PHYSICS 6-10

In an ideal pulley system, a woman lifts a 100-N crate by pulling a rope downward with a force of 25 N. For every 1-meter length of rope she pulls downward, the crate rises

0.25m Win=Wout (Fd)in=(Fd)out Dout=(Fin/Fout)(Din) =(25/100)(1) = 1/4

IN AN IDEAL PULLEY SYSTEM, A WOMAN LIFTS AN 80-N CRATE BY PULLING A ROPE DOWNWARD WITH A FORCE OF 20 N. FOR EVERY 2-METER LENGTH OF ROPE SHE​PULLS DOWNWARD, THE CRATE RISES

0.5 Win=Wout (Fd)in=(Fd)out Dout=(Fin/Fout)din=20/80x2

Suppose the potential energy of a drawn bow is 50 joules and the kinetic energy of the shot arrow is 40 joules

10 joules go to warming the bow (total energy of the drawn bow, which includes the poised arrow, is 50 joules. The arrow gets 40 joules and the remaining 10 joules warms the bow—still in the initial system)

WHEN A FARMER THROWS A 3-KG PUMPKIN AT A SPEED OF 4 M/S ONTO A COMPOST PILE, THE PUMPKIN'S MOMENTUM IS

12kgxm/s p=mv p=(3)(4) = 12

A certain machine is 30% efficient. This means the machine will convert​

30% of the energy input to useful work—​70% of the energy input will be wasted

A ladybug sits halfway between the rotational axis and the outer edge of the turntable. When the turntable has a rotational speed of 20 RPM and the bug has a tangential speed of 2 cm/s, what will be the rotational and tangential speeds of her friend who sits at the outer edge?

4 cm/s (V=rw) Rotational speed w of both bugs is the same, radical distance doubles, tangential speed doubles)

When two identical cars, one traveling twice as fast as the other, brake to a stop using ​old-fashioned brakes, the faster car will skid

4 times as far W=KE d=1/2Fmv^2 =1/2Fm(2v)^2 d=4(1/2Fmv^2)=4d1

WHEN YOU DO 2000 J OF WORK ON A CAR JACK AND 1200 J OF ENERGY IS LOST TO HEAT, THE EFFICIENCY OF THE JACK IS

40% Efficiency=useful energy output/total energy input E=2000-1200/2000 E=0.4

A SHINY SPORTS CAR AT THE TOP OF A VERTICAL CLIFF HAS A POTENTIAL ENERGY OF 1000 J RELATIVE TO THE GROUND BELOW. UNFORTUNATELY, A MISHAP OCCURS AND IT FALLS OVER THE EDGE. WHEN IT IS HALFWAY TO THE GROUND, ITS KINETIC ENERGY IS

500 J

WHICH OF THE FOLLOWING EQUATIONS IS MOST DIRECTLY USEFUL FOR SOLVING A PROBLEM THAT ASKS FOR THE DISTANCE A SPEEDING VEHICLE SKIDS IN COMING TO A STOP?

Fd=(1/2mv^2)

When a dish falls, will the change in momentum be less if it lands on a carpet than if it lands on a hard floor?

No both are the same (final momentum becomes zero in both cases, so both change by the same amount. Although the momentum change and impulse are the same, the force is less when the time of momentum change is extended. Be careful to distinguish among force, impulse, and momentum.)

How much power is required to raise a 200-kg crate a vertical distance of 2 m in a time of 4 s?​

P=W/t P=400/4=1000W

A pair of rulers standing nearly upright against a wall. What if one ruler has a massive hunk of clay stuck to its top end? Will it rotate to the floor in a longer or shorter time than the ruler with no clay?

The ruler with no clay will hit the ground first (More mass is concentrated at a further distance from the axis of rotation and tends to resist any change in its rotational motion)

Work is done in lifting a barbell. How much work is done in lifting a barbell that is twice as heavy the same distance?

Twice as much (W=Fx)

How much work is done on a 200-kg crate that is hoisted 2 m in a time of 4 s?​

W=Fx=mgh W=(200)(10)(2)=4000J

How much work is needed to lift a bag of groceries that weighs 300 N to a height of 2 m? How much work is needed to lift the bag twice as high?

W=Fy W=(300)(2)=600J W=(300)(4)=1200J

If the arrow travels to the right in a straight-line path with a kinetic energy of 40 J and penetrates one-tenth of a meter into a target, what is the average force exerted on the arrow?

W=KE 400N to the left

A fast-moving car hitting a haystack or hitting a cement wall produces vastly different results 1.Do both experience the same change in momentum?​ 2.Do both experience the same impulse?​ 3.Do both experience the same force?

Yes for 1 and 2 (STOPPING THE MOMENTUM IS THE SAME WHETHER DONE SLOWLY OR QUICKLY, THE FORCE IS VASTLY DIFFERENT. BE SURE TO DISTINGUISH AMONG MOMENTUM, IMPULSE, AND FORCE)

An average force of 50 N is exerted on a 4 kg cart for 2 s a. What is the impulse?​ b. What is the change in momentum?​ c. What is the change in velocity?​

a. Impulse I=Ft I=(50)(2) = 100Ns b. Change in momentum I=p=100Ns c. Change in velocity I=p=mv v=I/m v=100/4 = 25m/s

YOUR FRIEND SAYS THAT IMPULSE EQUALS MOMENTUM. YOUR FRIEND'S STATEMENT IS NOT CORRECT, AND THE MISSING WORD IS​

change I=Ft=p

When the speed of an object is doubled, its momentum​

doubles (p=mv)

A SIMPLE MACHINE CANNOT MULTIPLY​

energy 1000J=Win=Wout=1000J (Fdin)=(Fdout) 1000J=(500x2m)in=(250x4)=1000J

Truck A is moving toward identical Truck B that is at rest. Both trucks have the same mass. When they collide, both trucks merge together. Compared with the initial speed of truck A, the speed of the merged trucks is​

half Pi=Pf mv+mv=(m+m)vf mv+0=(m+m)vf mv=2mvf vf=v/2

When a car is braked to a stop, its kinetic energy is transformed to

heat

BIG FISH COASTS UP TO AND SWALLOWS A SMALL FISH AT REST. RIGHT AFTER LUNCH, THE SPEED OF THE BIG FISH IS

less than before

A moving object has

momentum energy speed

NETS USED TO CATCH FALLING BOULDERS ON THE SIDE OF ROCKY HILLSIDE ROADWAYS ARE MORE EFFECTIVE THAN RIGID FENCES BECAUSE THEIR BREAKAGE IS REDUCED BY​

more time to change the momentum of falling rocks

If you push against a stationary brick wall for several minutes, you do no work​

on the wall (W=Fx)

STANDING STILL ON A SKATEBOARD, YOU CATCH A BALL TOSSED HORIZONTALLY TO YOU. THE MASS OF THE BALL IS ONE-TENTH YOUR MASS. COMPARED WITH THE SPEED OF THE CAUGHT BALL, THE SPEED THAT BOTH YOU AND THE CAUGHT BALL ACQUIRE IS​

one-eleventh as much

A PENDULUM BOB SWINGS TO AND FRO. ITS KINETIC ENERGY AND ITS POTENTIAL ENERGY RELATIVE TO THE BOTTOM OF ITS SWING ARE THE SAME AT

one-half the vertical distance between the bottom and the top of the swing

WHEN BOTH THE FORCE AND TIME OF CONTACT ARE DOUBLED, THE IMPULSE ON AN OBJECT IS​

quadrupled I=Ft

Which vehicle has a smaller momentum? A 2000 kg car moving at 30 m/s or a 4000 kg light truck moving at 15 m/s.

same momentum (p=(2000)(30) = 60000) (p=(4000)(15) = 60000)

WHEN A FALLING OBJECT BOUNCES AS IT HITS THE GROUND

the impulse on it is greater than for stopping it.​ its change in momentum is greater than for stopping. (Total impulse is greater when objects bounce)

A LUNAR VEHICLE IS TESTED ON EARTH AT A SPEED OF 10 KM/H. WHEN IT TRAVELS AT THE SAME SPEED ON THE MOON, ITS MOMENTUM IS ​

the same (m on the moon is the same on earth)

A BALL IS TOSSED INTO A BALE OF HAY AND COMES TO A STOP. IF INSTEAD, IT COMES TO A STOP WHEN HITTING A STICKY SOLID WALL, THE IMPULSE NEEDED TO STOP IT IS​

the same I=p=m(vf-vi)

IF A CAR COMES TO A FULL STOP WHEN HITTING A HAYSTACK AND WHEN HITTING A STONE WALL, THE CHANGE IN MOMENTUM IS GREATER FOR HITTING THE

Same momentum change in both cases I=p=m(vf-vi)

Which vehicle has a greater momentum? A truck at rest or a man walking 3 mph?

The man (truck p=(1000)(0) = 0) (man p=(150)(3) = 450)