Chp 4: Forces

Question 1

define hooke’s law

Answer 1

Force is directly proportional to extension, provided that the elastic limit has not been exceeded

Question 2

define elastic limit

Answer 2

point beyond which the spring does not return to its original length when the load is removed

Question 3

define pressure

Answer 3

Pressure is defined as the force per unit area, where the force is acting at right angles to the area.

Question 4

define upthrust

Answer 4

Upthrust is the vertical upward force exerted on a body by a fluid when it is fully or partially submerged in the fluid, due to the difference in fluid pressure where the pressure at the bottom is larger than the pressure at the top.

Question 5

define archimedes’ principle

Answer 5

It states that the upthrust on a submerged object is equal and opposite to the weight of the fluid displaced by the object.

Question 6

state principle of moments

Answer 6

For a body to be in rotational equilibrium, the sum of all the clockwise moments about any point must be equal to the sum of all the anticlockwise moments about the same point.

Question 7

define moment

Answer 7

The moment of a force about a point is defined as the product of the force and the perpendicular distance from the point to the line of action of the force.

Question 8

define couple

Answer 8

a pair of equal and opposite parallel forces whose lines of action do not coincide

Question 9

define torque of a couple

Answer 9

product of one force and the perpendicular distance between the 2 forces.

Question 10

define centre of gravity

Answer 10

single point at which its entire weight of the body appears to act (ONLY FOR UNIFORM BODY)

Question 11

Explain why the upthrust acting on a human body when in air is normally ignored

Answer 11

The average person weighs about 600N and the upthrust in air of about 1N is less than 0.2% of the weight of the person, making it negligible.

Upthurst = m(f)g (since mass of air is small, thus upthrust is negligible????)

Question 12

What does it mean when an object floats in a fluid?

Answer 12

Means that upthrust = -(Weight of object)

Question 13

What does it mean when an object sinks in a fluid?

Answer 13

Means that upthurst < weight

Question 14

The position of centre of gravity is not at the midpoint of the raft. Suggest what this implies about the DISTRIBUTION OF MASS in the raft

Answer 14

The left side of the raft is heavier than the right side of the raft

Question 15

State the origin of upthrust.

Answer 15

The liquid exerts pressure forces on every point of the object. The liquid pressure increases with the depth of the liquid. Therefore, the pressure at the bottom of the object is larger than the pressure at the top of the object, since P=F/A there is a resultant upward force called upthrust acting on the object upwards.

Question 16

State the conditions required for a body to be in equilibrium.

Answer 16

The net external force in any direction acting on the body is zero. The net torque on the body about any axis is zero.

Question 17

Explain if the following systems are in equilibrium.
1.A ball falling at constant speed in air:

2.A satellite in a circular Earth orbit:

Answer 17

1. Since the ball is falling at constant speed, it has no acceleration. By N1L, there is no net force acting on the object. Since the object has no net torque and net force, it must be in equilibrium.
2. Since the satellite is in circular orbit, satelite is constantly changing direction. Since velocity is a vector, the velocity changes too. So, satellite must be accelerating. By N2L, there must be a net force acting on the satellite which is Fg providing Fc(by Earth on the satellite). Hence the satellite is NOT in equilibrium.

Question 18

State and explain the effect of air resistance on the acceleration of a ball falling through a long distance.

Answer 18

When object falls, air resistance on object is upwards. Since air resistance increases w speed, air resistance increases as object accelerate due to gravity. Air resistance increasing will decrease a so magnitude of mg = ma ==> terminal velocity

Question 19

Drag is sometimes referred to as fluid friction. Describe a way in which drag, and friction are similar, and away in which they differ.

Similarity:

Differences:

Answer 19

Similarity: Both are resistive forces which oppose motion. Hence, thermal energy will be dissipated.

Differences: Drag depends on speed while friction is independent of speed. Drag depends on the cross-sectional area of the body, shape of object and viscosity of the fluid medium but friction depends on the coefficient of friction (nature of the two solids in contact) and the magnitude of the normal contact force.

Question 20

A bungee jumper momentarily comes to rest at the bottom of the dive before he springs back upward. At that moment, is the bungee jumper in equilibrium? Explain your answer.

Answer 20

Since he is moving upwards at the next moment, there must be a net force acting upwards on him at the bottom of the dive. Therefore, he is not in equilibrium. If he was in equilibrium, he would remain at rest (according to Newton’s first law.)

Question 21

For the car, motion is impossible without friction. Discuss what is meant by friction and the direction in which it acts on the car. In your answer, suggest another example where friction is useful.

Answer 21

A frictional force is a resistive force that arises when two surfaces move relative to each other or are in attempted motion. Friction always opposes motion. Thermal energy is usually produced due to friction. As the wheels rotate, they exert a backward force on the road. By N3L, road will exert a forward frictional force of the same magnitude on the wheels. This enables the car to move.

Friction is useful in braking. When the brakes are applied, the wheels tend to stop rotating and rub against the road. Hence during braking, friction acts on the wheels in the opposite direction to the car’s motion. This decelerates the car and brings it to rest.

Question 22

Explain why, when ice floating in a jug of water melts, there is no change in the level of the water.

Answer 22

When ice floats, its weight of ice = weight of water displaced. When ice fully melts, weight of water produced = weight of ice = weight water displaced. Thus, when ice melts, the water formed will exactly replace the water displaced by the ice. There is no change in volume and so no change in water level.

Question 23

The beam has mass M and length 8.0 m. The cable makes an angle of 32o to the beam and has a tension T. A pile of bricks of total mass m is placed on the beam. Describe and explain what would happen to the tension T in the cable as the pile of bricks is slowly moved towards the end P of the beam.

Answer 23

By POM, taking pivot at O, as bricks moves towards the end P, clockwise moments increase. Hence the anticlockwise moments by T have to increase. So, T will increase since the perpendicular distance from T to O is fixed.