Forces in Action

Question 1

What is needed for an object to accelerate?

Answer 1

A net resultant force, if there is no resultant force acting on a object, then the object is in equilibrium

Question 2

What is the definition of a Newton?

Answer 2

The force that causes a mass of 1 kilogram to have an acceleration of 1 ms^-2

Question 3

What is the centre of mass?

Answer 3

A point where the entire weight of an object appears to act

Question 4

How do you find the centre of mass of a symmetrical object?

Answer 4

The centre of mass will be at its centre, the point where the symmetry lines of an object cross.

Question 5

How do you find the centre of mass of an irregular object?

Answer 5

1. Suspend the shape from a point
2. Use plumb line to mark a vertical line on the shape from the point
3. Suspend from another point
4. Use a plumb line to mark a vertical line on the shape from the second point
5. Where the lines intersect gives the position of the centre of mass

Question 6

When is an object stable?

Answer 6

An object is stable if the line of action of its weight falls within its base, an object will topple if it is outside of the base.

Stable objects have a low centre of mass and a wide base.

Question 7

What is drag?

Answer 7

The frictional force experienced by an object travelling through a fluid.

Question 8

What is a fluid?

Answer 8

A liquid or a gas (something that can flow)

Question 9

What 3 factors affect drag?

Answer 9

Viscosity of the fluid, speed of the object, shape and size of the object

Question 10

What 3 things do frictional forces do?

Answer 10

Always act in opposite direction to the motion of an object.

Can never speed things up or start moving something.

Convert kinetic energy to heat energy.

Question 11

What is terminal velocity?

Answer 11

The maximum speed reached by an object when the drag force on it is equal and opposite to the force causing the motion of the object..

Question 12

Describe and explain the motion of a skydiver before they opens the parachute.

Answer 12

1. Immediately after jumping
   
   • Speed = 0
   • Drag = 0
   • Net Force = Weight
   • Acceleration = g
2. Before terminal velocity (during free fall)
   
   • Speed has increased
   • Drag has increased
   • Net force has decreased (net force = weight - drag)
   • Net force F=ma therefore acceleration is less than g
3. At terminal velocity (during free fall)
   
   • Speed has increased to terminal velocity
   • Drag has increased
   • Net force = 0 because weight = drag
   • F=ma so acceleration is 0

Question 13

Describe and explain the motion of a skydiver after they open the parachute.

Answer 13

1. Immediately after parachute has opened
   
   • Drag has greatly increased
   • Net force is large and upwards
   • F=ma therefore acceleration is large and upwards
2. At a time before terminal velocity
   
   • Speed has decreased
   • Drag has decreased
   • Net force has decreased
   • F=ma therefore acceleration has decreased
3. At terminal velocity
   
   • Speed has decreased to terminal velocity
   • drag has decreased
   • net force = 0 because weight = drag
   • acceleration is 0

Question 14

What is density?

Answer 14

Mass per unit volume.

Question 15

What is pressure?

Answer 15

Force applied per unit area.

Question 16

What does pressure acting on an object depend on?

Answer 16

Depth of the object in the fluid, density and acceleration due to gravity.

Question 17

What is upthrust?

Answer 17

Upward force that fluids exert on objects that are completely or partially submerged in a fluid.

Question 18

What causes upthrust?

Answer 18

Top and bottom of a submerged object are at different depths. Since p = hpg, there is a difference in pressure which causes upthrust.

Question 19

What is Archimedes’ Principle?

Answer 19

An upthrust force in Newtons acting on a immersed object is equal to the weight of the fluid displaced.

Question 20

What is the equation for upthrust?

Answer 20

volume displaced * density of fluid * g

Question 21

What is a moment?

Answer 21

The product of the force and the perpendicular distance between the line of action of the force and the pivot point.

Turning effect of the force.

Question 22

When is an object in equilibrium?

Answer 22

• Resultant forces acting on it must be zero
• Resultant moment must be zero (sum of anticlockwise moments = sum of clockwise moments)
• Lines of action of the three forces must all pass through the same point

Question 23

What happens when the resultant moment of an object is not zero?

Answer 23

It will rotate with angular acceleration.

Question 24

What is the principle of moments?

Answer 24

For a body in equilibrium, the sum of clockwise moments about a point are equal to the sum of the anticlockwise moments about the same point. The resultant moment about a point must be zero.

Question 25

What is a couple?

Answer 25

A pair of equal and opposite parallel forces, whose lines of action do not cross and tent to produce rotation only.

Question 26

What does a couple produce?

Answer 26

A couple does not cause any resultant linear force, but does produce a turning effect which is called a torque.

Question 27

When are two forces a couple?

Answer 27

Equal in magnitude

Opposite in direction

Parallel to each other

Question 28

What is Torque?

Answer 28

The product of one of the forces in couple and the perpendicular distance between the forces.

Torque = One of the Forces x Perpendicular distances between forces