3.2 Forces in Action

Question 1

Define what mass is

Answer 1

Mass, which is a physical property of an object, is the amount of matter an object contains

Question 2

Describe what happens when a resultant force acts on an object

Answer 2

When a resultant (net) force is acting on an object, it will make the object accelerate in the direction of the net force

Question 3

What is the equation linking the mass, acceleration, and net (resultant force)?

Answer 3

F = ma
- Where Net Force (F) is measured in Newtons (N)
- Mass (m) is measured in kg
- Acceleration is measured in ms⁻²

Question 4

Using the formula F = ma, what happens if we give an object of 1kg mass an acceleration of 1ms⁻²?

Answer 4

Using F = ma, if we give an object of 1kg mass, an acceleration of 1ms⁻², then we say that a force of 1 Newton has been applied to it

Question 5

Define what weight is

Answer 5

Weight is defined as the gravitational force acting upon an object

Question 6

What is g?

Answer 6

g is the acceleration an object in free fall will have, which is 9.81ms⁻² on Earth, but can be different on other planets

Question 7

What formula can we use to determine the weight of an object?

Answer 7

W = mg, where:
- Weight (W) is measured in (N) Newtons
- (M) Mass is measured in kg
- (g) is the acceleration of an object in free fall on Earth - 9.81ms⁻²

Question 8

How do we derive the weight equation W = mg ?

Answer 8

We can derive the formula W = mg from F = ma, where:
- The Force is the Weight
- The acceleration is

Question 9

Describe what Tension is

Answer 9

Tension is the force within a stretched cable or rope

Tip: Tension acts away from the mass

Question 10

Describe what Normal Contact Force is

Answer 10

Normal contact force is the force arising when one object rests against another object. It acts at a 90° angle to the plane of contact.

Question 11

Define what Upthrust is

Answer 11

Upthrust is an upward buoyancy force acting on an object when it is in a fluid.

Definition: buoyancy - the ability or tendency of something to float in water or other fluid.

Question 12

What is friction

Answer 12

Friction is the force that arises when two surfaces rub against each other

Question 13

What are free body diagrams used for?

Answer 13

Free body diagreams are used to model all of the forces that are acting on an object

Question 14

What are the rules for drawing free body diagrams

Answer 14

→ Each force vector is represented by an arrow labelled with the force it represents
→ Each arrow is drawn to scale (the longer the arrow, the greater the force)

Question 15

Do questions on free body diagrams on a slope (Page 52)

Answer 15

Tip: It is often easier to resolve the forces in to the component acting parallel to the slope, and the component acting perpendicular to the slope, rather than horizontally and vertically

Question 16

What is drag?

Answer 16

Drag is the frictional force that opposes the motion of an object travelling through a fluid

Remember, air is also a fluid

In air, it is called air resistance

Question 17

What are the factors that affect the magnitude of the drag force on an object?

Answer 17

The 2 most important factors that affect the magnitude of the drag force:
- The speed of the object
- The cross-sectional area of the object

The other factors that affect the magnitude of the drag force on an object:
- Speed of the object
- Shape of the object
- Roughness or texture of the object
- The density of the fluid through which the object travels

Question 18

What happens to the drag force for an object with a large cross-sectional area

Answer 18

Objects with a large cross-sectional area experience a greater drag force

Question 19

Describe the relationship between the speed of an object travelling through a fluid, and its drag force

Answer 19

For an object travelling through a fluid, including air, the drag force is directly proportional to the square of speed

Question 20

Describe the motion of an object falling through air or any fluid

Answer 20

• At the instant an object starts to fall, there is no drag force on the object. The total force is equal to the weight, W = mg (tip: you can also think of g as the gravitational field strength force being the net force acting on the object)