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# Newton's second law of motion

AP.PHYS:
INT‑3.B (EU)
,
INT‑3.B.1 (EK)
NGSS.HS:
HS‑PS2‑1
,
HS‑PS2.A.1
,
HS‑PS2.A
,
HS‑PS2
Newton's second law of motion is F = ma, or force is equal to mass times acceleration. Learn how to use the formula to calculate acceleration. Created by Sal Khan.

## Want to join the conversation?

• If we have a car with a mass of 500 kg and a constant velocity 50 mph and it hits some wall what force will be applied to the wall? As the velocity is constant the acceleration would be zero and substituting in the 2nd law F = 500 x 0 = 0. Which is impossible - please explain.

Second question:
If we throw a ball in the space where there are no forces﻿ at all ( gravity , friction ...etc ) and we applied a 500 N force on the ball which had a mass of 2 KG, then:
500 = 2 x a
a = 250 meter per second

So in the first second the ball velocity would be 250 m /s and after 2 second the velocity would be 500 m /s and in the second n velocity would be 250n. The velocity will approach infinity which is impossible since nothing can go faster then the speed of light •   In the first question, the acceleration is not zero. It maybe zero before the car hits the wall, but when it hits the wall, the car's speed goes from 50 mph to 0 mph in a very short space of time. This is a large deceleration (i.e. acceleration in the opposite direction to its movement), hence it experiences a very large force in the opposite direction of movement. This is why having a large crumple zone at the front of the car is important as it allows the car to decelerate more slowly.

In the second question, as soon as you let go of the ball, it is no longer accelerating (since there is no force acting on it). It will continue at the same speed that it had when it left your hand. Similarly, on Earth, when you throw a ball, it is travelling fastest at the instant it leaves you hand. It slows due to friction with the air and then the ground.
• Why is it valuable to recognize scalar and vector values? I understand the difference between them, but I don't understand the practicality of it. Thanks. •   let's say your driving North at 50 mph for an hour (which is a vector because it has a magnitude, 50mph, and a direction, North), then you know you went 50 miles North, rather than just 50 miles in ay direction, and if you're like me then you might want to know which direction you're driving in.
• what exactly is a vector force? • I understand the whole math part of the formula (it's pretty simple), but can anyone tell me what he means by 5 m/s^2? is it just saying that this object of mass is moving at a speed of 5 meters per second? Why is seconds squared? • 5 meters per second is a rate, but acceleration is a change in rate, so 5 meters per second per second. this would look like 5m/sec/sec. If you apply algebra to this, that would be the same as 5m/sec *1/sec, because dividing is the same as multiplying by the reciprocal. multiply it out and you get 5m/sec^2.
• Can someone give me a quick explanation of what vectors and scalars are? • how do objects hit the floor at the same time • HI Jorge Garcia,

This only stands true when there is no air resistance present.

Suppose that an bowling ball and a tennis ball are dropped off a cliff at the same time. To understand this we must use Newton's second law - the law of acceleration (acceleration = force/mass). Newton's second law states that the acceleration of an object is directly related to the net force and inversely related to its mass. Acceleration of an object depends on two things, force and mass. This shows that the bowling experiences a much greater force. But because of the big mass, it resists acceleration more. Even though a bowling ball may experience 100 times the force of a tennis ball, it has 100 times the mass. So, the force/mass ratio (from the equation acceleration = force/mass) is the same for each. Therefore, the acceleration is the same and they reach the ground at the same time.

Hope that helps!
- JK
• hi there , I had a doubt in newtons laws of motion could you pls help me .....
a person kicks a 1kg football to score a goal. When he kicks a 1kg brick , his foot gets hurt .give a reason for it. thank you • What happens to the shape of the football and the brick when kicked? The football deforms and then elastically rebounds where as the brick is rigid and doesn't deform.

The deformation of the football increases the amount of time that the force of the kick is spread out so to transfer the momentum from the foot to the football is done at a slower rate over a longer time requiring a lower force.

The brick being rigid the momentum transfer has to occur quicker so there is more force on the foot and brick making it more painful and more likely to cause damage to the foot.
• I don't get one thing.
In the 1d motion I learnt that 2 objects irrespective of their mass will fall with the same velocity. But, according to the 2nd law of motion i.e. F=ma, force on a body is directly proportional to it's mass. And more the force, the greater the velocity of the object. • Am I correct?
F ∝ M & F ∝ A & Multiplication represents proportionality, and therefore F = M * A.

A better way to visualize everything is through A = F / M. Logically, doubling the force upon an object will double the acceleration of the object.

The unit kg * m/s^2 cannot be comprehended as kg * m/s^2 because you have created a new unit out of two independent properties: mass & acceleration. Kg * m/s^2 is a new unit that represents force, right?  