Cody seems a little reluctant to launch himself down this ramp at Newton’s Skate Park. It will be his first time down the ramp, and he knows from watching his older brother Jerod that he’ll be moving fast by the time he gets to the bottom. The faster he goes, the harder it will be to stop. That’s because of momentum.
What Is Momentum?
Momentum is a property of a moving object that makes it hard to stop. The more mass it has or the faster it’s moving, the greater its momentum. Momentum equals mass times velocity and is represented by the equation:
Momentum = Mass × Velocity
Q: What is Cody’s momentum as he stands at the top of the ramp?
A: Cody has no momentum as he stands there because he isn’t moving. In other words, his velocity is zero. However, Cody will gain momentum as he starts moving down the ramp and picks up speed.
Q: Cody’s older brother Jerod is pictured in the Figure below. If Jerod were to travel down the ramp at the same velocity as Cody, who would have greater momentum? Who would be harder to stop?
A: Jerod obviously has greater mass than Cody, so he would have greater momentum. He would also be harder to stop.
You can see an animation demonstrating the role of mass and velocity in the momentum of moving objects at this URL:
To calculate momentum with the equation above, mass is measured in (kg), and velocity is measured in meters per second (m/s). For example, Cody and his skateboard have a combined mass of 40 kg. If Cody is traveling at a velocity of 1.1 m/s by the time he reaches the bottom of the ramp, then his momentum is:
Momentum = 40 kg × 1.1 m/s = 44 kg • m/s
Note that the SI unit for momentum is kg • m/s.
Q: The combined mass of Jerod and his skateboard is 68 kg. If Jerod goes down the ramp at the same velocity as Cody, what is his momentum at the bottom of the ramp?
A: His momentum is:
Momentum = 68 kg × 1.1 m/s = 75 kg • m/s
- Momentum is a property of a moving object that makes it hard to stop. It equals the object’s mass times its velocity.
- To calculate the momentum of a moving object, multiply its mass in kilograms (kg) by its velocity in meters per second (m/s). The SI unit of momentum is kg • m/s.
At the following URL, review how to calculate momentum, and then solve the problems at the bottom of the Web page.
- Define momentum.
- Write the equation for calculating momentum from mass and velocity.
- What is the SI unit for momentum?
- Which skateboarder has greater momentum?
- Skateboarder A: mass = 60 kg; velocity = 1.5 m/s
- Skateboarder B: mass = 50 kg; velocity = 2.0 m/s