1.3 Dynamics

Question 1

An isolated system is one on which

Answer 1

no external forces act and no particles enter or leave

Question 2

Conservation of momentum

Answer 2

The vector sum of the momenta of the bodies in a system is constant provided there is no resultant force from outside the system

Question 3

Newton’s three laws of motion:

Answer 3

1. A body’s velocity will be constant unless a force acts upon it
2. The rate of change of momentum of a body is directly proportional to the resultant force acting upon it
3. If a body A exerts a force on body B, then B exerts an equal and opposite force on A

Question 4

Momentum, p=

Answer 4

mv

Question 5

A rider on an air track sits on a cushion of air so that

Answer 5

• there is zero resultant vertical force because the upward force on the rider due to air pressure is equal and opposite to the downward gravitational force
• there is zero frictional force on the rider and the force due to air resistance is very small

Question 6

If kinetic energy is lost in a collision

Answer 6

it is an inelastic collision

Question 7

If no kinetic energy is lost in a collision

Answer 7

it is an elastic collision

Question 8

It is harder to solve elastic collisions as there are two unknown velocities so

Answer 8

we need to solve simultaneous equations

Question 9

If several forces act on an object, it is the resultant force which is equal to

Answer 9

change in p/ change in t

Question 10

If change in p/ change in t = 0 then resultant force=

Answer 10

0 (this is Newton’s first and second laws)

Question 11

The gradient of a momentum-time graph is

Answer 11

the resultant force

Question 12

For two things to be a N3 pair…

Answer 12

1. They must be acting on different bodies

2. The forces must be of the same kind

Question 13

Static and dynamic friction…

Answer 13

oppose relative motion between surfaces

Question 14

If a surface would slide to the right, the frictional force on it is to the

Answer 14

left

Question 15

The normal force, Fn is

Answer 15

the force experienced if an object rests on a surface. The bodies experience a force at right angles to the surface (hence normal)

Question 16

An object on a slope will remain stationary as long as

Answer 16

the gradient isn’t too great. This is because static friction acts on the box up the slope, which counteracts the component of the weight (W cos theta) down the slope

Question 17

An object will start to accelerate down a slope if

Answer 17

(W cos theta) is larger than limiting friction, or Fr

Question 18

The force which opposes the relative motion when one surface slides over another is

Answer 18

dynamic friction

Question 19

Air resistance is one example of

Answer 19

drag