Lecture_12

Lecture_12 - Example: A 1.9-kg block slides down a...

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Example: A 1.9-kg block slides down a frictionless ramp, as shown in the figure below. The top of the ramp is 1.5 m above the ground, and the bottom of the ramp is 0.25 m above the ground. The block leaves the ramp moving horizontally, and lands a horizontal distance d away. Find d . Conservation of energy allows me to find the speed the block has when it leaves the ramp. E top 1.5 m bot top E E = E bot bot bot top top mgh mv mgh mv + = + 2 2 1 2 2 1 0.25 m ) ( 2 bot top bot h h g v = d m/s 95 . 4 ) 25 . 0 5 . 1 )( 8 . 9 ( 2 = = bot v From kinematics, d = v bot ·t. t is just the time the projectile is in the air, in other words, the time it takes the box to fall a height of 0.25 m. 2 1 a = Δ y Δ = 2 3 ) 25 . 0 0 ( 2 2 y y y a t s 23 . 0 8 . 9 t m 14 . 1 ) 23 . 0 )( 95 . 4 ( = = = t v d bot
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Ch. 7 Momentum 7.1 Impulse and Momentum Up to now, we have been dealing only with constant forces. But, in many instances the force on an object is not constant in time. Take collisions, for example. In a collision the forces are very complicated. In a collision, the forces involved may be very large, but only act for a very short period of time. These type of forces are called Impulse Forces. A car crashing into a wall would be an example of an impulse force. Typically, the impulse force is so large that it is the only significant force of the collision. Friction and gravity are insignificant. e define a quantity associated with the impulse force called the pulse We define a quantity associated with the impulse force, called the Impulse : t F I Δ = v I is the impulse. It is equal to the average force times the time over which the force acts. It is a vector quantity.
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o f t t t = Δ Before t o , the force is zero. fter e force is zero. Units? s] [N Time] [Force = × After t f the force is zero. Between t o and t f the force is large. There are two ways to generate a large impulse. Apply a large force for a very hort time or apply a small force for a very long time short time, or apply a small force for a very long time. In this chapter, we will only consider the former. As an example of an impulse force, let’s consider a crash test of a Mercedes.
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Example : If the time-averaged force acting on the Mercedes is -2.0 × 10 5 N. What is the impulse delivered to the car by the wall?
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Lecture_12 - Example: A 1.9-kg block slides down a...

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