Physics 1010 test 1

A railroad diesel engine weighs 4x as much as a freight car. If the diesel engine coasts at 5 km/h into a freight car that is initially at rest, how fast do the two coast after they couple together?

(4 kg)(5 Km/h)+(1 kg)(0 Km/h)=(4 kg +1 kg) v 20=5v v=4 km/h

What is the net force that acts on a 10-N falling object when it encounters 4 N of air resistance? 10 N of air resistance.

1. 6 N 2. 0 N

If you push a crate horizontally with 100 N across a 10-m factory floor, and friction between the crate and the floor is a steady 70-N, how much kinetic energy is gained by the crate?

100 N-70 N=30 N F(net)*Distance=KE 30 N*10 m=300 Nm

To tighten a bold, you push with a force of 80 N at the end of a wrench handle that is 0.25 m from the axis of the bolt. a) what torque are you exerting? b) if you move your hand inward to be only 0.10 m from the bolt, what force do you have to exert to achieve the same torque? c) Do your answers depend on the direction of your push relative to the direction of the wrench handle?

20 Newtons 20N=N*.1 divide both sides by .1 N=300

If there was no air drag, how fast would drops fall from a cloud 1 kilometer above the earth's surface?

D=1/2gt^2 D=1000 meters 1000=1/2(10)t^2 2000/10=t^2 Square root of 200=14.14 s V=gt V=(10)(14.14) V=141.4

When you pedal a bicycle, maximum torque is produced when the pedal sprocket arms are in the horizontal position, and no torque is produced when they are in the vertical position. Explain.

90 degree amount of force on horizontal, not real force on vertical.

What two units of measurement are necessary for describing speed.

Distance over time (m/s)

Distinguish between velocity and acceleration.

Acceleration deals with the change of velocity over time while velocity is just distance over time.

What is the net force acting on a 1-kg ball in free fall?

Acceleration=f(net)/mass 10 m/s/s=f(net)/1 kg =10N

Two people who weigh the same climb a flight of stairs. The first person climbs them in 30 s, while the second person climbs them in 40 s. Which person does more work? Which uses more power?

Both do same amount of work, but the first one has more power because it gets done faster.

What does it mean to say that a moving object has inertia. Give an example.

By saying a moving object has inertia, it means the object will resist change but it will change when acted upon by an outside force. Ex:car turning the corner.

Consider the swinging-balls apparatus. If two balls are lifted and released, momentum is conserved as two balls pop out the other side with the same speed as the released balls at impact. But the momentum would also be considered if one ball popped out at twice the speed. Can you explain why this never happens?

Can't create energy out of something else.

A lever is used to lift a heavy load. When a 50-N force force pushes one end of the lever down 1.2 m, the load rises 0.2 m. Calculate the weight of the load.

N*.2m=50N*1.2m=300

In order to slide a heavy cabinet across the floor at constant speed, you exert a horizontal force of 600 N. is the force of friction between the cabinet and the floor greater than, less than, or equal to 600 N? Defend your answer.

Five between floor and cabinet is 600 N. (velocity is constant, net force is 0).

Consider a heavy accelerate resting on the bed of a flatbed of a truck. When the truck accelerates, the crate also accelerates and remains in place. Identify the force that accelerates the crate.

Friction is holding the cart in place.

For the pulley system shown, what is the upper limit of weight the strongest man can lift?

His own body weight.

Is a fine for speeding based on one's average speed or one's instantaneous speed? Explain.

Instantaneous speed because that is the speed a person is going at a given moment.

Is it possible to move in a curved path in the absence of a force? Defend your answer.

No, because in order to change direction a force must be applied to it.

As you stand on the floor, does the floor exert an upward force against your feet? How much force does it exert? Why are you not moved upward by this force?

Support force is same as your weight. It doesn't lift up because it only exerts as much force as you are exerting on it.

Before the time of Galileo and Newton... stone dropped on moving ship, where would it land from what you you know about inertia.

It is already traveling at the speed of the boat so it would fall right below the mast.

Consider a ball at rest in the middle of a toy wagon. When the wagon is pulled forward, the ball rolls against the back of the wagon. Interpret this observation in terms of Newton's first law.

It remains at rest until acted upon by the wagon.

In an orbiting space shuttle, you are handed two identical boxes, one filled with sand and the other filled with feathers. How can you determine which is which without opening the boxes?

Judge the boxes by their inertia. The one less prone to being moved is filled with sand.

A moving car has kinetic energy. If it speeds up until it is going four times as fast, how much kinetic energy does it have in comparison?

KE=1/2mv^2

On a playground slide, a child has potential energy that decreases by 1000 J while her kinetic energy increases by 900 J. What other form of energy is involved, and how much?

Kinetic energy is 100 J. There is friction (heat).

A space probe may be carried by a rocket into outer space. What keeps the probe moving after the rocket no longer pushes it?

Law of inertia (object in motion resisting change in motion) Also mechanical equilibrium (if moving, motion continues without change).

What is meant by the lever arm of a torque?

Lever arm is the distance from the axis or rotation. Provides leverage for torque.

Which is more fundamental, mass or weight? Which varies with location?

Mass. Weight changes with location.

Consider a pair of forces, one having a magnitude of 20 N and the other a magnitude of 12 N. what maximum net force is possible for these two forces? What is the minimum net force possible.

Maximum: 32 N Minimum: 8 N

Dan and sue cycle at the same speed. The tires on Dan's bike are larger in diameter than those on sue's bike. Which wheels, if either, have the greater rotational speed?

She has greater rotational speed, because her tires are smaller, and she is at the same speed. (Because they are going the same speed, the tangential velocity is the same.)

Explain how the conservation of momentum is a consequence of Newton's third law?

Since according to momentum conservation :- Initial momentum = Final momentum Here, Initial momentum = 0 Final momentum = 2m*v1-m*v2=0 [v1 =velocity of 2m mass, v2 =velocity of m mass] therefore, 2*v1=v2 or v1=0.5v2 So, Heavier cart move half the speed of Lighter cart.

Distinguish between speed and velocity.

Speed is how fast something goes, velocity is both speed and direxfuon

If air resistance can be neglected, how does the acceleration of a ball that has been tossed straight upward compare with its acceleration if simply dropped?

The ball decelerates on its ascent, then accelerates at the same rate which it decelerated.

When a driver applies brakes to keep a car going downhill at constant speed and constant kinetic energy, the potential energy of the car decreases. Where does this energy go? Where does most of it go with a hybrid vehicle?

The energy goes to the brakes of a non-hybrid car. Hybrid cars have a system that stores that energy when the brakes are in use.

Which is greater, an acceleration from 25 km/h to 30 km/h or one from 96 km/h to 100 km/h if both occur during the same time?

The first because it has a greater acceleration.

Three identical blocks are pulled, as shown, on a horizontal friction-less surface. If tension in the rope held by the hand is 30 N, what is the tension in the other ropes?

The left rope has a tension of 10 N, the middle has 20 N.

What is the net force on a cart that is pulled to the right with 100 pounds and to the left with 30 pounds?

The net force is 70 pounds to the right.

You sit at the middle of a large turntable at an amusement park as it is set spinning and then allowed to spin freely. When you crawl toward the edge of the turntable, does the rate of the rotation increase, decrease, or remain unchanged? What physics principle supports your answer.

The rate of rotation decreases. By re-distributing the mass of the rotating system in a manner of spreading it out, you increase the rotational inertia of the system. Angular momentum, the product of rotational inertia and rate of rotation, must remain constant as per a conservation law, unless a net external torque acts on the system. Since rotational inertia increases, the spin rate must decrease.

On which of these hills does the ball roll down with increasing speed and decreasing acceleration along the path?

The second. it has a quicker acceleration rate and then it decreases faster.

Consider a book that weighs 15 N at rest on a flat table. How many newtons of support force does the table provide? What is the net force on the book in this case?

There are 15 newtons of support force. There are 0 N of net force (because it isn't moving).

You are driving north on a highway. Then, without changing speed, you round a curve and drive east. a) does your velocity change? b) do you accelerate? Explain.

Velocity changes because your direction changes. Acceleration is also a vector. As you round the corner acceleration is always pointing towards the center of curvature of the bend. This vector is at right angles to the curve. As the curve is changing directions so is the direction of acceleration. Also acceleration was zero before you entered the curve.

Consider the two forces action on the person who stands still - namely, the downward pull of gravity and the upward support of the floor. Are these forces equal and opposite? Do they form an action-reaction pair? Why or why not?

Yes, they are an equal and opposite reaction force pair. They are both in the system with no outside forces.

Can an object reverse its direction of travel while maintaining a constant acceleration? If so, give an example. If not provide an explanation.

Yes. When a ball is thrown straight up in the air, it has a constant acceleration towards the earth due to gravity. The ball slows down in its upward motion as the acceleration due to gravity acts upon it. The ball will eventually stop moving up, change direction, and start moving down. All this time, the acceleration remains constant.

In terms of center of gravity, support base, and torque, why can you not stand with heels and back to a wall and then bend over to touch your toes and return to your stand-up position.

You cannot stand with heels and back to a wall and then bend over to touch your toes and then stand up again because in doing so you would move your center of gravity so that it was no longer over your feet, which are your support base. This causes gravity to apply a torque to your body so you will fall over.

Fill in the blanks: The standard International unit for mass is the ____. The Standard International unit for force is the_______.

_______=Kilometer _______=Newton

How much acceleration does a 747 jumbo jet of mass 30,000 kg experience in takeoff when the thrust for each of four engines is 30,000 N?

a=30000(4)/30000 =4 m/s/s

Sprinting near the end of a race, a runner with a mass of 60 kg accelerates from a speed of 6 m/s to a speed of 7 m/s in 2 sec. a) what is the runner's average acceleration during this time? b) to gain speed, the runner produces a backward force on the ground so that the ground pushes the runner forward, providing the force necessary for acceleration. Calculate this average force.

a=m/t/t a=7-6/2 =1/2 or .5 m/s/s a=f(net)/mass .5=f(net)/60 30=N

The centers of gravity of the three trucks parked on a hill are shown by the Xs which truck will tip over?

bottom one because it's center balance is not over it's majority of mass.

A racing car on a flat circular track needs friction between the tires and the track to maintain its circular motion. How much more friction is required for twice the speed?

centripital force=mv^2/r the mass and radius of the track are constant, which means when only the velocity, (or speed) changes, that change is squared. In other wards, the race car would have 4 times the friction.

A boxer punches a sheet of paper in midair and brings it from rest up to a speed of 25 m/s in 0.05 s. If the mass of the paper is 0.003 kg, what force does the boxer exert on it.

f(net)=a*m a=m/s/s (a=25 m/s/0.05 s)=500 f(net)=500*.003=1.5N

Solve for the force of friction action on a 50 kg carton sliding at 4 m/s if it stops in 3 s.

f=(50)*(4 m/s / 3 s) 50*(4/3) f=66.67N

When the girl in figure 7.14 jacks up a car, how can applying so little force produce sufficient work to raise the car?

fD=Fd. The girl used a little bit of force, over an increased distance, to move the car a little bit with a larger force. (because of the machine)

Is it an inward force or an outward force that is exerted on the clothes during the spin cycle of an automatic washing machine?

inward force centripetal force is occurring.

Using the definitions of momentum and kinetic energy, p=mv and ke=(1/2)mv^2, show, by algebraic manipulation, that you can write ke=p^2/2m. This equation tells us that, if two objects have the same momentum, the one of less mass has the greater kinetic energy.

ke=(1/2)mv^2 p=mv or v=p/m ke=(1/2)m(p/m)^2

Forces of 3.0 N and 4.0 N act at right angles on a block of mass 2.0 kg. How much acceleration occurs.

look at answer to 42 to see the force. a=f(net)/mass a=5/2.0 a=2.5 m/s/s

A firefighter of mass 80 kg slides down a vertical pole with an acceleration of 4 m/s^2. What is the friction force that acts on the firefighters?

mg-f=m*a 80(10)-f=80(4) 800-f=3200/(-800) -f=-480 f=480

A lunar vehicle is tested on Earth at a speed of 10 Km/h. When it travels as fast on the moon, is its momentum more, less, or the same?

p=mv

If a skater who is spinning pulls her arms in so as to reduce her rotational inertia by half, by how much will her angular momentum increase? By how much will her rate of spin increase? (why are answers different)

the angular momentum would stay the same because there is no unbalanced torque action on her. Her spin (rotational speed) would double (2x) when the rotational inertia is reduced by half. The difference between them is because they are affected by different things.

Neglect the weight of the meter-stick and consider only the two weights hanging from its ends. One is a 1-kg weight and the other is a 3-kg weight, as shown. Where is the center of mass of this system (the point of balance)? How does your answer relate to torque?

torque=lever arm*force t(l)=La(l)*f(l)= La(l)*(1 kg* 10 m/s/s)=10 t(r)=la(r)*f(r)=la (r)*(3 kg* 10 m/s/s)=30 t(l)=t(r) la(l)*10=la(r)*30 divide by 30 la(1)*1/3 N=la(r) la(l)+la(r)=1 meter la(l)+(1/3 la(1))=1 Meter 4/3 la(l)=1 meter times both sides by 3/4. la (l)=3/4 meters.

A ball is thrown straight up with an initial speed of 30 m/s. How high does it go, and how long is it in the air?

v(f)=v(o)+gt final velocity=0, v(o)=30 m/s g=-10 t=? 0=30-10t t=3 seconds. d=(1/2)gt^2 d= (1/2)(10)(3^2)=45 meters

If your friend pushes a lawnmower four times as far as you do while exerting only half the force, which one of you does more work? How much more?

w(me)=(1)f*(1)d=1, w(friend)=(1/2)f*(4)d=2 Friend does two times the amount of work I do because my friend goes 4 times the distance.

If raindrops fall vertically at a speed of 3 m/s and you are running at 4 m/s, how fast do they hit your face?

x=3 y=4 x^2+y^2=z^2 9+16=25 square root of 25=5 m/s/s