Physics Review
A compact car and an 18-Wheeler both starting from rest accelerate at the same rate to a final velocity. A. Which Engine did more work? B. Which had more KE? C. Is it possible for them to have the same KE?
A. 18-Wheeler because W: 1/2mv^2, assuming the truck weighs more and the car weighs less, the work will always be greater in the truck due to the larger mass. B. The 18-Wheeler because KE:.5(m)(Vf)^2, since the have the same Vf the vehicle with the greater mass will have the most KE. C. Yes, but only at rest. Since the Vf=0 will make the KE zero since anything times zero is zero.
Why are cars with anti-lock brakes safer than cars without?
Anti lock brakes prevent wheels from locking and skidding. Skidding is kinetic friction, Anti lock brakes quickly apply and release the brakes over and over. This way, the tires are constantly grabbing the road and letting go before skidding. This would be static friction. Static Friction is greater than kinetic friction Therefore, anti lock brakes allow you to stop over less distance than skidding and also allow the driver to steer the car slowly rather than skidding out of control.
To the right is a FBD of an unknown object. Describe all the possible types of motion for the object?
Check the packet for the FBD. The object will either be stationary or moving at a constant velocity in any direction.
How can you use a spring powered dart gun to find the spring constant of the spring that launches the dart? You are allowed to use a digital scale and a tape measure. You must use words and math!!
First you set up the equation for energy. Pes:Kef: .5(k)(d)^2=.5(m)(v)^2. you have to measure the delta x and the mass of the dart too. Then you have to do projectile motion. You measure the delta dx and time the shot. Then you put the time into math rep 1 and find the velocity for Vx. Vf becomes Vi for the energy equation, then just plug and chug. It's hard to explain but it's basically a work and energy projectile motion problem.
Supposing a snowmobile is equipped with a flare launcher which is capable of launching a flare vertically (relative to the snowmobile). If the snowmobile is in motion and maintains a constant horizontal velocity after the launch, then where will the flare land (neglect air resistance)? EXPLAIN.
Flare will land in the snowmobile because the velocity in the X is the same as the snowmobile, therefore they will have have the same ΔT so they will travel the same Δ D. Plus, X and Y are independent!!!!
You are standing on a scale in an elevator that is initially at rest. Explain what will happen to the reading on the scale when the elevator starts to move up, when it is traveling at a constant velocity, and when it slows down at the top. Draw FBD's to help represent your explanation.
Fn=mg+ma a+ = Forces go up Fn= mg + m(-a) a- = Forces go down Fn= mg + m(0) a0 = Fn =mg *check picture on google doc for detailed image*
Using only a tape measure and a stopwatch, how can you find the initial speed and angle of a football that is kicked on a level surface? You must use words and math!
Measure the distance the football travels and the time from start to finish. Use math rep #1 to find the velocity in the x. Then use math rep #3 to find the initial velocity in the y-axis. Next use pythagorean theorem to find velocity, then sub in these values for inverse tangent to find the angle. Initial velocity y/initial velocity x.
You are standing on a skateboard. Would it be a good idea to jump off a skateboard and try to propel yourself forward?
No, MsVis + MyViy = MsVfs + MyVfy 0=mVfs + Myvfy you would have very little velocity, so you would fall
All of the sudden, our sun becomes a black hole. This means that its mass will stay the same, but its radius will become very very small. What will happen to our solar system?
Radius stays the same. Fg= m^1m^2/r^2 so everything in the equation will remain the same. Nothing will happen to the solar system but we will cease to exist without the sun.
Two people are holding a rope between them. One person has a mass m and the other has a mass 2m. Describe the motion of each person if the more massive person pulls on the rope. Be sure to use words like "three times" as much. Please use words and math to explain your answer.
Same force, different mass. Person with lighter mass will have double the acceleration even though their forces are equal but in opposite direction because of 3rd law.
Suppose you roll a bowling ball into a pillow and the ball stops. Now suppose you roll it against the spring and it bounces back in the opposite direction with the same speed. Which object causes a greater impulse, the pillow or spring? EXPLAIN.
Spring: -m(vf-vi) The spring's impulse is greater than the pillows impulse. Pillow: -mvi
A 100 kg person walks a 15 m long incline and ends up 5 m above the ground. A 50 kg person walks up a 30 m long incline and ends up 10 m above the ground. The first person takes half as much time as the second person. Please explain who does more work. Also decide which person uses more power.
THEY DO THE SAME WORK! Although the one who does it in less time has more power. w=mgh w=100(9.8)(5) w=50(9.8)(10) same work, person 1 produces more power because half as much time.
During the year we smashed a cinder block with a sledgehammer while lying on a bed of nails. Explain why Mr. G was NOT hurt during the demonstration.
Tafelski did not overcome inertia of the block. If you hit the block quickly the block doesn't move because you're not taking the time to overcome the blocks inertia. (i think).
Why were we unable to break an egg while throwing it as hard as we could into a sheet while at the same time, the egg broke if we just dropped it on the ground?
The bed sheet cushions the throw of the egg and increases the amount of time it travels which in effect, decreases the amount of force on the egg. However, the egg broke when we dropped it because the ground did not move to cushion the egg and had a smaller time traveled which increased the amount of force on the egg.
A massive ball is suspended on a string and pulled by another string as shown below. Which string will break and why?
The bottom string is more likely to break if it is pulled quickly.But if the string is pulled slowly the top string will break because the entire weight of the ball is being put on the string.
How can you use a record player to find the coefficient of friction between a record and another object? EXPLAIN using words and math.
The equation is μFn= mv^2/r, and Fn = mg so it becomes μmg= mv^2/r and the masses cancel out. So the equation becomes μg= v^2/r... then you use 2πr/T, you measure the radius on the turntable, you use the radius in which the object on the turntable begins to slide off, and time for one revolution, (time for 10 revolutions divided by 10 for more accuracy) then you have the velocity and plug that back into the equation μg= v^2/r
A projectile is thrown straight up. What are TWO ways you can find the initial velocity of the projectile? You can use a stopwatch.
The first way is to measure the projectile at its maximum height. At the max height, the vf would be 0, we could measure the distance and using the stopwatch we could measure the time it takes for the projectile to get to its max height. Then use math rep 2 to find vi The second way is to use math rep 3 to get vi, you have t,d, and a, Remember d is zero because it travels back down to the same position
Two projectiles are shot with the same initial velocity and from the same height. They both land at the same height they were fired at. Explain how it is possible to fire two projectiles and have them travel the same horizontal distance if they are fired at two different angles.
The two projectiles would have to be fired at complementary angles such as 30-60 or 45-45. Any two angles that add up to 90. Example: The one shot at 30 degrees will hit the ground first. It spends less time in the air, but had a larger horizontal velocity. The one shot at 60 degrees will hit the ground second spending more time in the air while having a smaller horizontal velocity.
You are stranded on a frozen pond and there is little friction between you and the surface. You shout to your friend for some assistance and he slide you a heavy stone the come to rest at the base of your feet. How can you use this stone to get a safety? What are the principles behind your answer?
Throw or slide the rock so you move in the opposite direction that it is thrown. MrVir + MyViy = MrVfr + MyVfy 0 + 0 = MrVfr + MyVfy -MyVfy = MrVir
Two objects are traveling with the same initial velocity across a smooth surface. Object A has a mass "m" and object B has a mass "5m." Object A has twice the surface area of object B. They are both made of the same material and have the same coefficient of kinetic friction. Which object will slide farther before coming to a stop? Explain using FBD's, words and mathematical representations.
Travel the same. Surface area does not affect force of friction and mass divides out of equation. -uFN=ma FN=mg substitute in, mass cancels out, and acceleration for both is -ug. They will have the same Vi and the same acceleration to they will have the same delta d.
How is it that an object can accelerate while traveling at a constant speed, but not at a constant velocity?
Velocity > vector of magnitude and direction Speed > only has magnitude (not direction) Example: A satellite in space (in circular orbit) can be traveling at a constant speed while accelerating toward center but is not constant because the direction of the velocity that is changing and speed is constant while velocity is changing.
Surprisingly we can use only a tape measure to find the speed of a potato coming out of a potato gun. Explain how we can do this. Assume the potato will be shot perfectly horizontal.
Viy=0. We would then use the tape measure to find Δdx and Δdy, we know that a=-9.8m/s^2 in the y due to gravity, then you can use MR#3 to find Δt once you find Δdy, and because Δt is the same for x and y you can use MR#1 to find the Vx.
While making a left turn in your car, you feel like your body is being pushed right.
While the car turns left, your body continues to travel straight until acted upon by a force that makes you go left. The car moving left beneath you makes you feel as if you being pushed right.
While traveling on a rollercoaster, you feel lighter while going over the top of hill and heavier while traveling at the bottom of the hill?
You feel heavier at the bottom and lighter at the top. When drawing the FBD's you have to remember that in order to travel in a circle, you need a net force towards the center of the circle. Based on this principle, you can see in your FBD why you feel lighter or heavier. Remember that your weight "feels" like whatever the force of the seat on you is doing. The normal force (force of seat) is changing giving you the sensation of heavy or light.
