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2. Newton's lawPhysicsGrade 11

The knowledge strand is the same as the footer in the textbook. For gr11 and 12 we can omit this. ForceA force is anything that can cause a change to objects. Forces can: change the shape of an object

accelerate or stop an object

change the direction of a moving object.

The unit of force is the Newton (N)Forces can be classified as either contact forces or a non-contact forces.

A contact force must be in touch or be in contact with an object to cause a change. Examples are: push and pull forces and the force of the wind to turn a windmill.

A non-contact force does not have to touch an object to cause a change. Examples are gravity, electricity and magnetism.

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Different types of forces in physics

The normal force, , is the force exerted by a surface on an object in contact with it.

The normal force is always perpendicular (at a right angle) to the surface.

Frictional force is the force that opposes the motion of an object in contact with a surface and it acts parallel to the surface the object is in contact with.

Frictional forces always act parallel to surfaces.

Tension is the magnitude of the force that exists in objects like ropes, chains and struts that are providing support.

More about frictional forcesThe magnitude of the frictional force depends on the surface and the magnitude of the normal force. Different surfaces will give rise to different frictional forces, even if the normal force is the same. Frictional forces are proportional to the magnitude of the normal force.

Every surface has a constant factor, the coefficient of friction. Since static and kinetic friction have different magnitudes we have different coefficients for the two types of friction: for static friction and for kinetic friction.

Static friction varies up to a maximum value while kinetic friction stays constant. We use the following two equations to calculate frictional forces:

Force diagramsForce diagrams are sketches of the physical situation you are dealing with, with arrows for all the forces acting drawn on the system. When drawing force diagrams remember the following: Make your drawing large and clear.

You must use arrows and the direction of the arrow will show the direction of the force.

The length of the arrow will indicate the size of the force. Arrows of the same length indicate forces of equal size.

Draw neat lines using a ruler. The arrows must touch the system or object.

All arrows must have labels. If necessary create a key on the side to show the forces.

The labels must indicate what is applying the force, on what the force is applied and in which direction

If the values of the forces are known, these values can be added to the diagram or key.

Free body diagramsIn a free-body diagram, the object of interest is drawn as a dot and all the forces acting on it are drawn as arrows pointing away from the dot.

Resolving forces into componentsWe have looked at resolving forces into components. There is one situation we will consider where this is particularly useful, problems involving an inclined plane. It is important because the normal force depends on the component of the gravitational force that is perpendicular to the slope.We can use the following two equations to find the components:

Finding the resultant forceWe can find the resultant force quite easily be following a few simple guidelines. Draw a free body diagram.

Resolve all forces into components parallel to the x- and y-directions.

Calculate the resultant in each direction, and , using co-linear vectors.

Use and to calculate the resultant .

Newton's laws

Newtons First Law An object continues in a state of rest or uniform motion (motion with a constant velocity) unless it is acted on by an unbalanced (net or resultant) force.This property of an object, to continue in its current state of motion unless acted upon by a net force, is called inertia.An ice skater pushes herself away from the side of the ice rink and skates across the ice. She will continue to move in a straight line across the ice unless something stops her. Objects are also like that. If we kick a soccer ball across a soccer field, according to Newtons first law, the soccer ball should keep on moving forever!

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Newtons Second Law If a resultant force acts on a body, it will cause the body to accelerate in the direction of the resultant force. The acceleration of the body will be directly proportional to the resultant force and inversely proportional to the mass of the body.Newton's laws

Mathematically this is:

Newton's laws Newton's second lawForce is a vector quantity. Newtons second law of motion should be applied to the y- and x-directions separately. You can use the resulting y- and x-direction resultants to calculate the overall resultant as we saw in the chapter on vectors.We can use Newton's second law to solve problems involving: Objects accelerating along a surface with or without frictional forces.

Two connected objects with a tension force. The tension force may be at an angle.

Objects being pulled along, with the pulling force applied at an angle.

Problems involving objects on an inclined plane.

Problems involving lifts and rockets.

Newton's laws

Newtons Third Law If body A exerts a force on body B, then body B exerts a force of equal magnitude on body A, but in the opposite direction.

Using Newton's third law we can determine action-reaction pairs of forces. These have the following properties: the same type of force acts on the objects; the forces have the same magnitude but opposite direction; and the forces act on different objects.

Forces in equilibrium

EquilibriumAn object in equilibrium has both the sum of the forces acting on it and the sum ofthe moments of the forces equal to zero.We mentioned that resultant forces cause objects to accelerate in a straight line. If an object is stationary or moving at constant velocity then either, no forces are acting on the object, or

the forces acting on that object are exactly balanced.

In other words, for stationary objects or objects moving with constant velocity, the resultant force acting on the object is zero.

Newton's laws

Newtons Law of Universal Gravitation Every point mass attracts every other point mass by a force directed along the line connecting the two. This force is proportional to the product of the masses and inversely proportional to the square of the distance between them.

Newton's laws Newton's law of universal gravitationAn important point to note about this law is that it is always attractive, and it depends only on the masses involved and the distance between them.This law also involves the universal gravitational constant:

We also note that for any large object we use the distance from the centre of the object(s) to do the calculation.

And finally we note that we can find the acceleration due to gravity for Earth (and for any planet) by using:

On Earth this value comes out to be

Weight and massMass is a scalar and weight is a vector.Mass is a measurement of how much matter is in an object and is measured in kg.Weight is a measurement of how hard gravity is pulling on that object and is measured in N.Your mass is the same wherever you are. Your weight depends on how strong a gravitational force is acting on you.We can use the following equation to calculate weight:

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Measuring mass

Newton's law of universal gravitation comparative problemsA common application of this law is to solve comparative problems. The following strategy will help you solve these equations: Write out equations and calculate all quantities for the given situation

Write out all relationships between variable from first and second case

Write out second case

Substitute all first case variables into second case

Write second case in terms of first case

For more practice see:www.everythingscience.co.zaShortcode: ESBM3

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