# Forces

Question 3

What force is required to give mass m = 40 kg acceleration a = 2 m/c2 ?

Answer

60 N

80 N

40 N

20 N

Question 4

What force is required to keep mass m = 20 kg

moving with a constant speed along straight line?

Answer

100 N

80 N

60 N

No force required

Question 5

Two boxes have the same size, but one box is empty and the other has books inside.

The same force is applied to each box. What box will have the bigger acceleration?

Answer

Empty box

Box with books

Boxes will have the same acceleration

Impossible to make a conclussion

Question 6

According to the Second Newton's Law,

if you drop the ball it will fall down with acceleration

(because of gravity force).

Gravity on a Moon is less than on Earth.

If you drop the same ball on the Earth and on the Moon

Answer

acceleration on the Earth will be bigger than on the Moon

acceleration on the Earth will be less than on the Moon

acceleration will be the same on the Earth and on the Moon

ball will not fall down on the Moon

Question 8

Gravity force applied to the monitor is 50 N.

Monitor is on a horizontal surface of table and doesn't move.

What is the magnitude and direction of surface reaction?

Answer

50 N directed horizontally

50 N directed down

50 N directed up

There is no surface reaction because monitor doesn't move

Question 9

Imagine a book on the horizontal surface. without motion (at rest).

Gravity force pushing the book down with force 10 N,

but the book is remaining on the surface without motion (at rest).

What is the Net Force applying to the box?

Answer

The Net Force is 10 N and directed up

The Net Force is 10 N and directed down

The Net Force is 20 N

The Net Force is zero

Question 10

Following two forces applying to the object:

one 30 N to the right (------------------->)

another 10 N to the left (<-----)

What is the Net Force?

Answer

-----> 10 N

----------> 20 N

---------------> 30 N

10 N <-----

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#### Solution Summary

This solution explains how to solve several problems dealing with forces.

Velocity, conservation of momentum, force, force of gravity

1. A ball is thrown in the air with enough force so that it goes straight up for several seconds.

a. What is the velocity of the ball 1 sec before it's highest point?

b. What is the change in velocity during this 1 sec interval?

c. What is the velocity 1 sec after it reaches it's highest point?

d. what is the change in velocity during this 1 sec interval?

e. What is the change in velocity during the 2 sec interval?

f. What is the acceleration of the ball during any of these time intervals and at the moment the ball has a zero velocity?

2. An airplane is flying horizontally with speed 1000 km / h ( 280 m/s) when the engine falls off. Neglecting air resistance, if it takes 30 sec for the engine to hit the ground:

a. How high is the plane?

b. How far horizontally will the engine travel while it falls?

c. If the airplane somehow continues to fly as if nothing happened, where is the engine relative to the airplane at the moment the engine hits the ground?

3. A bicycle has wheels with a circumference of 2m. What is the linear speed of the bicycle when the wheels rotate at 1 revolution per second?

4. Before going into orbit, an astronaut has a mass of 55kg, While in orbit, a measurement determines that a force of 100N causes her to move with an acceleration of 1.90 m/s2. to regain her original weight, should she diet, or eat more?

5. A railroad car weighs four times as much as a freight car. If the railroad car coasts at 5 km/h into the freight car that is initially at rest, how fast do the two coast together after they are coupled?

6. A 60kg skydiver moving at terminal speed falls 50m in 1 sec. What power is the skydiver expending on the air?

7. To tighten a bolt, you push with a force of 80N at the end of a wrench handle that is 0.25m from the axis of the bolt:

a. What torque are you exerting?

b. If you move your hand to 0.10 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?

8. The value of g at the earth's surface is about 9.8m/s2. What is the value of g at a distance from the earth's center that is 4 times the earth's radius?

9. A 3 kg newborn at the earth's surface is gravitationally attracted to earth with a force of about 30N.

a. Calculate the force of gravity with which the baby on earth is attracted to the planet Mars, when Mars is closest to earth. (The mass of Mars is 6.4 x 1023 kg, and it's closest distance is 5.6 x 1010 meters)

b. Calculate the force of gravity between the baby and the physician who delivers it. Assume the physician has a mass of 100kg and is 0.5 m from the baby.

c. How do the forces compare?

10. Does the speed of a falling object depend on its mass? Does the speed of a satellite in orbit depend on its mass? Defend your answer.

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