directions are called balanced forces. Balanced forces acting on an object do not change an object s motion. Balanced forces have a net force of 0.

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8 Friction The force that two surfaces exert on each other when they rub against each other. The strength of the force of friction depends on two factors: 1. how hard the surfaces push together 2. the types of surfaces involved Example: Smooth surfaces produce less friction than rough surfaces. Surfaces tightly pressed together produce more friction than surfaces loosely pressed together.

9 Types of Friction Static: Acts on objects that are not moving. Sliding: Occurs when two solid surfaces slide over each other. Rolling: Occurs when an object rolls across a surface (easier to overcome than sliding friction). Fluid: Occurs when a solid object moves through a fluid. (easier to overcome than sliding friction).

10 Gravity The force that pulls object towards each other. Law of universal gravity states that gravity acts between all objects in the universe, meaning any two objects in the universe attract each other. Two factors affect gravity: mass & distance Remember mass is the measure of the amount of matter in an object.

11 Free Fall When the only force acting on an object is gravity, the object is said to be in free fall. In free fall, the force of gravity is an unbalanced force, which causes an object to accelerate. All objects in free fall accelerate at the same rate regardless of their masses.

12 Air Resistance Air resistance is a type of fluid friction. It is an upward force exerted on falling objects. Falling objects with greater surface area experience more air resistance. Air resistance increases with velocity. Something called terminal velocity is reached when the force of air resistance equals the weight of the object. Terminal velocity is the greatest velocity that a falling object reaches.

13 Force & Motion State of motion is an important concept. These are the four states we will consider: Constant: 1. Motionless 2. Moving at constant speed in a straight line Changing: 1. Increasing or decreasing speed 2. Changing direction at a constant speed

14 Newton s 1st Law of Motion An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless acted upon by an unbalanced force. Whether an object is moving or not, it resists any change to its motion. This tendency of an object to resist a change in motion is called inertia. Inertia depends on mass. The greater and object s mass, the greater its inertia, and the greater the force required to change its motion.

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16 Newton s Second Law of Motion An object acted upon by a constant unbalanced force, moves with constant acceleration in the direction of the unbalanced force. Acceleration depends on the object s mass and on the net force acting on the object. Acceleration = net force/mass (A = F/M) Force = mass x acceleration (F= MA)

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18 Newton s Third Law of Motion If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. Another way to think about this law is that for every action there is an equal and opposite reaction.

19 Action-Reaction Pairs Example 1: When you jump, you push on the ground with your feet. This is an action force. The ground pushes back on your feet with an equal and opposite force. This is the reaction force. You move upward when you jump because the ground is pushing you. Example 2: A kayaker moves forward by exerting an action action force on the water with a paddle. The water pushes back on the paddle with an equal reaction force that propels the kayak forward.

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21 Detecting Motion Can you always detect motion when paired forces are in action? No. For example, when Earth s gravity pulls on an object you cannot detect Earth s equal and opposite reaction because Earth s inertia is so great that its acceleration toward the pencil is too small to notice.

22 Do Action-Reaction Forces Cancel? Why don t the action and reaction forces in Newton s third law of motion cancel out? After all they are equal and opposite. The action and reaction forces do not cancel out because they are acting on different objects.

23 Momentum All moving objects have what Newton called a quantity of motion which today is known as momentum. Momentum = Mass x Velocity Momentum is described by both its quantity and direction. The more momentum a moving object has, the harder it is to stop.

24 Conservation of Momentum The law of conservation of momentum states that, in the absence of outside forces, the total momentum of objects that interact does not change. In other words the amount of momentum is the same before and after they interact. The total momentum of any group of objects remains the same, or is conserved unless outside forces act on the object. Outside forces do include gravity and friction.

25 Collisions with Two Moving Objects Before the collision, the blue car moves faster than the green car. Afterward, the green car moves faster. The total momentum stays the same. Momentum is conserved the momentum of one train car decreases while the momentum of the other increases.

26 Collisions with One Moving Object When the green car is at rest before the collision, all of the blue car s momentum is transferred to it. Momentum is conserved. Momentum is conserved all of the momentum has been transferred from the blue car to the green car.

27 Collisions with Connected Objects If the two cars couple together, momentum is still conserved. Together, the cars move slower than the blue car did before the collision. Momentum is conserved since the mass is doubled and the velocity is divided in half, the total momentum remains the same.