QotD Make a list of what types of forces we have on Earth?

4-1 Force A force is a push or pull. An object at rest needs a force to get it moving; a moving object needs a force to change its velocity.

4-2 Newton’s First Law of Motion Newton’s first law is often called the law of inertia. An object in motion stays in motion and an object at rest stays at rest, unless a force acts upon it.

4-3 Mass Mass is the measure of inertia of an object. In the SI system, mass is measured in kilograms. Mass is not weight: Mass is a property of an object. Weight is the force exerted on that object by gravity. If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.

4-4 Newton’s Second Law of Motion Newton’s second law is the relation between acceleration and force. Acceleration is proportional to force and inversely proportional to mass. (4-1)

4-4 Newton’s Second Law of Motion Force is a vector, so is true along each coordinate axis. The unit of force in the SI system is the newton (N).

Your turn: What average force is required to stop an 1100-kg car in 8.0 s if the car is traveling at 95 km/hr?

ConcepTest 4.1aNewton’s First Law I ConcepTest 4.1a Newton’s First Law I 1) there is a net force but the book has too much inertia 2) there are no forces acting on it at all 3) it does move, but too slowly to be seen 4) there is no net force on the book 5) there is a net force, but the book is too heavy to move A book is lying at rest on a table. The book will remain there at rest because:

There are forces acting on the book leaving no net force There are forces acting on the book, but the only forces acting are in the y-direction. Gravity acts downward, but the table exerts an upward force that is equally strong, so the two forces cancel, leaving no net force. ConcepTest 4.1aNewton’s First Law I ConcepTest 4.1a Newton’s First Law I 1) there is a net force but the book has too much inertia 2) there are no forces acting on it at all 3) it does move, but too slowly to be seen 4) there is no net force on the book 5) there is a net force, but the book is too heavy to move A book is lying at rest on a table. The book will remain there at rest because:

A hockey puck slides on ice at constant velocity. What is the net force acting on the puck? ConcepTest 4.1bNewton’s First Law II ConcepTest 4.1b Newton’s First Law II 1) more than its weight 2) equal to its weight 3) less than its weight but more than zero 4) depends on the speed of the puck 5) zero A hockey puck slides on ice at constant velocity. What is the net force acting on the puck?

constant velocity not accelerating no net force The puck is moving at a constant velocity, and therefore it is not accelerating. Thus, there must be no net force acting on the puck. A hockey puck slides on ice at constant velocity. What is the net force acting on the puck? ConcepTest 4.1bNewton’s First Law II ConcepTest 4.1b Newton’s First Law II 1) more than its weight 2) equal to its weight 3) less than its weight but more than zero 4) depends on the speed of the puck 5) zero Follow-up: Are there any forces acting on the puck? What are they? A hockey puck slides on ice at constant velocity. What is the net force acting on the puck?

1) a net force acted on it 2) no net force acted on it 3) it remained at rest 4) it did not move, but only seemed to 5) gravity briefly stopped acting on it ConcepTest 4.1cNewton’s First Law III ConcepTest 4.1c Newton’s First Law III You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why?

1) a net force acted on it 2) no net force acted on it 3) it remained at rest 4) it did not move, but only seemed to 5) gravity briefly stopped acting on it continued moving forwardinitial state of motion The book was initially moving forward (since it was on a moving bus). When the bus stopped, the book continued moving forward, which was its initial state of motion, and therefore it slid forward off the seat. ConcepTest 4.1cNewton’s First Law III ConcepTest 4.1c Newton’s First Law III You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why? Follow-up: What is the force that usually keeps the book on the seat?

ConcepTest 4.1dNewton’s First Law IV ConcepTest 4.1d Newton’s First Law IV 1) the force pushing the stone forward finally stopped pushing on it 2) no net force acted on the stone 3) a net force acted on it all along 4) the stone simply “ran out of steam” 5) the stone has a natural tendency to be at rest You kick a smooth flat stone out on a frozen pond. The stone slides, slows down and eventually stops. You conclude that:

some forceto slow it down and stop it After the stone was kicked, no force was pushing it along! However, there must have been some force acting on the stone to slow it down and stop it. This would be friction!! ConcepTest 4.1dNewton’s First Law IV ConcepTest 4.1d Newton’s First Law IV 1) the force pushing the stone forward finally stopped pushing on it 2) no net force acted on the stone 3) a net force acted on it all along 4) the stone simply “ran out of steam” 5) the stone has a natural tendency to be at rest You kick a smooth flat stone out on a frozen pond. The stone slides, slows down and eventually stops. You conclude that: Follow-up: What would you have to do to keep the stone moving?

4-5 Newton’s Third Law of Motion Newton’s third law: Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. (Every action has an equal and opposite reaction)

4-5 Newton’s Third Law of Motion Helpful notation: the first subscript is the object that the force is being exerted on; the second is the source. This need not be done indefinitely, but is a good idea until you get used to dealing with these forces. (4-2)

4-6 Weight – the Force of Gravity; and the Normal Force Weight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight is:

4-6 Weight – the Force of Gravity; and the Normal Force An object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there? The force exerted perpendicular to a surface is called the normal force.

Your turn! A 20.0-kg box rests on a table. (a) What is the weight of the box and the normal force acting on it? (b) A 10.0-kg box is placed on top of the kg box. Determine the normal force that the table exerts on the 20.0-kg box and the normal force that the 20.0-kg box exerts on the kg box.

4-7 Solving Problems with Newton’s Laws – Free-Body Diagrams When a cord or rope pulls on an object, it is said to be under tension, and the force it exerts is called a tension force.

QotD: Without looking at your notes, what are Newton’s 3 Laws of Motion?

4-8 Applications Involving Friction, Inclines Static friction is the frictional force between two surfaces that are not moving along each other. Static friction keeps objects on inclines from sliding, and keeps objects from moving when a force is first applied.

4-8 Applications Involving Friction, Inclines For kinetic – sliding – friction, we write: is the coefficient of kinetic friction, and is different for every pair of surfaces.

4-8 Applications Involving Friction, Inclines

The static frictional force increases as the applied force increases, until it reaches its maximum. Then the object starts to move, and the kinetic frictional force takes over. 4-8 Applications Involving Friction, Inclines

An object sliding down an incline has three forces acting on it: the normal force, gravity, and the frictional force. The normal force is always perpendicular to the surface. The friction force is parallel to it. The gravitational force points down. If the object is at rest, the forces are the same except that we use the static frictional force, and the sum of the forces is zero.

Your turn! A box slides down a 25° ramp with an acceleration of 1.08 m/s 2. Determine the coefficient of kinetic friction between the box and the ramp.

QotD A 2-kilogram block slides down a 30  incline as shown below with an acceleration of 2 meters per second squared. Draw the FBD and find the frictional force along the plane.

Coefficient of Friction Lab Question: Which material will have the highest coefficient of friction? Hypothesis: Procedure: 1.Find the mass of the block 2.Set the block on the ramp 3.Slowly raise the ramp until the block slides 4.Measure the angle of the ramp 5.Repeat for other side Data Table: angle (wood) angle (carpet) Analysis: 1.Using sum of forces, calculate F N and F g 2.Calculate the coefficient of static friction