9. Forces and their Effects

Question 1

What is a force?

Answer 1

A push or pull acting on an object due to an interaction with another object.

Question 2

What are the two categories that all forces can be split into?

Answer 2

• Contact forces (objects touching).
• Non contact forces (objects separated).

Question 3

Give three examples of contact forces.

Answer 3

1) Friction
- Resistive forces between moving surfaces.
- Acts opposite to object’s motion.
- Air resistance is a type of friction.
- Lubrication reduces heat loss due to friction.

2) Tension

3) Normal contact force
- Acts in the opposite direction to the force an object applies on a surface.

Question 4

Give three examples of non-contact forces.

Answer 4

1) Gravitational
- Acts between masses.

2) Electrostatic
- Acts between charges.

3) Magnetic fields
- Magnetic poles exert non-contact forces on each other.

Question 5

Is a force a vector or a scalar quantity? Why?

Answer 5

Vector because it has both magnitude and direction.

Question 6

What is a scalar quantity?

Answer 6

• A quantity that only has a magnitude.
• A quantity that isn’t direction dependent.

Question 7

What is a vector quantity?

Answer 7

A quantity that has both magnitude and direction.

Question 8

Give three examples of vector quantities.

Answer 8

• Velocity
• Displacement
• Force

Question 9

Give examples of scalar quantities.

Answer 9

• Temperature
• Time
• Mass
• Speed
• Distance
• Energy

Question 10

How are vectors represented?

Answer 10

• Vector arrows
• Length indicates magnitude
• Arrow indicates direction

Question 11

How can you calculate the resultant magnitude and direction of forces using a scale drawing?

Answer 11

• Draw the component forces as scaled arrows, joined tip-to-tail.
• Draw a line connecting the two open ends.
• Measure the length of this line and convert into the magnitude.
• Measure the angle between the resultant line and the vertical/horizontal to find the direction.

Question 12

How does lubricating levers/gears improve efficiency?

Answer 12

There is less resistance; less energy is wasted/dissipated (as heat) overcoming frictional forces.

Question 13

What is a free body diagram and what does it show? (learn how to draw)

Answer 13

• Shows the direction of forces that are present in a situation.
• The reaction force always acts at the normal to the line of contact, from the point of contact.
• Friction acts in the opposite direction to movement, along line of contact.
• Weight always acts downwards, acting from the centre of mass.

Question 14

What is a scale drawing and what does it show? (learn how to draw)

Answer 14

• The length of each arrow represents its size (in relation to the other forces acting on the object). So direction with larger arrows show resultant force.
• If arrows are in opposite directions with equal length (equal in magnitude but opposite in direction), the forces cancel out so the object is in equilibrium so travels at a constant velocity.

Question 15

Forces on a skydiver, explain these changes.

Answer 15

Forces that act are air resistance and weight.
- Initially, the skydiver has no air resistance and the only force acting on him is weight.
- As he falls, he accelerates, increasing his speed.
- This makes air resistance increase.
- Therefore the resultant force decreases.
- Therefore, acceleration decreases as F=ma, so he is not speeding up as quickly.
- Eventually weight and air resistance are equal and balanced, so there is no resultant force.
- So there is no acceleration and terminal velocity is reached.

Question 16

Forces on a vehicle, explain these changes.

Answer 16

• Initially, low air resistance and thrust is only hindered by friction.
• Air resistance increases, decreasing resultant force.
• Eventually the car is travelling at terminal velocity, where the thrust is balanced by drag and friction, so no resultant force acts.
• (This is immediately changed when more thrust is added, as it now becomes the resultant force until the drag increases to balance it again.)