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Force and Acceleration

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1 Force and Acceleration
Newton’s Second Law Force and Acceleration

2 5.1 Force causes acceleration.
Recall that acceleration = change in velocity/time interval. This chapter focuses on the cause of acceleration: Force. A force is a push or a pull. Most often there is more than one force acting on an object. The combination of forces acting on an object is the net force.

3 Force causes acceleration.
Acceleration is proportional to net force. F ~ a To increase the acceleration of an object you must increase the net force on the object.

4 5. 2 Mass resists acceleration.
More massive objects resist a change in their motion. Inertia prevents changes from occurring without a net force present. a ~ 1/m Thus acceleration and mass are inversely proportional.

5 5.3 Newton’s Second Law Newton was the first to realize that the acceleration produced when an object moves depends not only on how hard it is pushed or pulled but also on the object’s mass. The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

6 Newton’s Second Law a = F/m Or F = ma

7 Questions Two forces act on a book resting on a table: its weight and the support force from the table. Draw a free body diagram of the book. Does a force of friction act as well? Suppose a high-flying jet cruises with a constant velocity when the thrust from its engines is a constant N. What is the acceleration of the jet? What is the force of air resistance acting on the jet?

8 5.4 Friction Friction is a force that opposes motion.
It acts on materials that are in contact with one another. Friction is due to irregularities in the surfaces of the materials. Depends on the kinds of materials in contact Depends on how much the surfaces are pressed together. Friction is present with solids, liquids and gases.

9 Friction If the force of friction just balances the force applied, the object moves with constant velocity. If air friction is equal to the force of gravity on an object, the object is at its terminal velocity and it no longer accelerates.

10 Applying force --- pressure
No matter how you place a book on a table, the force of the book on the table is the same. Why? There is a difference in the amount of pressure the book applies to the table however which is dependent upon the area of contact. What is true about the pressure of the book if the area of contact is small?

11 5.5 Applying force --- pressure
The force of a car can be determined by finding the pressure that each of its tires exerts on the road. The surface area of contact must be known. From the pressure of each of the tires and the surface area of contact, force of each tire can be calculated. P = F/A

12 5.6 Free Fall Explained Galileo showed that falling objects accelerate equally, regardless of their masses. This is strictly true if air resistance is neglected. Galileo couldn’t say why that was true, however. The reasoning is simple. Recall that mass and weight are proportional. Followers of Aristotle believed that heavier objects would accelerate faster than lighter ones. They only considered the weight, not the greater mass as well. See page 67.

13 Free Fall Explained If you were on the moon and dropped a hammer and a feather from the same elevation at the same time, would they strike the surface of the moon at the same time?

14 5.7 Falling and Air Resistance
If air resistance is considered, then the net force exerted on falling objects is different. A feather is buoyed up by air currents and reaches terminal velocity quickly whereas a rock of similar size is really unaffected by air currents. It does not reach terminal velocity and continues to accelerate. Terminal velocity occurs when the force of air resistance is equal to the weight of the object. Thus net force is zero. It is possible to change the speed of a falling object by altering its orientation in the fall.

15 Falling and Air Resistance
If a heavy person and a light person open their parachutes together at the same altitude and each wears the same size parachute, who will reach the ground first? If the force of air resistance is the same for a falling baseball and a falling tennis ball, which will have the greater acceleration? A skydiver jumps from a high altitude balloon. As she falls faster and faster through the air, does air resistance increase, decrease, or remain the same? Does the net force on her increase, decrease, or remain the same? As she falls faster and faster, does her acceleration increase, decrease, or remain the same?

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