FORCES NEWTONS LAWS OF MOTION WHAT IS NEWTONS

FORCES NEWTON’S LAWS OF MOTION

WHAT IS NEWTON’S FIRST LAW OF MOTION? • If an object is not moving, it will not start moving until a force acts on it. • If an object is moving, it will continue at a constant velocity until a force acts to change its speed or its direction.

• Newton’s first law of motion states that an object at rest will remain at rest unless acted upon by a nonzero net force. • An object moving at a constant velocity will continue moving at a constant velocity unless acted upon by a nonzero net force.

INERTIA • All objects, moving or not, resist changes in motion. • Resistance to change in motion is called inertia. • Newton‘s first law of motion is also called the law of inertia. • Inertia explains many common events.

INERTIA DEPENDS ON MASS • Some objects have more inertia than others. • The greater the mass of an object, the greater its inertia, and the greater the force required to change its motion. • A full backpack is harder to move than an empty one because it has more mass and therefore more inertia.

• Around and Around Lab • Homework is pages 46 & 47

WHAT IS NEWTON’S SECOND LAW OF MOTION? • Which is harder to push, a full shopping cart or an empty one? • Who can cause a greater acceleration on a shopping cart, a small child or a grown adult?

CHANGES IN FORCE AND MASS • Suppose you increase the force on a cart without changing its mass. • The acceleration of the cart will also increase. • Your cart will also accelerate faster if something falls out. • This reduces the mass of the cart, and you keep pushing just as hard.

DETERMINING ACCELERATION • Newton’s second law of motion states that an object’s acceleration depends on its mass and on the net force acting on it. • This relationship can be written as follows. • Acceleration = net force / mass

• This formula can be rearranged to show much force must be applied to an object to get it to accelerate at a certain rate. • Net force = mass x acceleration

• Acceleration is measured in meters per second (m/s 2). • Mass is measured in kilograms (kg). • Newton’s second law shows that force is measured in kilograms times meters per second (kg x m/s 2). This unit is also called the newton (N), which is the SI unit of force. • One newton is the force required to give a 1 kg mass an acceleration of 1 m/s 2.

• Newton’s Second Law Lab • Homework is pages 48 -51

WHAT IS NEWTON’S THIRD LAW OF MOTION? • If you leaned against a wall and it didn’t push back on you, you’d fall through. • The force exerted by the wall is equal in strength and opposite in direction to the force you exert on the wall. • Newton’s third law of motion states that 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 state Newton’s third law is that for every action there is an equal but opposite reaction.

ACTION-REACTION PAIRS • Pairs of action and reaction forces are all around you. • When you walk, you push backward on the ground with your feet. • Think of this as an action force. (It doesn’t matter which force is called the “action” force and which is called the “reaction” force. )

• The ground pushes forward on your feet with an equal and opposite force. • This is the reaction force. • You can only walk because the ground pushes you forward!

DETECTING MOTION • If you drop your pen, gravity pulls the pen downward. • According to Newton’s third law, the pen pulls Earth upward with an equal and opposite reaction force. • You see the pen fall. • You don’t see Earth accelerate toward the pen.

• Remember Newton’s second law. • If mass increases and force stays the same, acceleration decreases. • The same force acts on both Earth and your pen. • Since Earth has such a large mass, its acceleration is so small that you don’t notice it.

DO ACTION-REACTION FORCES CANCEL? • You have learned that two equal forces acting in opposite directions on an object cancel each other out and produce no change in motion. • So why don’t the action and reaction forces in Newton’s third law of motion cancel out as well? • Action and reaction forces do not cancel out because they act on different objects.

• Interpreting Illustrations Lab • Homework is pages 52 -55