Chapter 12 Forces & Motion

Forces Section 12-1

Force A push or pull that acts on an object A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. Measured in newtons (N) One newton is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second.

Combining Forces Net force – the overall force acting on an object after all the forces are combined. When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion When an unbalanced force acts on an object, the object accelerates.

Friction A force that opposes the motion of objects that touch as they move past each other. There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.

Static Friction The friction force that acts on objects that are not moving. Always acts in the direction opposite to that of the applied force.

Sliding Friction A force that opposes the direction of motion of an object as it slides over a surface. This force is less than static friction. It takes less force to keep an object moving than to start it moving.

Rolling Friction The friction force that acts on rolling objects. Rolling friction is 100 to 1000 times less than the force of static or sliding friction. That’s why we use dollies to move heavy things.

Fluid Friction Opposes the motion of an object through a fluid. Fluid friction acting on an object moving through air is known as air resistance.

Gravity A force that acts between any two masses It is an attractive force – it pulls objects together Earth’s gravity acts downward toward the center of Earth.

Falling Objects Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration Terminal velocity – the constant velocity of a falling object when the force of air resistance equals the force of gravity.

Projectile Motion The motion of a falling object (projectile) after it is given an initial forward velocity. This is a curved path The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

Newton’s First and Second Laws of Motion Section 12-2

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. Sometimes called the law of inertia. Inertia – the tendency of an object to resist a change in its motion. An object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed.

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. Mass – a measure of the inertia of an object and depends on the amount of matter the object contains Net Force Mass Acceleration =

Weight and Mass Weight is the force of gravity acting on an object. Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object Weight = Mass x acceleration due to gravity

Newton’s Third Law Section 12-3

Newton’s Third Law According to Newton’s third law of motion, whenever an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. Song

Momentum Momentum – the product of an object’s mass and its velcoity An object has a large momentum if the product of its mass and velocity is large. Momentum = Mass x Velocity

Conservation of Momentum 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 in momentum of another object – momentum is conserved.

Universal Forces Section 12-4

Electromagnetic Forces Electromagnetic force is associated with charged particles. Electric force and magnetic force are the only forces that can both attract and repel.

Nuclear Forces Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together. Strong nuclear force is a powerful force of attraction that acts only on the neutrons and protons in the nucleus, holding them together. Weak nuclear force acts only over a short range (shorter than the range of the strong nuclear force).

Gravitational Force An 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.

The Earth, Moon, and Tides Centripetal force – a center-directed force that continuously changes the direction of an object to make it move in a circle. Centripetal force causes the moon to orbit the earth. The gravitational pull from the moon creates two bulges in Earth’s oceans. This causes two high tides and two low tides on the Earth each day.

Satellites Satellites are kept in orbit by centripetal force when they are in a high orbit. Satellites in low orbits are slowed by friction in the Earth’s atmosphere. They eventually reenter the Earth’s atmosphere and burn up.