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Force and Motion. Forces…. What happens to objects when they are acted on by unbalanced forces?. a push. a pull. a turn. A force is a …push, pull, or a turn. You can feel forces and see their effects , but you cannot see forces.
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Forces… What happens to objects when they are acted on by unbalanced forces?
a push a pull a turn A force is a …push, pull, or a turn. You can feel forces and see their effects, but you cannot see forces. Forces make things happen or stop them from happening.
Effect 1. A force can change the shape of an object. Effect 2.A force can change the speed of an object, making it faster or slower. Effect 3.A force can change the direction of an object. Forces affect objects in 3 ways…
Forces have magnitude and direction ! • Forces are vector quantities…remember F = ma. • The unit of force is the newton, N. • 1 N = .2248 pounds of force, lbf..
The combination of all forces acting on an object is called the net force. Whenever the net force on an object is zero, the object is said to be in mechanical equilibrium. An upward force on an object is called a support force and balances the weight of an object on a surface. More about forces…
Objects at rest… • Objects at rest are said to be in static equilibrium while objects moving at constant speedin a straight line are said to be in dynamic equilibrium. • In order for objects to be at rest, the sum of the forces acting on that object must equal 0.
Types of forcesForces start motion, stop motion, or change motion… Balanced • Do not cause a change in motion. • They are equal in size and opposite in direction. • The forces can move toward each other or opposite each other. The net force on the object is 0.
Types of forcesForces start motion, stop motion, or change motion… Unbalanced • ALWAYS cause a change in motion. • They are not equal and opposite. One force is always greater than the others. • The combined force is equal to the difference in the two forces. The resultant force is always directed in the direction of the largest force. • These forces can be in opposite direction or in the same direction. The net force on the object is greater than 0.
Friction Forces… What happens when you rub your hands together?
Friction Forces… Heat is generated as a result of friction. Friction occurs when two surfaces move against each other. Friction opposes the direction of motion. Which types of surface generates more friction, smooth or rough surfaces?
Types of friction… • Rolling Friction…riding a bike or skateboard • Sliding Friction…cartilage against bone • Static friction…pushing or pulling an object • Fluid friction…rowing a boat. Air resistance is fluid friction. • What are some cases in which you would want to increase friction?....decrease friction?
Net Force • The net force is the vector sum of all the forces that act upon an object. That is to say, the net force is the sum of all the forces, taking into account the fact that a force is a vector and two forces of equal magnitude and opposite direction will cancel each other out.
Finding Net Force…. • Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.
Forces… How do mass and proximity relate to gravitational force? Are mass and weight the same thing?
Gravity…a fundamental force in the universe • Gravity is one of the four fundamental forces of the universe. Others are strong and weak nuclear forces and electromagnetic forces. • Gravity always acts toward the center of an object such as earth. • Gravity changes depending on where you are in the universe.
Weight is a function of gravity… • w = mg m = mass and g = 10m/s2 • Weight is a force measured in Newtons. • 1 N = 1 kg m s2
●Mass is a measurement of the amount of matter something contains. ● Mass is measured by using a balance comparing a known amount of matter to an unknown amount of matter. ●Mass of an object doesn't change when an object's location changes. ● Weight is the measurement of the pull of gravity on an object. Mass vs. Weight… ● Weight is measured on a scale. ●Weight does change with location.
What happens to weight and mass when gravity changes? Mass is constant.No matter where you are in the universe your mass is the same as it is on earth. Your weight will vary depending on where you are in the universe. Smaller objects than earth, such as the moon, will have less gravity than earth whereas larger objects, such as Saturn, will have more gravity.
And finally… May the force be with you!
The Laws of Motion according to… Sir Issac Newton
Forces…a review • A force is a push, a pull, or a turn. • Friction, drag due to air resistance, gravity, and weight are forces. • Unbalanced forces acting on an object cause them to change speed or direction. • Large objects have more inertia than small objects.
Inertia and Mass • Inertia is the tendency of an object to resist changes in motion. • Mass is the amount of matter in an object. • Mass is a quantitative measure of inertia. • Inertia, like mass, does not depend on the position or location of an object.
1st Law of Motion…the Law of Inertia • An object at rest remains at rest, and an object in motion continues in motion at a constant speed in a straight line, unless acted on by a nonzero net force. • …the shorter version…”the velocity of an object remains constant unless acted on by an unbalanced force.” • Smaller mass objects are easier to set in motion (accelerate) by an unbalanced force. Large masses resist net forces and do not accelerate much.
Which is harder to stop? The larger mass has more inertia. It has more resistance to changes in motion!
Is a net force needed to keep an object in motion? No! An object in motion stays in motion at constant velocity forever! (Unless a new net force acts…)
Motion… What causes an object to accelerate?
2nd Law of Motion…Law of Acceleration • F = ma • Net Force = Mass x acceleration • A net force causes an object to accelerate in the direction of the net force.The acceleration isdirectly proportional to thenet force and inversely proportional to theobject’s mass.
2nd law continued… • When mass is in kilograms and acceleration is in m/s2, the unit of force is in Newtons (N). • One Newton is equal to the force required to accelerate one kilogram of mass at one meter/sec2.
Try these 2nd law problems… • Determine the acceleration when a 100 N force is applied to a 5 kg mass? • If a 5 kg object accelerates at 2 m/s2 , how much force was applied to the object? • A 500 N force accelerates an object at 2 m/s2 . Determine the mass of the object.
3rd Law of Motion…Action and Reaction • When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude, but opposite in direction. • Shorter version… “For every action there is an equal and opposite reaction.”
3rd Law…Action and Reaction • According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.
3rd Law…Action and Reaction • Two forces are acting on two different objects. • The two forces DO NOT cancel each other out. The unbalanced forces produce MOTION. • The only way for two forces to cancel each other out is for them to be equal and opposite and act on the same object!
The 3rd Law in Nature… • Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. • The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backward) is opposite the direction of the force on the fish (forward).
Motion is Relative • Can you think of a time that you thought you were moving and you weren’t or vice versa?
How do we know that motion has occurred? • We use a frame of reference. It is best that this is a stationary object.
