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NEWTON’S 2ND LAW AND THE FORCE OF

GRAVITY

The student will be able to state Newton’s Second Law and apply it in qualitative and quantitative terms to explain the effect of forces acting on objects.

HOW WILL THE MARBLE ROLL WHEN IT LEAVES THE TRACK?

1. Which object has the most inertia?a) A Featherb) A textbookc) Mr. Findlayd) An Elephant

2. Fuzzy dice are hanging from the rear-view mirror of a car that is travelling forward at constant speed. The dice are:

a) Angled toward the back of the carb) Angled toward the front of the carc) Hanging straight downd) It cannot be determined

Review: Newton’s First Law

3. Fuzzy dice are hanging from the rear-view mirror of a car that is travelling forward and speeding up. The dice are:

a) Angled toward the back of the carb) Angled toward the front of the carc) Hanging straight downd) It cannot be determined

4. Fuzzy dice are hanging from the rear-view mirror of a car that is travelling forward and slowing down. The dice are:

a) Angled toward the back of the carb) Angled toward the front of the carc) Hanging straight downd) It cannot be determined

Review: Newton’s First Law

5. A block of weight 4.0 N is suspended from the ceiling by a piece of string. What is the magnitude of the tension in the string?

a) 4.0 Nb) Less than 4.0 Nc) Greater than 4.0 Nd) Zero

6. A block of weight 4.0 N is being lifted at constant velocity by a piece of string. What is the magnitude of the tension in the string?

a) 4.0 Nb) Less than 4.0 Nc) Greater than 4.0 Nd) zero

REVIEW: NEWTON’S FIRST LAW

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NEWTON’S 2ND LAW OF MOTION AND

WEIGHT

If the net force on an object is not zero, the object will be accelerated in the direction of the net force.

i.e. The more massive the object, the harder it is to change its motion (the higher its inertia).

Newton’s Second Law of Motion

A weightlifter lifts a 165-kg weight by exerting a force of 1.8 kN [up]. The force of gravity on the object is 1.6 kN [down]. Draw a FBD of the weight. What is the net force on the weight? What is the acceleration of the weight?

Example

1.8 kN

1.6 kN

A car is travelling at 25 m/s [forward] when the driver slams on the brakes and stops the car in 3.0 s. Calculate a) the acceleration of the car

b) the net force needed to cause that acceleration if the mass of the car is 1200 kg.

Example Using Equations of Motion

The formula can also be used to calculate the weight, , of an object, which represents the magnitude of the force of gravity acting on it.

If the acceleration, , is due to the acceleration due to gravity, ,

Where the mass, , is measured in kg.Weight is how much force of gravity is pulling down

on an object.Weight is therefore measured in Newtons.The direction is always [down].

Weight (The Force of Gravity)

What is the weight of a 140-kg person?

Example

1. Calculate the weight of the objects of given mass below.

a) Oxygen gas molecule: 5.356 x 10-26 kg5.25 x 10-25 N

b) Penny: 0.00235 kg0.023 N

c) Tennis ball: 0.057 kg0.56 N

d) Student: 61 kg598 N

e) Newborn Elephant: 105 kg1030 N

Practice