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35 Cards in this Set
- Front
- Back
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How does impulse differ from force!
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Impulse- a force that produces motion suddenly
force- strength exerted to make movement --> Force causes impulse |
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2 ways to increase impulse
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-increase the force
- increase the time interval |
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For the same force, why does a long cannon impart more speed to a cannonball than a smaller cannon?
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It will have more momentum coming from the long cannon due to the force acting for more time
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Is the impulse-momentum relationship related to Newton's second law?
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It is a direct result; Impulse= Change in the Momentum
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To impart the greatest momentum to an object, should you exert the largest force possible, extend that force for as long a time as possible, or both? Explain.
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Both, for then the quantity Ft is greater.
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When you are in the way of a moving object and an impact force is your fate, are you better off decreasing its momentum over a short time or over a long time? Explain.
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The force of impact will be less if momentum changes over a long time. By making time long, F will be smaller.
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Which undergoes the greatest change in momentum: (1) a baseball that is caught, (2) a baseball that is thrown, or (3) a baseball that is caught and then thrown back, if the baseballs have the same speed just before being caught and just after being thrown?
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(3) There is twice the momentum change for the baseball that is caught and then thrown back. A change in stopping, and the same change in returning.
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In the preceding question, in which case is the greatest impulse required?
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Greatest impulse also for (3) because of the greater change in momentum.
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Can you produce a net impulse on an automobile by sitting inside and pushing on the dashboard? Can the internal forces within a soccer ball produce an impulse on the soccer ball that will change its momentum?
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No for both, for the forces, and hence impulses, are internal.
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Is it correct to say that if no net impulse is exerted on a system, then no change in the momentum of the system will occur?
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Correct
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What does it mean to say that momentum (or any quantity) is conserved?
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It means that in any interaction where external impulses don’t occur, the momentum of a system remains the same before and after the interaction. In general it means that the magnitude of the quantity remains unchanged while other changes may take place.
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When a cannonball is fired, momentum is conserved for the system of cannon + cannonball. Would momentum be conserved for the system if momentum were not a vector quality? Explain.
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No, the momentum would not be conserved if momentum were not a vector quantity because then it would not cancel. That is, the momentum of the cannon + the momentum of the cannonball would be twice the momentum of the cannon and it would have increased from zero.
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Distinguish between an elastic collision and an inelastic collision. For which type of collision is momentum conserved?
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An elastic collision is one where rebound is without lasting deformation or the generation of heat. An inelastic collision is one where deformation does occur, or heat is generated. Momentum is conserved for both types.
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Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?
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Momentum is completely transferred from car A to car B. Since the masses are the same, the speed of Car B is equal to the initial speed of car A.
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If the equally massive cars of the previous question stick together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A?
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Again, the momentum of car A is transferred, this time to both cars. Since twice the mass is moving after collision, the speed is half.
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To bring a supertanker to a stop, its engines are typically cut off about 25 km from port. Why is it so difficult to stop or turn a supertanker?
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A supertanker has an enormous mass so, with any velocity at all, its momentum will be very large and it will require a force acting over a very long time to bring it to a stop.
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In terms of impulse and momentum, why are nylon ropes, which stretch considerably under stress, favored by mountain climbers?
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Nylon ropes, which stretch considerably under tension, are favored by mountain climbers because the stretch and take time in coming to a stop when mountain climbers accidently fall.
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A person can survive a feet-first impact at a speed of about 12 m/s on concrete; 15 m/s on soil; and 34 m/s on water. Why the difference values for different surfaces?
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A person survives a fall feet first onto different surfaces because of how he stops. On concrete, he stops instantly. On soil, he stops almost instantly, but the soil has some give, so it is not instantaneous. The water does not stop a person but slows him down gradually. The ability to survive the impact depends on the rate of negative acceleration.
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A lunar vehicle is tested on Earth at a speed of 10km/h.When it travels as fast on the Moon, is its momentum?
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Momentum = mass X velocity; We know mass is always constant & since the velocity is also same so momentum will always be the same at that speed no matter whether on earth or on moon
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If you throw a raw egg against a wall youll break it but if you throw it with the same speed into a sagging sheet it wont break explain?
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This isn't actually wordplay, it's physics. For the shell of the egg to break requires a high IMPULSE, or instantaneous force. With the wall, all the force to stop the egg occurs almost at the same time, so the impulse is very high, breaking the egg. With the sheet however, the force to stop the egg is applied over a much greater period of time. Therefore, the force is lower at each instant of the collision, meaning a vastly reduced impulse. Since the impulse is much less than what is required to break the shell, the egg does not break.
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Why is it difficult for a firefighter to hold a hose that ejects large amounts of high-speed water?
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The water exiting the hose is being pushed forward by the water behind it in the hose, so it pushes back on that water (and the hose). Since it is a large mass of water being ejected at a high speed, the impulse it feels is large and forward, and the water then provides a large impulse back on the hose, which makes it hard to hold.
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Why are the impulses that colliding objects exert on each other equal and opposite?
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Newton’ s third law and momentum is conserved
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If only external force can change the velocity of a body how can the internal force of the brakes bring a car to rest?
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A car moves forward by exerting a (backward) force on the road which then exerts a (forward) force on the car. The brakes of a car cause the tires to exert a force on the road and the road then exerts a force on the tires or the car. This is another example of action-reaction pairs of forces described by Newton's Third Law of Motion.
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A fully dressed person is at rest in the middle of a pond on perfectly frictionless ice and must get to shore. How can this be accomplished?
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Momentum must be conserved. The person must take off some clothing and throw it in the opposite direction to which he needs to go. The person has no momentum to start off with, so after he's thrown his clothing the total momentum of person and clothing must stil be zero. But this is still true if the person's momentum is non-zero provided it has the same magnitude and opposite direction as the clothing's momentum.
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Your friend says that the law of momentum conservation is violated when a ball rolls down a hill and gains momentum. What do you say?
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The Law of Momentum Conservation states that if no external forces act on a system, then the system’s momentum will not change. If you treat the ball as your system, it is clear that the gravitational force is acting on the ball, provides an impulse, and therefore changes the ball’s momentum. But this is not a violation of the Conservation of Momentum because there is an external force acting.
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A billiard ball will stop short when it collides head-on with a ball at rest. The ball cannot stop short, however, if the collision is not exactly head-on -- that is, if the second ball moves at an angle to the path of the first. Why? (Hint: Consider momentum before and after the collision along the initial direction of the first ball and also in a direction perpendicular to this initial direction.)
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Momentum is a vector. Momentum is conserved along any direction and the direction perpendicular to that direction. Consider the direction perpendicular to the initial velocity of the incomming billiard ball. Initially, there is no momentum along that direction. But, after the collision, if the second billiard ball moves at some angle, it does have momentum along that direction. If the incoming billiard ball simply stopped then momentum would not be conserved. We would, in that case, go from having no momentum in this perpendicular direction to having momentum in that direction. This can not happen. So that means such a collision can not happen.
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When vertically falling sand lands in a horizontally moving cart, the cart slows. Ignore any friction between the cart and the tracks. Give two reasons for this, one in terms of a horizontal force acting on the cart and one in terms of momentum?
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When vertically falling sand lands in a horizontally moving cart, the cart slows. As the sand falls on the cart and the sand acquires the speed of cart, the cart is applying force on the sand in a direction opposite to the direction of the motion of the cart. Thus, the cart slows down. Also, as the sand is falling on the cart, there is no net horizontal force on the sand + cart system. There is no initial speed of the sand in the horizontal direction. So as the sand falls on the cart, the mass of cart is increasing, so in accordance with the law of conservation of momentum, its speed decreases.
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Suppose that there are three astronauts outside a spaceship and that they all decided to play catch. All the astronauts weight the same on Earth and are equally strong. The first astronaut throws the second one toward the third one and the game begins. Describe the motion of the astronauts as the game proceeds. How long will the game last?
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As the 2nd astronanut moves toward the 3rd one, the 1st astronaut moves away from the 3rd with the same speed as the 2nd one is moving toward the 3rd one. As the 3rd astronaunt catches the 2nd astronaut, both start moving with half the initial speed of the 2nd astronaut. When the 3rd astronaunt thwos the 2hd astronaut, his speed bcomes half the speed of the 1st astronaut. The direction of speed for the 3 astonauts is shown in the following figure in this situation:
← 1st (u) ← 2nd (u/2) → 3rd (3u/2) |
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What is the momentum of a 50-kg carton that slides at 4 m/s across an icy surface?
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momentum = mass * velocity; momentum = 50 * 4; momentum = 200 kg m/s
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What impulse occurs when an average force of 10 N is exerted on a cart for 2.5 s?
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Average force is(F) =10N; Time taken by the cart to exert is(t) =2.5s
Impulse(J) = force * time: = Ft = (10 N)(2.5 s) = 25 Ns |
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What is the impulse of an 8kg ball rolling at 2m/s when it bumps into a pillow and stops?
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Mass of the ball is(Δm) =8kg; Speed of the ball(v) = 2m/s
Impulse = change in momentum Ft = Δmv Ft = (8 kg)(2 m/s) Ft = 16 kgm/s = 16 Ns |
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A 2kg blob of putty moving at 3m\s slams into a 2kg blob at rest. Calculate the speed of the two stuck together?
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2 kg X 3 m/s + 2 kg X 0= 4kg X v
→ V= 3/2 m/s = 1.5 m/s |
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A car crashes into a wall at 25 m s and is brought to rest in 0.1 s. calculate the average force exerted on a 75 kg test dummy by the seat belt.
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impulse= 25 m/s X 75 kg; 1,875 kg X m/s
Force X time= 1,875 kg X m/s Force= 1,875 kg/ 0.1 s Force= 18,750 N |
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A 5-kg fish swimming 1 m/s swallows an absent-minded 1-kg fish swimming toward it at a velocity that brings fish to a halt immediately after lunch. What is velocity of the smaller fish before lunch?
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M = mass of the larger fish; V = velocity of the larger fish
m = mass of the smaller fish; v = velocity of the smaller fish (5 kg)(1 m/s) = (1 kg)(x m/s); = 5 m/s |
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Two automobiles, each of mass 1000 kg, are moving at the same speed, 20 m/s, when they collide and stick together. In what direction and at what speed does the wreckage move a) if one car was driving north and one south? b) if one car was driving north and one east?
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-Since m1 = m2 = 1000kg, v1 going n with (+20m/s in +y), v2 going s (-20m/s in-y)
Initial momentum is m1v1 + m2v2 = 1000x20 + 1000x(-20) = 0 - Speed and direction of the cars after collision is 20 root 2 in north east direction. |
