Force, motion and gravitation

Question 1

What is an object’s mass? What is Newton’s first law?

Answer 1

Its measure of inertia (the capacity of that object to remain motionless or to move with a constant velocity if the sum of the forces acting upon it is zero.

This definition is from Newton’s first law: A body remains at rest or in motion with a constant velocity unless acted upon by an external force

Question 2

What point on an object always has the simplest motion of all the points of that object?

How is this different form the centre of gravity?

Answer 2

The centre of mass of that object. This is the point that can be described as it moves through space

The centre of gravity is like the centre of mass, but it can change depending on the gravitational forces working on it and its position in space and relative to other objects.

Question 3

When is an object in stable equilibrium?

Answer 3

If the centre of gravity is as low as possible and any change in orientation will lead to an elevation of the centre of gravity

An object is in unstable equilibrium if the COG is high relative to the support point or surface and any change in orientation will lead to lowering of the COG

Question 4

What is ‘weight?’ Give the formula for weight

Answer 4

A force (ie Newton’s pounds). It is a vector (unlike mass), though it is proportional to mass.

It is the product of the mass by the vector gravitational acceleration ‘g’

W = m x g

Question 5

What is Newton’s second law? How can it find acceleration (formula)? What is the limitation of this?

Answer 5

The sum of all the exterior forces acting upon the centre of mass of a system is equal to the product of the mass of the system by the acceleration of its centre of mass.

Therefore, if there is a net force, the object must accelerate. It is a vectorial equality which asserts that a net force against an object must result in acceleration.

ΣF = m x a

This only works for the centre of mass of a system in translational motion

Question 6

What is Newton’s third law?

Answer 6

For every action there is an equal and opposite reaction. If one object exerts a force, F, on a second object, the second object exerts a force, F’, on the first object. F and F’ have opposite direction, but the same magnitude.

Question 7

Give the equation of free fall motion derived from Newton’s Second Law

Answer 7

ΣF = ma

ΣF represents all the forces acting on the object, m is the mass of the object and a is the acceleration of the centre of mass of the object (which can be replaced by g) to give the following equation:

g = Kg(Mass of Earth / distance squared from centre of earth)

Question 8

What three values of gravitational acceleration must be memorized?

Answer 8

32 ft/s^2

980 cm/s^2

9.8 m/s^2

Question 9

How is terminal velocity of a free falling object reached?

Answer 9

Terminal velocity is reached when the drag force (resistance of air from location of earth, shape and size of the object and velocity of the object) increases to a point where it is equal to the force of gravity.

Question 10

What is projectile motion? What are the two components of it?

Answer 10

Motion of any object fired or launched at some angle from the horizontal

• Vertical component (affected by g)
• Horizontal component (independent of g)

Question 11

When calculating motion of an object down a slop (due to gravity), do you use mass or weight? Why?

Answer 11

Weight, mass is irrelevant in this case.

Question 12

What are the forces acting on a pulley system?

Answer 12

• Acceleration from the masses of the objects attached

- Tension in the string (which is a force)

Question 13

How can you find the final velocity of an object if you are given:

• Initial velocity
• Acceleration
• Time

Answer 13

Final velocity = Vi + at

Question 14

How can you find the final velocity of an object if you are given:

• Initial velocity
• Acceleration
• Displacement

Answer 14

Vf^2 = Vi^2 + 2ay

Question 15

What does the force of gravity (10 m/s^2) actually stand for? (eg. how is it calculated)

Answer 15

Like any other gravitational force it is F=mg,

where g = G(Mass of Earth)/(Earth radius)^2

Question 16

What is the downward ACCELERATION of ANY object in freefall?

Answer 16

10 m/s^2

This is because the force of gravity is proportional to the mass, while acceleration is inversely proportional to the mass. Against the earth, the mass of the object is negligible.

Question 17

A car goes around a curve to the left. What force provides the centripetal force?

Answer 17

The driver turns her wheels to the left, so the wheels exert a force on the road to the right. By Newton’s third law of motion, the road exerts a force to the left on the wheels, turning the car left. In this case, friction provides the centripetal force.

(Think what would happen if there were no friction)

Question 18

You are driving a car and veer to the left, your groceries in the passenger seat hit the passenger door. Why?

Answer 18

This is NOT due to centripetal force. It is simply Newton’s first law. The groceries are travelling in a linear direction when the car door veers into the grocery’s path.

Question 19

What is tangential acceleration? Give the equation for it.

Answer 19

An object in circular motion is acted on by tangential acceleration, which is responsible for changing the speed of the object.

It is (change of speed)/(change of time)

Centripetal acceleration is always pointing towards the middle of the circle. Tangential acceleration points in the direction of movement and is trigonometrically summed with centripetal acceleration to yield the overall acceleration of an object in circular motion.

Question 20

A 10 kg mass is moving along a surface at a uniform velocity of 2 m/s. The mass is opposed by a 50N frictional force as it moves.

g = 10 m/s^2

What is the coefficient of friction between the mass and the surface?

Answer 20

μ = (frictional force)/(normal force)

μ = (50 N)/(100N)

μ = .5

Question 21

A mass (M) is lifted from rest at a constant acceleration (a). The tension in the cord would be what:

A. Mg
B. Ma
C. Mg + Ma
D. Mg - Ma

Answer 21

C. Mg + Ma

Weight (mg) points downward and tension (T) points upward. The net vertical force therefore is:

F = T - Mg = Ma

T = Mg + Ma