Chapter 12 Forces and Motion

Force: Can cause a resting object to move, or it can accelerate a moving object by changing it’s speed or direction. Force is measured in Newtons (N). 1N= 1kg x m/s²

Net Force The overall force acting on an object after all the forces are combined.

Forces: Balanced Unbalanced When the net force = 0 there is no change in motion of an object. An object will accelerate when the net force is not zero.

Friction: Static Friction Sliding Friction Friction force that is acting on an object that is not moving. ALWAYS acts in the direction opposite to the applied force. Friction force that opposes the direction of motion of an object as it slides over a surface. Sliding friction is less than static friction, therefore less force is needed to keep an object moving once it has started.

Which picture shows sliding friction? Static Friction?

Friction: Rolling Friction Fluid Friction Occurs when both the shape of an object and the shape of the surface that the object is on is slightly bent out of shape. Force is about 100 to 1000 times less than static or sliding friction. Force that opposes the motion of an object through a fluid. Fluid friction acting on an object moving through air is known as Air Resistance.

Which picture shows rolling friction? Fluid Friction?

Gravity: Earth’s gravity acts downward toward the center of the Earth. Acceleration of Gravity is 9.8 m/s²

Falling Objects Gravity is the force that causes objects to accelerate down toward the Earth. Air Resistance is the friction force that reduces the acceleration of a falling object by acting in the opposite direction of the falling object. Terminal Velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

Projectile Motion The combination of an initial forward velocity and the downward vertical force of gravity that causes an object to follow a curved path.

Newton’s Three Laws of Motion

Newton’s Laws of Motion An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force. Force equals mass times acceleration (F = ma). For every action there is an equal and opposite reaction.

Newton’s 1st Law of Motion Motion of an object does not change as long as the net force acting on the object is zero. Also known in the law of inertia. Inertia is the tendency of an object to resist a change in its motion.

What does this mean? Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force. If the object was sitting still, it will remain sitting still. If it was moving at a constant velocity, it will keep moving in a constant velocity. It takes force to change the motion of an object.

Newton’s 2nd Law of Motion The acceleration of an object is equal to the net force acting on it divided by the objects mass. Force equals mass times acceleration. F=ma. Acceleration is directly proportional to the net force acting on it. Acceleration = Net Force/Mass

Newton’s 2nd Law of Motion If you double the mass, you double the force. If you double the acceleration, you double the force. What if you double the mass and the acceleration? (2m)(2a) = 4F Doubling the mass and the acceleration quadruples the force. So . . . what if you decrease the mass by half? How much force would the object have now?

Newton’s 2nd Law of Motion Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force. Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.

Mass vs. Weight Mass Weight Measure of the inertia of an object. Depends on the amount of the matter the object contains. Force of gravity acting on an object.

Newton’s 3rd 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. In other words, for every action there is an equal and opposite reaction.

Newton’s 3rd Law of Motion Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

Action and Reaction Forces Hammer hitting a nail. Pushing against the wall. Oars in the water as a canoe moves in the water. Nail hitting the hammer The wall pushing back. Water pushing against the oar.

Momentum The product of an object’s mass and it’s velocity. Momentum = Mass X Velocity

Conservation of Momentum According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change. In a closed system, the loss of momentum of one object equals the gain of momentum of another object- the momentum is conserved.

Universal Forces Electric and magnetic force are two different aspects of the electromagnetic force. Electric and Magnetic Forces are the only forces that can attract and repel.

Electromagnetic Forces Electric Forces Magnetic Forces Act between charged objects or particles. Such as in Electrons and Protons. Objects with different charges attract each other, where as objects with like charges repel each other. Act on certain metals, on the poles of magnets, and on moving charges. Magnets have two poles, North and South Poles. Two poles of opposite poles attract to each other, while the same poles repel each other.

Nuclear Forces Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together. Explains how protons are held together in the nucleus of an atom.

Nuclear Forces Strong Nuclear Force Weak Nuclear Force Powerful force that only acts on the protons and neutrons in the nucleus. Holds the protons and neutrons together. Attractive force that acts only over a short range.

Gravitational Forces The attractive force that acts between any two masses. Newton’s law of universal gravitation states that every object in the universe attracts every other object.

Gravitational Forces Earth Moon Gravitational Force of the Earth keeps the moon in orbit. Works much like a string tied around an eraser. Centripetal Force is the center-directed force that continuously changes the direction of an object to make it move in a circle. The moon’s gravitational force acts on the Earth as well. High Tide and Low tide of the ocean.